WO2005108939A1 - Cavity ringdown spectroscopy with swept-frequency laser - Google Patents

Cavity ringdown spectroscopy with swept-frequency laser Download PDF

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Publication number
WO2005108939A1
WO2005108939A1 PCT/AU2005/000683 AU2005000683W WO2005108939A1 WO 2005108939 A1 WO2005108939 A1 WO 2005108939A1 AU 2005000683 W AU2005000683 W AU 2005000683W WO 2005108939 A1 WO2005108939 A1 WO 2005108939A1
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Prior art keywords
cavity
ringdown
optical
light
frequency
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PCT/AU2005/000683
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French (fr)
Inventor
Brian John Orr
Yabai He
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Macquarie University
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Priority claimed from AU2004902516A external-priority patent/AU2004902516A0/en
Application filed by Macquarie University filed Critical Macquarie University
Publication of WO2005108939A1 publication Critical patent/WO2005108939A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
    • G01J3/433—Modulation spectrometry; Derivative spectrometry
    • G01J3/4338—Frequency modulated spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
    • G01N2021/391—Intracavity sample

Definitions

  • the present invention relates to optical absorption spectroscopy and in particular is directed to a method and apparatus for optical detection using an optical ringdown-cavity cell.
  • Background of the Invention The detection of trace and weakly absorbing gas-phase species is of importance in scientific, industrial, medical, agricultural and environmental spectroscopic sensing applications.
  • optical cavity ringdown (CRD) laser absorption spectroscopy has become a new analytical technique for dete ⁇ nination of such trace concentrations. The technique is simple, quick, versatile and an accurate way to acquire weak optical absorption spectra, the method typically being able to make optical absorption measurements with sensitivities of the order of 10 " 7 per cm of sample.
  • CRD spectroscopy involves injecting coherent light into an optical cavity acting as a high- finesse stable optical resonator (e.g., a Fabry-Perot optical cavity) formed by at least two highly reflecting input and output mirrors. A portion of the light incident on one of the mirrors enters the optical cavity and is multiply reflected inside the cavity.
  • a high- finesse stable optical resonator e.g., a Fabry-Perot optical cavity
  • the intensity of radiant energy injected into the resonator decreases in time following an exponential decay with a ringdown time (which depends on the reflectivity of the mirrors, their separation and the speed of light in the optical cavity).
  • a ringdown time which depends on the reflectivity of the mirrors, their separation and the speed of light in the optical cavity.
  • the radiant energy decrease is accelerated at those wavelengths where optical absorption by the sample occurs; this results in a shorter ringdown time (at such wavelengths).
  • An optical absorption spectrum for the sample gas is obtained by placing a photodetector after the output mirror to detect light emerging from the optical cavity and plotting the energy loss rate ⁇ "1 versus the wavelength (or, alternatively, frequency) of the cavity-ringdown light and subtracting the corresponding spectrum of the empty optical cavity.
  • This energy loss rate ⁇ "1 is also known as the cavity-ringdown decay rate, which is the reciprocal of the ringdown time ⁇ (obtained from the time profile of the intensity of light emerging from the optical cavity).
  • the shape or profile of the resulting absorption spectrum changes if the composition of the absorbing species present in the optical cavity changes.
  • the cavity-ringdown decay rate ⁇ "1 increases linearly as optical absorbance or optical absorption coefficient of the sample.
  • CRD spectroscopy has the advantage that it increases the effective optical absorption path- length by many orders of magnitude relative to the physical length of the cavity, owing to multiple reflections by the highly reflective cavity mirrors and optical resonance in that cavity.
  • CRD spectroscopy is highly sensitive because it measures time decay and not amplitude of light, and is therefore not affected by fluctuations in amplitude (optical intensity) of the light generated by the coherent light source.
  • CRD spectroscopy is generally capable of detecting optical absorption of a sample with very high sensitivity, routinely (in typical instruments) of the order of 10 "7 - 10 "9 per cm. In certain circumstances, particularly for detecting very low concentrations of absorbing species in gases, it is desirable to achieve greater sensitivities. Detection at greater sensitivities has been reported by using instruments with more specialised optical and electronic components but is not readily attainable with typical current CRD spectroscopic systems, owing to problems such as optical feedback and noise present in detection electronics.
  • CRD spectroscopy performed with a continuous-wave (cw) laser has some advantages relative to its pulsed-laser counterpart, in that higher spectroscopic resolution can be attained and in that inexpensive, compact, single-mode tunable semiconductor diode laser sources may be used.
  • cw continuous-wave
  • the spatial beam profile of such a laser is generally superior, which helps to couple the laser light into the ringdown cavity.
  • AO acousto-optic
  • EO electro-optic
  • continuous-wave or “cw” pertain to any laser or other coherent light source that continuously delivers optical power without interruption, including any such laser or coherent light source with an optical frequency or amplitude that is varied in time, in a controlled fashion (e.g., by frequency modulation or amplitude modulation or frequency sweeping).
  • Various implementations of cw-CRD spectroscopy have depended primarily on strategy (a), combined in some cases with (b) and (c) to ensure that the optical cavity and the light are in resonance.
  • AO and EO switches tend to be expensive, cumbersome, and limited in their spectral range, so that there is some practical advantage in eliminating them in applications where compactness, portability, cost, and wavelength range are considered.
  • strategy (b) it has not always been straightforward to achieve reliable, reproducible electronic control of laser amplitude and frequency, as required for high-resolution cw-CRD-spectroscopic applications.
  • a recently preferred approach in some laboratories has therefore been to implement strategy (c), thereby avoiding the inherent complexity and expense of an AO or EO switch.
  • This approach (c) has been described in detail by Y. He and B. J. Orr: "Rapidly swept, continuous- wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi- wavelength sensing of gases", Applied Physics B 75 (2002) 267 - 280.
  • the swept-cavity OHD cw-CRD spectroscopic technique is also described in International Patent Publication No.
  • CEAS cavity-enhanced absorption spectroscopy
  • cw- ICOS continuous-wave integrated cavity output spectroscopy
  • a further distinctive refinement of the rapidly-swept cw-CRD spectroscopic technique reported by He and Orr (op. cit.) comprises an optical-heterodyne-detected (OHD) approach that is intrinsically simpler than other OHD variants of cw-CRD spectroscopy employing at least one active EO or AO modulator to generate OHD sidebands.
  • OHD optical-heterodyne-detected
  • the backward- propagating light actively reflected off the rapidly swept cavity may be envisaged as a heterodyne beat signal between the directly reflected part of the incident laser light field and the backward- propagating ringdown light field, given that the moving mirror of the rapidly swept cavity Doppler-shifts the frequency of the intracavity light.
  • One of the principal advantages of the swept-cavity OHD cw-CRD technique is that it enables "single-ended" detection with both the optical transmitter and receiver naturally collocated in a single console that can be widely separated from the rapidly swept ringdown cavity module.
  • This multi-wavelength swept-cavity OHD cw-CRD approach which is intrinsically and uniquely reliant on a rapidly swept ringdown cavity and a single-ended OHD transmitter-receiver configuration, offers a substantial multiplex advantage for multi-species spectroscopic sensing applications.
  • a disadvantage of the swept-cavity OHD cw-CRD spectroscopic technique is that it relies on either a slow, continuous scan of a single laser wavelength or on the availability of several suitable fixed-wavelength lasers for multiplex detection.
  • Object of the invention It is an object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages or at least provide a suitable alternative.
  • a method for optical detection comprising: - generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - coupling, into an optical cavity defined by at least two reflectors and containing between those reflectors a sample, at least a portion of the beam of coherent light, such that cavity- ringdown light is generated by the optical cavity each time the frequency of the coherent light reaches a value that is resonant in the optical cavity, the intensity of the cavity-ringdown light being associated with build-up and ringdown decay of intracavity radiation caused when the frequency of the coherent light respectively reaches said value and departs therefrom; - decoupling at least a portion of the cavity-ringdown light from the optical cavity; and - detecting, during a period of time, a signal attributable to a property of the sample and derived from a variation in intensity of the cavity-ringdown light decoupled from the optical cavity during the period of time,
  • the exponential ringdown decay during this period of time may be characterised by a ringdown time ⁇ .
  • a cavity-ringdown decay rate may be calculated from the ringdown time.
  • the cavity-ringdown decay rate may be expressed as ⁇ "1 .
  • the phrase, "a signal ... derived from a variation in intensity of the cavity-ringdown light,” as used in this specification, shall be taken to include any coherent combination of the cavity- ringdown light with any part of the aforesaid beam of coherent light.
  • One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time.
  • either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a variation in intensity, rather than the cavity-ringdown decay rate.
  • either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a variation in intensity, rather than the cavity-ringdown decay rate.
  • the variation in intensity of the decoupled cavity-ringdown light and/or the frequency thereof may be associated with a variation in a parameter of the sample.
  • the parameter of the sample may be the concentration of a constituent thereof or a pollutant therein.
  • the frequency or a series of resonant frequencies may be characteristic of a specific chemical compound whereas the rate at which the intensity of the decoupled light decays may be associated with its concentration.
  • the variation in intensity of the cavity-ringdown light typically decays exponentially and may have superimposed oscillations owing to optical interference effects. This may be measured by a photodetector and recorded as the amplitude of an output signal obtained from the photodetector.
  • the decay period may vary from tens of nanoseconds to hundreds of micro-seconds. A convenient decay period has been found to be about 5 ⁇ s, preferably from about 1 ⁇ s to about 100 ⁇ s.
  • the beam of coherent light may be generated by a laser, a nonlinear-optical frequency converter such as a difference-frequency generator, a sum-frequency generator, or an optical parametric generator.
  • the beam of coherent light may be a single-longitudinal-mode frequency output such as that obtainable from an external grating cavity semiconductor diode laser.
  • the range of optical frequencies over which the frequency of the coherent light may be swept may typically be over two or more successive resonance frequencies of the optical cavity.
  • the interval of successive resonance frequencies of the optical cavity equals the speed of light divided by the round-trip cavity length.
  • the interval of successive resonance frequencies of the optical cavity is about 300 MHz. Therefore, the range of frequencies, over which the frequency of the coherent light may be swept to cover two cavity resonance frequencies, may be from about 600 MHz to about 900 MHz.
  • the range of frequency sweep may be up to many GHz or THz within the tunability of the light source to cover a larger frequency range.
  • the range of frequency sweep may be a few MHz and may be swept over just one single cavity resonance frequency.
  • rapidly swept is meant that the amount by which the frequency of the coherent light is changed during the ringdown (decay) time ⁇ of the optical cavity exceeds 1 / (2 ⁇ ).
  • the cavity-ringdown light decoupled from the optical cavity may be combined, usually outside of the optical cavity, with a portion of the beam from the swept-frequency coherent light source, and may be split therefrom.
  • the combination may form a coherent light beam with oscillatory characteristics suitable for optical-heterodyne detection.
  • the method according to this aspect of the invention may further include the step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means.
  • the coherent swept-frequency light may be interrupted, deflected or terminated after it has been coupled into the optical cavity, and it may be restored again after a second period of time.
  • the second period of time is preferably several times the cavity-ringdown time ⁇ (typically tens of microseconds), in order to allow each cavity-ringdown decay process to be completed.
  • the electronic means may include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source.
  • a suitable acousto-optic modulator can typically deflect or switch a light beam within about 100 ns.
  • a method for detecting one or more parameters of a sample located within an optical cavity defined by at least two reflectors comprising: - generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - coupling into the optical cavity at least a portion of the beam of coherent light, such that cavity-ringdown light is generated when the frequency of the coherent light reaches a value that is resonant in the optical cavity; - decoupling at least a portion of the cavity-ringdown light from the optical cavity; - detecting, during a period of time and for at least some of the resonant frequencies of the range of optical frequencies of the beam of coherent light, a signal attributable to a property of the sample and derived from the temporal variation in intensity of cavity-ringdown light decoupled from the optical cavity, associated with build-up and ringdown decay of intracavity radiation; and - determining a value of one or more parameters of the sample, from the
  • the exponential ringdown decay during this period of time may be characterised by a ringdown time ⁇ .
  • a cavity-ringdown decay rate may be calculated from the ringdown time.
  • the cavity-ringdown decay rate may be expressed as ⁇ "1 .
  • the phrase, "a signal ... derived from the temporal variation in intensity of cavity- ringdown light,” as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light.
  • One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time.
  • either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of the beam from the swept-frequency light source, which may be split therefrom.
  • the combination may form a coherent light beam with oscillatory characteristics suitable for optical-heterodyne detection.
  • the method according to this aspect of the invention may also include the further step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means immediately after it has been coupled into the optical cavity and by restoring it when the cavity-ringdown process has been concluded.
  • the electronic means may include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source.
  • a method for identifying and/or quantifying chemical species in a sample located within an optical cavity defined by at least two reflectors comprising: - generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - coupling into the optical cavity at least a portion of the beam of coherent light, such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - decoupling at least a portion of the cavity-ringdown light from the optical cavity; - detecting, during a period of time, a signal attributable to a property of the chemical species and derived from the temporal variation in intensity of the cavity-ringdown light decoupled from the cavity, associated with build-up and
  • the exponential ringdown decay during this period of time may be characterised by a ringdown time ⁇ .
  • the cavity-ringdown decay rate may be expressed as ⁇ "1 .
  • the phrase, "a signal ... derived from the temporal variation in intensity of the cavity- ringdown light,” as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light.
  • One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time.
  • either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of a beam from the swept-frequency light source.
  • the combination may form a coherent light beam with oscillatory characteristics suitable for optical-heterodyne detection.
  • the method according to this aspect of the invention may also include the further step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means by interrupting, deflecting or terminating the coherent swept-frequency light immediately after at least one instant when it is coupled into the optical cavity.
  • the electronic means may, as before, include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source.
  • a method for measuring spectroscopic properties of a sample located within an optical cavity defined by at least two reflectors, separated from one another by a cavity length comprising: (a) generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; (b) coupling into the optical cavity at least a portion of the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; (c) decoupling at least a portion of the cavity-ringdown light from the optical cavity; (d) measuring a signal attributable to a property of the sample and derived from the temporal variation in intensity of such cavity-ringdown light during a period of time, such variation in intensity
  • the exponential ringdown decay during this period of time may be characterised by a ringdown time ⁇ .
  • the cavity-ringdown decay rate may be expressed as ⁇ "1 .
  • the phrase, "a signal ... derived from the temporal variation in intensity of such cavity- ringdown light,” as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light.
  • One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time.
  • either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward ⁇ propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • the variation of the cavity length may be performed either in controlled discrete steps or by slow, continuous scanning.
  • the method according to this aspect of the invention may further include a step (g) wherein steps (b) to (f) are repeated to electronically sample coarsely resolved discrete points in the absorption spectrum of the sample.
  • coarsely resolved is meant, for a given cavity length, the frequency interval of the data points of the spectrum is set by the interval of the successive cavity-resonance frequencies of the optical cavity.
  • the method according to this aspect of the invention may also include a step (h) wherein steps (b) to (g) are repeated many times to assemble a more extensive, but still coarsely resolved, absorption spectrum of the sample. The method may be repeated to electronically sample finely resolved discrete points in the absorption spectrum of the sample.
  • the frequency interval of the data points of the spectrum is less than the interval of the successive cavity-resonance frequencies of the optical cavity.
  • the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of a beam from the swept-frequency light source so as to form a combined beam with oscillatory characteristics suitable for optical-heterodyne detection.
  • the method according to this aspect of the invention may also include the further step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means after at least one instant when it is coupled into the optical cavity.
  • the coherent light beam may be restored when the cavity- ringdown process has been concluded.
  • the electronic means may, as before, include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source.
  • a method for measuring spectroscopic properties of a sample located within an optical cavity defined by at least two reflectors separated from one another by a cavity length comprising: (a) generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies using a swept-frequency coherent light source that is tunable over at least one and optionally over at least two optical frequency ranges; (b) coupling into the optical cavity at least a portion of the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; (c) decoupling at least a portion of the cavity-ringdown light from the optical cavity; (d) measuring, during a period of time, a signal attributable to a property of the sample and derived from the temporal variation in intensity of such cavity-ringdown light, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation; (e
  • the exponential ringdown decay during this period of time may be characterised by a ringdown time ⁇ .
  • the cavity-ringdown decay rate may be expressed as ⁇ "1 .
  • the phrase, "a signal ... derived from the temporal variation in intensity of such cavity- ringdown light,” as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light.
  • One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time.
  • either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • the method according to this aspect of the invention may further include a step (h) wherein the cavity length of the optical cavity is adjusted before measuring another coarsely resolved set of discrete points in a different portion of the sample's abso ⁇ tion spectrum, corresponding to a different portion of the SF cycle.
  • This variation of the cavity length may be performed either in controlled discrete steps or by slow, continuous scanning.
  • the method according to this aspect of the invention may optionally include a further step (i) in which steps (b) to (h) are repeated many times to assemble a finely resolved abso ⁇ tion spectrum of the sample.
  • the abso ⁇ tion spectrum may conveniently be representative of the parameter of the sample.
  • the swept-frequency coherent light source may be tunable over at least one and optionally over two or more optical frequency ranges.
  • a single range may be from about 1 MHz to many GHz or THz. Different ranges may be separated or may have overlapping optical frequencies (or wavelengths).
  • the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of a beam from the swept-frequency light source. The combination may form a coherent light beam with oscillatory characteristics suitable for optical-heterodyne detection.
  • the method according to this aspect of the invention may also include the further step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means by interrupting, deflecting or terminating the coherent swept-frequency light immediately after at least one instant when it is coupled into the optical cavity.
  • the electronic means may, as before, include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source.
  • an optical detection system comprising: - a coherent light source capable of generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - an optical cavity, defined by at least two reflectors and adapted to, in use, contain between those reflectors a sample, the optical cavity being optically coupled, in use, to at least a portion of the beam of coherent light, such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler to decouple at least a portion of the cavity-ringdown light from the optical cavity; and - a photodetector to detect, during a period of time, a signal attributable to a property of the sample and
  • the exponential ringdown decay during this period of time may be characterised by a ringdown time ⁇ .
  • a signal ... derived from a temporal variation in intensity of such cavity- ringdown light shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light.
  • One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time.
  • either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • the optical detection apparatus or system may further include an electronic processor to convert an output from the photodetector to a cavity-ringdown decay rate.
  • the cavity-ringdown decay rate may be expressed as ⁇ "1 .
  • the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of the beam from the swept-frequency light source.
  • the combination may form a coherent light beam with oscillatory characteristics suitable for optical-heterodyne detection.
  • the method according to this aspect of the invention may also include the further step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means by interrupting, deflecting or terminating the coherent swept-frequency light immediately after at least one instant when it is coupled into the optical cavity.
  • the electronic means may, as before, include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source.
  • an optical detection system for detecting a parameter of a sample comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the
  • the exponential ringdown decay during this period of time may be characterised by a ringdown time ⁇ . From the ringdown time ⁇ , a cavity-ringdown decay rate may be calculated. The cavity-ringdown decay rate may be expressed as ⁇ "1 .
  • the phrase, "a signal ... derived from the temporal variation in intensity of such cavity- ringdown light,” as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light.
  • One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time.
  • either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of the beam from the swept-frequency light source so as to form a coherent light beam with oscillatory characteristics enabling optical-heterodyne detection.
  • an optical system for identifying and/or quantifying at least one chemical species in a sample comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of optical frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the frequency of the coherent light reaches such values, a signal attributable to a property of the chemical species and derived from the temporal variation in intensity of such cavity-ringdown light decoupled from the cavity,
  • the exponential ringdown decay during this period of time may be characterised by a ringdown time.
  • the ringdown time may be expressed as ⁇ .
  • a cavity- ringdown decay rate may be calculated from the ringdown time ⁇ .
  • the cavity-ringdown decay rate may be expressed as ⁇ "1 .
  • the phrase, "a signal ... derived from the temporal variation in intensity of such cavity- ringdown light,” as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light.
  • One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time.
  • either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • an optical system for spectroscopically measuring a parameter or a property of a sample, the method comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of optical frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the frequency of the coherent light reaches such values, a signal attributable to the parameter or the property and derived from the temporal variation in intensity of such cavity-ringdown light decoup
  • the exponential ringdown decay during this period of time may be characterised by a ringdown time.
  • the ringdown time may be expressed as ⁇ .
  • a cavity- ringdown decay rate may be calculated from the ringdown time ⁇ .
  • the cavity-ringdown decay rate may be expressed as ⁇ ⁇ ⁇
  • the phrase, "a signal ... derived from the temporal variation in intensity of such cavity- ringdown light,” as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light.
  • One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time.
  • either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • an optical system for measuring spectroscopic properties of a sample comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of optical frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the frequency of the coherent light reaches such values, a signal attributable to a property of the sample and derived from the temporal variation in intensity of such cavity-ringdown light decoupled from the cavity, such variation in intensity
  • the exponential ringdown decay during this period of time may be characterised by a ringdown time.
  • the ringdown time may be expressed as ⁇ .
  • a cavity- ringdown decay rate may be calculated from the ringdown time ⁇ .
  • the cavity-ringdown decay rate may be expressed as ⁇ " ⁇ .
  • a signal ... derived from the temporal variation in intensity of such cavity- ringdown light shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light.
  • One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time.
  • either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate.
  • the resonance frequencies of the cavity may be altered by varying the cavity length either in controlled discrete steps or by continuous scanning, which may be slow.
  • the coherent light may be generated by a coherent light source selected from a continuous- wave (cw) coherent light source (i.e., with amplitude or intensity that does not vary rapidly in time) and a quasi-continuous-wave (quasi-cw) or long-pulse coherent light source, that is able to generate coherent light over a range of optical frequencies.
  • a coherent light source selected from a continuous- wave (cw) coherent light source (i.e., with amplitude or intensity that does not vary rapidly in time) and a quasi-continuous-wave (quasi-cw) or long-pulse coherent light source, that is able to generate coherent light over a range of optical frequencies.
  • cw continuous- wave
  • quadsi-cw quasi-continuous-wave
  • long-pulse coherent light source that is able to generate coherent light over a range of optical frequencies.
  • coherent light sources include lasers, continuous and quasi-continuous light sources such as nonlinear-optical devices of various forms, including laser-pumped wavelength converters, optical parametric oscillators (OPOs), optical parametric generators (OPGs), optical parametric amplifiers (OP As); second- harmonic generators (SHGs); third-harmonic generators (THGs); sum-frequency generators (SFGs); difference-frequency generators (DFGs); stimulated Raman scattering (SRS) devices, and the like.
  • OPOs optical parametric oscillators
  • OPGs optical parametric generators
  • OP As optical parametric amplifiers
  • SHGs second- harmonic generators
  • SHGs third-harmonic generators
  • FSGs sum-frequency generators
  • DFGs difference-frequency generators
  • SRS stimulated Raman scattering
  • the descriptions "continuous-wave” or “cw” are taken to apply to any laser or other coherent light source that continuously delivers optical power without interruption, including any such laser or coherent light source with an optical frequency or amplitude that is varied in time, in a controlled fashion (e.g., by frequency modulation or amplitude modulation or frequency sweeping).
  • the laser may be selected from infrared lasers, diode lasers, and single-mode tunable continuous-wave (cw) dye lasers. Suitable lasers also include distributed-feedback lasers, external optical-cavity lasers and optical-fibre lasers.
  • the laser can be monochromatic or wide- spectrum, provided that means are provided to control or select its wavelength, which control or selection may be part of the laser itself or of an external component situated elsewhere in the system. Conveniently, the laser has an optical bandwidth of less than 10 MHz.
  • One suitable form of a quasi-cw coherent light source is a long-pulse laser. Its optical power needs to be established at a constant plateau level well in advance of the point in the frequency-sweep cycle at which the optical cavity comes into resonance with the light, while optical power builds up in the cavity.
  • the coherent light source may be a continuous-wave (cw) tunable laser, which may be a semiconductor diode laser that is able to generate continuous-wave (cw) coherent light over a range of optical frequencies during a swept-frequency (SF) cycle.
  • the range of optical frequencies that is generated during a broad SF cycle may be from about 1 MHz to about 10 THz.
  • a single coherent light source may be used, although in some embodiments more than one coherent light source may be desirable (e.g., two, three or more lasers). In such events, each coherent light source may have a different wavelength (or set of SF-cycled wavelengths) to advantageously cover a combined wavelength range which is larger than can be obtained when only one laser is used.
  • the performance advantages of one light source i.e., such as high frequency stability
  • Diode lasers are typically small and inexpensive, enabling an apparatus to be designed for portability and compactness.
  • Continuous-wave (cw) tunable lasers are generally superior to pulsed lasers due to the narrower optical bandwidth of the cw tunable laser output and the narrowband filtering role of the ringdown optical cavity.
  • the laser suitably generates light comprising a wavelength corresponding to an optical abso ⁇ tion region of interest in a sample, preferably a gaseous sample.
  • the continuous-wave (cw) tunable laser used emits infrared, ultraviolet, or visible wavelengths, typically infrared. Accordingly, the term light as used in the description of the invention is intended to include light or radiation at these wavelengths. The term light is to be inte ⁇ reted broadly and may include light within other wavelengths of the wavelength spectrum.
  • the ringdown-cavity cell may be a linear or folded optical cavity or a ring resonator such as adopted in US Patent 5,912,790 by way of reference.
  • the ringdown-cavity cell is preferably a linear cell of two reflectors, suitably two highly reflective dielectrically-coated input and output mirrors having a concave curvature designed to retain light in the optical cavity. Suitable radii of curvature of the mirrors are in a range of 0.01 to 10 metres, typically about 1 metre.
  • An optically stable, low-loss optical cavity is formed from the two mirrors being aligned with respect to the optical axis of the cell, the mirror separation typically being less than twice the radius of curvature.
  • Suitable distances between the optical-cavity mirrors are of the order of 10 to 1000 mm. The distance may be adjusted to optimise separation of low-order transverse optical-cavity modes to the longitudinal cavity modes.
  • This description of the invention is concerned primarily with the longitudinal modes of the optical ringdown cavity, but the transverse-mode structure of the cavity is also within the scope of this invention.
  • the coupling of input coherent-light-source/laser radiation into the lowest-order transverse modes (which correspond to the longitudinal or axial modes) of the cavity may be preferred and may be achieved by using a suitable combination of lenses or mirrors to match the input-beam geometry to the cavity-mode geometry.
  • Suitable mirrors for use in the invention are well known in the art. They may be suitable for use over a wide range of the visible, ultraviolet and infrared spectrum. Suitable mirrors typically have reflectivity greater than 99.95% and up to 99.999% to enable most radiation entering the optical cavity to be reflected over path-lengths of the order of about 10 4 times the physical length of the cell, thereby providing a long effective optical abso ⁇ tion path-length within the optical cavity and providing a ringdown decay time of a few microseconds.
  • a sample may be provided in the cell.
  • the sample is preferably a gas. It may be introduced into the cell in the vicinity of the optical axis of the ringdown-cavity cell.
  • gases which can be detected by use of the method and apparatus of the invention include carbon dioxide (C0 2 ), carbon monoxide (CO), water vapour (H 2 0), nitric oxide (NO), oxygen (0 2 ), hydrogen fluoride (HF), nitrous oxide (N 2 0), hydrogen cyanide (HCN), methane (CH 4 ), light alkanes (C n H 2n+2 ), ethylene (C 2 H 4 ), acetylene (C 2 H 2 ), ethanol (C 2 H 5 0H), acetaldehyde (CH 3 CHO), ketones such as acetone, (CH 3 ) 2 CO, chloroform (CHC1 3 ) and ammonia (NH 3 ).
  • gas-phase chemical agents such as nerve gas, phosgene, and vapour associated with common explosives can be detected for security pu ⁇ oses if suitable mid-infrared tunable lasers are available.
  • Combinations of gases can also be detected and, if necessary in order to do so, more than one coherent light source, which may be continuous-wave sources, may be used. These sources may have wavelengths corresponding to wavelengths of the optical abso ⁇ tion regions of the gases of interest.
  • the ringdown-cavity cell can be formulated as a separate module from the remaining apparatus so that it can be used in remote sensing applications.
  • Certain properties of solid and liquid samples can also be measured using CRD spectroscopy by depositing small amounts of a sample to the coating surface of at least one ringdown-cavity mirror/reflector.
  • an extra environmentally sensitive mirror e.g., by evanescent-wave coupling
  • the means for directing a portion of said coherent light/radiation emitted from the continuous-wave coherent-light-source/laser to the ringdown-cavity cell may ensure that the light/radiation emitted from the coherent-light-source/laser follows an optical path into the ringdown-cavity cell.
  • suitable means for directing a portion of said light/radiation is provided by an optical circulator which has the advantage of being able to route one portion of the light/radiation by optical fibre from the coherent-light-source/laser to the ringdown-cavity cell and to route the light/radiation back-reflected by the optical cavity by optical fibre to a photodetector, thereby preventing the light/radiation back-reflected by the optical cavity returning to the coherent-light-source/laser.
  • the optical circulator may be of a type widely used in fibre-optical telecommunication networks.
  • the light/radiation is preferably coupled to the input of the ringdown-cavity cell by means of a combination of any one of at least one reflector, a polarisation-control means, an optical circulator means, an optical isolator means and a lens system (suitably a 5-cm to 5-m focal-length lens, either single or compound, more typically a 40-cm focal-length lens) which may collect and may direct the light/radiation into the optical cavity.
  • the polarisation-control optics means may function to direct backward-propagating light to the detection means (suitably a photodetector) and to control the polarisation of light incident on the detecting means and/or on the input of the ringdown-cavity cell.
  • the polarisation-control optics means may comprise a polarising prism/beam splitter, waveplate, optical circulator or Faraday rotator.
  • the polarisation-control optic means comprises a polarising beam splitter and a Faraday rotator, or a polarising beam splitter and a quarter-wave plate or an optical circulator.
  • the rotation angle of a Faraday rotator and thickness of a quarter-wave plate as well as the performance of the optical circulator are wavelength dependent.
  • they may be designed for working in a narrow wavelength region. They may be specified by their designed centre wavelengths. Accordingly, it is desirable to select them on the basis of the wavelengths of interest, because operating at wavelengths far away from the designed wavelengths will result in a reduction of coupling efficiency of the backward-propagated light to the detecting means and an increasing amount of the backward-propagated light may reach the coherent-light-source/laser causing disruption or damage.
  • a combiner/multiplexer an optical switch, a switch module or an optical-fibre splitter so as to distribute the coherent-light-source/laser radiation and to return cavity-ringdown light or to switch between the various wavelengths of multiple coherent light sources or lasers.
  • optical energy is built up and stored in the optical cavity when the frequency moves into the vicinity of a resonance frequency of the cavity.
  • the ringdown decay of light energy that is built up and stored in the optical cavity during the short resonance interval is observable after the frequency of the coherent radiation has moved off resonance, because the input light is then effectively blocked, owing to the absence of resonant interaction within the cavity, by the highly reflective optical cavity reflectors/mirrors during the (relatively long) off-resonance interval.
  • the decay of light energy is gradual with a ringdown time constant (that depends on the reflector/mirror reflectivity and the abso ⁇ tion of the optical medium in the optical cavity.
  • this build-up and ringdown-decay event repeats each time during a SF cycle that the frequency of the coherent radiation moves across a cavity resonance frequency.
  • the length of the optical cavity may be varied incrementally or decrementally by an optical-length adjuster, in order to obtain additional ringdown-time data at different cavity-resonance frequencies after the coherent-light-source/laser has emitted light/radiation at a range of optical frequencies during a SF cycle.
  • This incrementally (or decrementally) stepped cavity-length (ISCL) adjustment may occur between successive SF cycles.
  • An alternative approach is to vary the length of the optical cavity by continuous scanning,of an analog control input to the optical-length adjuster, in a manner that is synchronised with successive SF cycles and is sufficiently slow that many SF cycles (e.g., at least 10) occur before the cavity length traverses one free spectral range (FSR, corresponding to half the wavelength of the light).
  • This slow, synchronously scanned cavity-length (SSCL) adjustment may either occur continuously during and between successive SF cycles or it may be controlled to occur only during portions of the SF cycle when cavity-ringdown signals are recorded.
  • Either form of optical-length adjustment may be provided by a piezoelectric translator (PZT) operated to provide movement of at least one of the reflectors/mirrors constituting the optical-cavity, so as to vary the length thereof.
  • the reflector/mirror may conveniently be mounted on a cylindrical PZT device controlled by an electronic control circuit, so that the reflector/mirror can be moved either in a prescribed incremental fashion between one SF cycle and the next, using the ISCL procedure, or by slow, continuous scanning using the SSCL procedure.
  • Suitable optical-cavity length changes in the ISCL procedure are of the order of from about 1 nm to about 10 ⁇ m.
  • the maximum useful cavity-length scan range in the SSCL procedure is one free spectral range (FSR) of the cavity (i.e., half the wavelength of the light).
  • FSR free spectral range
  • the optical-cavity resonance frequency changes as one of the optical-cavity mirrors is incrementally moved between SF cycles, using the ISCL procedure, or continuously scanned, using the SSCL procedure. Depending on the distance of mirror movement, the amount of frequency shift is typically in the range of 10 kHz to 100 MHz.
  • a ringdown signal may be generated while still using light/radiation that is continuous-wave (i.e., with amplitude or intensity that does not vary rapidly, apart from its SF characteristics).
  • a fast optical switch such as an electro- optic modulator, a Pockels cell, an electro-optic Kerr cell or an acousto-optic modulator.
  • the system of the invention may be made more compact, more robust and less expensive. Moreover, in this embodiment it is not necessary to lock the optical-cavity length and the coherent-light-source/laser wavelength to each other.
  • the means for directing the coherent light and the means for combining the coherent light and the cavity-ringdown light may be the same.
  • the frequency of coherent light emitted from the swept-frequency coherent light source changes monotonically and may match many successive discrete resonance frequencies of the ringdown cavity. This may create many sequential build-up and ringdown- decay events. Each discrete matching frequency defines the frequency of the cavity-ringdown light/radiation for each corresponding build-up and ringdown-decay event.
  • an intense detectable signal (a beat signal) may be obtained which is the product of two fields containing a strong radiation component corresponding to the intense light/radiation coupled with a weak build-up and ringdown-decay component of light/radiation emitted by the ringdown cavity.
  • the coherent light that may be used as the intense light/radiation for combining with the cavity-ringdown light/radiation is preferably coherent light emitted from the coherent light source which either. (a) has not passed through the cavity cell but has been back-reflected off the cell and/or (b) coherent light that has pu ⁇ osely been redirected (e.g., by at least one beam splitter).
  • the cavity-ringdown light which is combined is either cavity-ringdown light emerging from the input mirror or reflector and/or cavity-ringdown light emerging from the output mirror or reflector.
  • the method of the invention enables optical signals related to the ringdown time ⁇ to be detected at a high modulation frequency, determined by the SF sweep rate, rather than at the relatively low frequency of the intrinsic ringdown decay signal, thereby enabling more sensitive detection.
  • the cavity-ringdown light or radiation which is combined is either cavity-ringdown light/radiation emerging from the input mirror or reflector and/or cavity-ringdown light or radiation emerging from the output mirror or reflector.
  • the cavity-ringdown light or radiation is fixed at a cavity-resonance frequency, while the much more intense emitted SF light or radiation has a continuously shifting frequency.
  • an intense detectable wave may be obtained, which is the product of weak cavity-ringdown light/radiation field coupled with a strong SF coherent-light-source/laser light or radiation field which varies in accordance with the swept frequency shift.
  • the method of the invention thereby enables optical signals related to the ringdown time ⁇ to be detected at a high modulation frequency, determined by the SF sweep rate, rather than at the relatively low frequency of the intrinsic ringdown decay signal to enable more sensitive detection.
  • the preferred mode of detection is described in common parlance as optical-heterodyne detection.
  • the Macquarie Dictionary (2 nd Edition, The Macquarie Library Pty Ltd) defines heterodyne as denoting or pertaining to a method of receiving continuous-wave radiotelegraph signals by impressing upon the continuous radiofrequency oscillations another set of radiofrequency oscillations of a slightly different frequency, the interference resulting in fluctuations or beats of audiofrequency.
  • the Macquarie Dictionary (2 nd Edition, The Macquarie Library Pty Ltd) defines homodyne as a radio receiver which demodulates an amplitude- modulated signal by the process of mixing the carrier signal with the sidebands.
  • the means for optical-heterodyne detection of the combined light/radiation may conveniently comprise at least one photodetector, typically a square-law photodetector. In some embodiments at least two photodetectors may be required.
  • the photodetector(s) may be coupled to a preamplifier (suitably with a bandwidth of up to 500 MHz, more preferably 10 - 100 MHz) and a data-acquisition device such as an A/D converter, a digital oscilloscope or a boxcar integrator system.
  • the photodetector detects the combined light/radiation measuring the interaction of a sample contained within the optical cavity with intracavity light/radiation. The combined light/radiation is converted by the photodetector into a corresponding signal.
  • Suitable photodetectors include photomultiplier tubes and semiconductor photodiodes. Avalanche, Si, Ge, AIGaAs, InGaAs and HgCdTe photodiodes are suitable.
  • the photodetector(s) may be located before and/or after the ringdown-cavity cell.
  • the photodetector is located before the ringdown-cavity cell and is preferably located along an optical light path different to that of the incoming light/radiation. This may be achieved by use of a polarising beam splitter and a polarisation controller or by use of an optical circulator.
  • the photodetector is located after the ringdown-cavity cell, prior to reaching the photodetector, a portion of the emitted light/radiation is suitably directed by means of a beam splitter and a number of reflectors prior to combination with cavity-ringdown light/radiation emerging from the output of the ringdown-cavity cell, suitably by means of beam splitters.
  • two photodetectors are used, one located before the ringdown-cavity cell and one after the ringdown- cavity cell.
  • the photodetector located before the ringdown-cavity cell suitably detects the combined light/radiation as above, and the second photodetector detects cavity-ringdown light/radiation emerging from the output of the ringdown-cavity cell.
  • Means for determining a parameter of a sample is suitably a data analysis and recording device such as a computer.
  • Various parameters of interest which can be determined from the photodetector signals include cavity-ringdown decay rates and the time dependence of the light intensity.
  • the ringdown-decay rate of the combined light/radiation is suitably determined by amplifying, rectifying and digitising the electronic signal and fitting the waveform to an envelope with an exponential decay, using a suitable algorithm.
  • the output of the combined light/radiation may be suitably processed by a demodulating logarithmic amplifier (e.g., Analog Devices, model AD8307) and subsequently fit the waveform to a straight line, using a suitable algorithm.
  • a demodulating logarithmic amplifier e.g., Analog Devices, model AD8307
  • the ringdown decay curve and or the slope of its demodulated logarithm are indicative of the optical abso ⁇ tion of a sample.
  • the level of trace species in a sample or the optical abso ⁇ tion spectra of a known composition may be determined.
  • the ringdown decay curve can also be correlated to other parameters aside from abso ⁇ tion, including light scattering, reflectivity and dielectric relaxation or any other parameter that causes an energy change as a result of the interaction between the intracavity light/radiation and the sample.
  • abso ⁇ tion including light scattering, reflectivity and dielectric relaxation or any other parameter that causes an energy change as a result of the interaction between the intracavity light/radiation and the sample.
  • the cavity-ringdown decay rate at multiple discrete cavity resonance frequencies of the optical cavity may be determined to obtain a coarsely resolved spectrum that is electronically sampled in a single SF cycle over the output wavelength range of the coherent light source; this can typically be accomplished in less than 1 second, including time for signal-averaging to enhance the signal-to-noise ratio.
  • the method of the invention may also be used to determine the presence of more than one trace species in a sample (each trace species having different characteristic optical abso ⁇ tion wavelengths) or alternatively, to confirm the presence of a trace species having multiple characteristic optical abso ⁇ tion wavelengths.
  • Improvement of frequency resolution in the abso ⁇ tion spectrum of the sample may be achieved by using the ISCL procedure to record multiple coarsely resolved CRD spectra at successive incrementally or decrementally shifted resonance frequencies of the optical cavity and by combining in a single more finely resolved plot all the cavity-ringdown decay rates and their corresponding frequencies.
  • the resonance frequencies of the optical cavity may be shifted incrementally or decrementally by changing the cavity length such as by displacing a cavity mirror by a piezoelectric translator.
  • similar outcomes may be achieved by slow, continuous scanning using the SSCL procedure.
  • a triangular (or ring) ringdown-cavity is assembled by introducing a third mirror, the reflectivity of which is designed to be sensitive to its environment, such as temperature, pressure, humidity, external electric or magnetic fields or pressure/concentration of a particular chemical species (e.g., one that adsorbs on the surface of the third mirror).
  • the intracavity optical field may be coupled via an evanescent wave to a medium or environmental conditions that are external to the optical cavity itself.
  • the angle between the incident and reflected beams on the mirror/reflector connected to the PZT device is small, to minimise displacement of the beam path when PZT-induced adjustments occur.
  • the optical cavity may be formed by four or more mirrors.
  • optical fibres are used to transmit the light/radiation over various portions of the beam path, for example, between the coherent-light- source/laser and the ringdown-cavity cell.
  • the means for directing a portion of said light/radiation comprises an optical fibre and/or the means for combining a portion of said light/radiation emitted with cavity-ringdown light/radiation comprises an optical fibre.
  • the optical-cavity cell may be remotely isolated from the light source and detection system which may be provided in a single transmitter-receiver unit, thereby facilitating scientific, environmental, agricultural, medical and industrial monitoring applications.
  • the PZT device may be controlled by a separate voltage source, by electrical connection to the control means or by wireless means.
  • Suitable optical fibres include silica fibres (which have a cut-off wavelength of about 1.8 ⁇ m) and fluoride glass fibres (which can operate up to 3- ⁇ m wavelengths).
  • silica fibres which have a cut-off wavelength of about 1.8 ⁇ m
  • fluoride glass fibres which can operate up to 3- ⁇ m wavelengths.
  • the ringdown-cavity resonance characteristics of the cavity cell may be varied by incrementally or decrementally altering the optical path-length at the end of a SF cycle and then applying another SF cycle (or repetitive SF cycles, to enable signal-averaging) to the cavity cell, using the ISCL procedure.
  • This advantageously enables successive CRD measurements to be sampled at multiple sets of cavity-resonance frequencies so that the coarsely resolved spectroscopic data from these frequency sweeps can be combined to obtain more finely resolved CRD-spectroscopic results.
  • similar outcomes may be achieved by slow, continuous scanning using the SSCL procedure.
  • the resonance properties of a frequency-swept coherent-light-source/laser and optical cavity length that is fixed (or incrementally varied, using the ISCL procedure, or continuously scanned, using the SSCL procedure) also simplify CRD spectroscopy with a swept-frequency cw coherent-light- source/laser in that they eliminate the need for a fast optical switch and avoid locking of the optical-cavity length and coherent-light-source/laser wavelength to each other. Nevertheless, one or more optical switches may still be advantageous in some applications.
  • ultra-sensitive, high-resolution, accurate CRD spectroscopy is obtained with relatively simple, inexpensive, compact apparatus suitable for use in the field or at industrial sites or in clinical situations.
  • Figure 1 is a schematic drawing of an optical detection system according to ⁇ one embodiment of the present invention
  • Figures 2 (a) to 2(c) are graphs of simultaneously recorded signals measured with a swept- frequency coherent-light-source/laser and an optical cavity of fixed optical cavity length, Figure 2(a) being a representation of a build-up and ringdown signal detected in the forward-propagating or transmitting direction of the optical cavity
  • Figure 2(b) is a representation of an optical- heterodyne signal prepared by combining a build-up and ringdown signal propagating in the backward direction from the optical cavity, with swept-frequency coherent-light-source/laser light reflected from the front mirror of the optical cavity
  • Figure 2(c) represents the demodulated logarithm of the optical-heterodyne signal of Figure 2(b)
  • Figures 3(a) to 3(g) depict a sequence
  • FIG. 1 there is shown a schematic diagram of one embodiment of an optical detection system 10 in accordance with the invention, for measuring a spectroscopic property of a sample.
  • the optical detection system 10 comprises a coherent light source in the form of continuous-wave (cw) tunable swept-frequency (SF) coherent-light-source/laser 12 that emits coherent light of which the optical frequency can be varied over a range from about 1 MHz to many GHz or THz.
  • cw continuous-wave
  • SF swept-frequency
  • the cw tunable SF coherent-light-source/laser 12 is optically coupled to an optical resonator in the form of a ringdown-cavity measuring cell 14.
  • the ringdown-cavity measuring cell 14 is provided with a pair of highly reflective mirrors 16.1, 16.2 facing one another and defining an optical cavity 18 between them. Suitable mirrors typically have a reflectivity greater than 99.95% and up to 99.999%.
  • the surfaces of the mirrors 16.1, 16.2 that face one another are concave, each mirror 16.1, 16.2 having a radius of curvature of about 1 m.
  • the mirrors 16.1, 16.2 are aligned with respect to an optical axis 20.
  • the distance between the mirrors 16.1, 16.2 is initially set at about 0.5 m, but this distance can be varied by an optical-length adjuster in the form of a piezoelectric translator (PZT) 22, that is attached to the mirror 16.2, for moving the mirror 16.2 incrementally or continuously along the optical axis 20, relative to the mirror 16.1.
  • the ringdown-cavity measuring cell 14 is designed to operate according to the cavity- ringdown principle, as described in the following review articles: "Cavity-Ringdown Spectroscopy - An Ultratrace-abso ⁇ tion Measurement Technique", edited by K. W. Busch and M. A. Busch, ACS Symposium Series (1999) No.
  • a gaseous sample 24 is introduced into the ringdown-cavity measuring cell 14, so that it constitutes an optical medium in the optical cavity 18.
  • the optical system 10 is designed to detect the presence of trace chemical species in the sample 24.
  • the sample 24 is directed into the ringdown-cavity measuring cell 14 by sample handling means 26.
  • Two transparent windows 14.1 and 14.2 are provided in opposite ends of the wall of the ringdown-cavity measuring cell 14, so as to allow coherent light to enter and leave the ringdown- cavity measuring cell 14.
  • the ringdown-cavity measuring cell 14 is optically coupled to the cw tunable SF coherent- light-source/laser 12 by single-mode optical fibres 28, 30 and an optical coupler in the form of an optical circulator 32.
  • the ends (not shown) of the optical fibres 28, 30 are preferably cut at an angle (typically 8 degrees, as widely used in fibre-optic technology) to avoid interference caused by the returning reflection from the end facets of the fibres.
  • a lens (or system of lenses) 34 is provided in the optical path of the light being emitted by the cw tunable SF coherent-light-source/laser 12 towards the ringdown-cavity measuring cell 14, to collect and direct such light into the optical cavity 18 of the ringdown-cavity measuring cell 14.
  • the lens system 34 comprises a microscope objective in a suitable optical-fibre micropositioner.
  • the lens system 34 and optical fibre 30 are also able to collect and transmit both backward-propagating intracavity light and reflected incident SF coherent-light-source/laser light from the mirror 16.1 of the ringdown-cavity measuring cell 14.
  • the optical circulator 32 directs backward-propagating light from the ringdown-cavity measuring cell 14 towards a photodetector 36 (also designated as PD1 in Figure 1), via a line 32.1 (that may for example be an optical fibre or a freely propagating light beam).
  • the optical circulator 32 also suppresses backward-propagating light from returning to the cw tunable SF coherent-light-source/laser 12, and protects the SF coherent-light-source/laser 12 from interruption or damage by the returned light.
  • An additional optical isolator (not shown in Figure 1) may also be integrated into the SF coherent-light-source/laser 12, and/or into the optical fibre 28 and/or be inserted between the SF coherent-light-source/laser 12 and the optical circulator 32, to provide further suppression of back-reflected light reaching the SF coherent-light-source/laser 12.
  • Another optical detector in the form of a photodetector 38 is provided on the remote side of the ringdown-cavity measuring cell 14, to detect light that has passed through the optical cavity 18, the mirror 16.2 and the window 14.2.
  • a further optical fibre may be provided for transmitting light emerging from the window 14.2 to the photodetector 38 (PD2).
  • the photodetectors 36 (PD1) and 38 (PD2) detect light by converting it into corresponding electrical signals.
  • the photodetector 36 (PD1) converts the combined beam 32.1 from the optical circulator 32 into a corresponding electrical signal. This beam has oscillatory characteristics suitable for optical-heterodyne detection, as explained above under "Summary of the Invention".
  • optical-heterodyne beat signal in the previous swept-cavity case is generated by interference of Doppler-shifted cavity-ringdown light emitted from the cavity with fixed-frequency light reflected from the front mirror of the cavity
  • optical-heterodyne beat signal in the swept-frequency case here may be generated by interference of fixed-frequency cavity-ringdown light emitted from the cavity with swept-frequency light reflected from the front mirror of the cavity.
  • the signal from photodetectors 36 (PD1) or 38 (PD2) may also be used optionally for swept-frequency (SF) cavity-enhanced abso ⁇ tion spectroscopy (CEAS), in which either the peak amplitude for build-up and decay of the transmitted CRD signal or the peak-to-peak amplitude of the OHD CRD signal is measured, rather than the cavity-ringdown decay rate.
  • SF CEAS approaches are different from the swept-cavity CEAS technique, as outlined above in the "Background of the Invention" section and explained in the following article by Y. He and B. J.
  • a swept-frequency (SF) variant of integrated cavity output spectroscopy may also be used optionally, either by integrating the signal profile for build-up and decay of the transmitted CRD signal or by rectifying and integrating the full-wave OHD CRD signal, rather than measuring the cavity-ringdown-decay rate or the SF CEAS signal.
  • SF ICOS approaches are different from the modulated-cavity cw-ICOS technique, as outlined above in the "Background of the Invention" section and explained in the following article by A. O'Keefe, J. J. Scherer and J. B. Paul: "cw Integrated cavity output spectroscopy", Chemical Physics Letters 307 (1999) 343 - 349.
  • the cw-ICOS signal in the previous swept-cavity case is generated by build-up and decay of light with fixed laser frequency and modulated optical-cavity length
  • the SF ICOS signal in the swept-frequency cases here may be generated by build-up and decay of light with fixed optical-cavity length and swept coherent-light-source/laser frequency.
  • the output of the photodetector 36 (PD1) has the profile as is shown in Figure 3(a). It is sent to an electronic unit 40 via a line 36.1 and is there processed to obtain the demodulated envelope of the oscillatory signal as is shown in Figure 3(b).
  • An electronic data-acquisition module 42 is provided to collect and store the output signal, as shown in Figure 3(b), of the electronic circuit 40 via a line 40.1.
  • the same data-acquisition module 42 is also provided to collect and store the output signal, as shown in Figure 3(c), of the photodetector 38 (PD2) via a line 46.4.
  • a suitable data-acquisition module 42 may be a digital oscilloscope or an analog-to-digital converter.
  • the module 42 is in communication with the coherent-light-source/laser 12 via a line 46.3.
  • a signal representing the status of the coherent- light-source/laser 12, as is shown in Figure 3(e) is transmitted along this line 46.3.
  • a data- processing and computer .control unit 44 including control electronics (not shown separately) is coupled to the data-acquisition module 42 by means of a line 46.6.
  • the control electronics of the processing-and-control unit 44 are connected, via a connection 46.1, to the piezoelectric translator 22, to enable controlled incremental, decremental or continuous movement of the mirror 16.2.
  • the control electronics of the processing-and-control unit 44 are also connected to the sample handling means 26, via a line 46.5.
  • the processing-and-control unit 44 is also coupled to the SF coherent-light-source/laser 12, as is shown by line 46.2, to control the frequency of the light emitted by the cw tunable SF coherent-light-source/laser 12 and to analyse the data detected by photodetectors 36 (PD1) and 38 (PD2) that are recorded by the data-acquisition module 42.
  • the cw tunable SF coherent-light-source/laser 12 emits light of which the frequency varies over a range of optical frequencies during each swept-frequency (SF) cycle.
  • a portion of the emitted SF coherent-light-source/laser light then enters the optical cavity 18 where it is reflected between the highly reflective mirrors 16.1 and 16.2 a great number of times.
  • the distance between the two mirrors 16.1 and 16.2 may be held constant while the SF coherent-light- source/laser 12 emits light of varying frequency during each SF cycle. Alternatively, it may be varied continuously.
  • the forward-propagating cavity- ringdown light emerging through the mirror 14.2 is detected by the photodetector 38 (PD2). This light has the profile of the signal shown in Figure 2(a). Cavity-ringdown light also emerges in the backward-propagating direction through the mirror 14.1 to the optical circulator 32, where it is combined with incident SF coherent-light- source/laser light that has been back-reflected off the mirror 16.1, towards the optical circulator 32.
  • the optical circulator 32 transmits the combined light via path 32.1 to the photodetector 36 (PD1).
  • This combined beam emerging from the optical circulator 32 via path 32.1 is detected by a square-law photodetector 36 (PD1) to yield an optical-heterodyne-detected (OHD) signal as is shown in Figure 2(b).
  • An optical electric field may be associated with each of the various light paths in Figure 1 , at various points in the system.
  • a forward-propagating optical electric field E L may be associated with the coherent light emitted, in use, by the cw tunable SF coherent-light-source/laser 12 through the optical fibres 28 and 30, the optical circulator 32 and the lens system 34, to the ringdown-cavity measuring cell 14.
  • a forward-propagating cavity-ringdown optical electric field E F exp(-t/2 ⁇ ) maybe associated with the light which, in use, is transmitted through the mirror 16.2 to the photodetector 38 (PD2).
  • an optical electric field Ei may be associated with the light which constitutes the back-reflected portion of the field E which, in use, returns from the mirror 16.1 and propagates in a backward direction.
  • the field Ei propagates together with a backward-propagating cavity-ringdown optical electric field E B exp(-t/2 ⁇ ) via the focusing lens system 34, the optical fibre 30 and the optical circulator 32, via path 32.1 to the photodetector 36 (PD2).
  • the optical electric fields Ei and E B exp(-t/2 ⁇ ) are detected by photodetector 36 (PD1).
  • the optical fields represented by Ei and E B exp(-t/2 ⁇ ) vary rapidly with time at their respective optical frequencies.
  • the field E B exp(-t/2 ⁇ ) decays relatively slowly due to the cavity- ringdown effect, as is shown explicitly.
  • the former (rapid) time-dependences lead to oscillatory interference effects between Ei and E B exp(-t/2 ⁇ ) as their frequency difference varies; such effects are not shown explicitly in Equation (1) below, but they are evident in Figure 2(b).
  • the latter (slow) time-dependence of E B exp(-t/2 ⁇ ) leads to ringdown decay, as is shown explicitly in Equation (1) below.
  • the signal (S) of interest is therefore of the form: ⁇ S> oc I Ei + E B exp(-t/2 ⁇ )
  • the first line of Equation (1) indicates that the signal (S) is related to the square of the combined electric fields of the light propagating backwards from the ringdown cavity, which offers the advantage that the signal arising from the combined fields is of a much higher intensity than the electric light field E B exp(-t/2 ⁇ ) of the backward-propagating cavity-ringdown light, because the amplitude of Ei is much greater than that of E B .
  • 2 exp(-t/ ⁇ ) is equivalent to the forward-propagating CRD signal term
  • the optical-heterodyne-detected signal detected by the photodetector 36 (PD1) decays much slower (twice as slowly) than the forward-propagating ringdown signal due to E F that is detected by the photodetector 38 (PD2). Because the invention relies on the use of cavity- ringdown times, this increased decay time further enhances detection sensitivity. Furthermore, because of the higher frequency domain of the optical-heterodyne-detected signal, low-frequency technical noise can be removed more efficiently by means of electronic high-pass filtering, should it be considered necessary to do so.
  • the frequencies of the optical fields Ei and E B exp(-t/2 ⁇ ) differ because the former varies continuously as the optical frequency of the coherent-light-source/laser 12 is swept, whereas the optical frequency of E B exp(-t/2 ⁇ ) remains at the frequency at which the optical cavity resonates.
  • Figures 2(a) to 2(c) show a comparison of three simultaneously recorded signal profiles derived from a system as shown in Figure 1.
  • Figure 2(a) represents the build-up and ringdown- decay signal of the transmitted light in the optical cavity 18, as detected by the photodetector 38 (PD2).
  • Figure 2(b) represents the backward-propagating optical-heterodyne cavity-ringdown signal as detected by the photodetector 36 (PD1).
  • Figure 2(c) represents the demodulated logarithm of the envelope of optical-heterodyne detected cavity-ringdown signal processed by the electronic processing unit 40.
  • the preferred section of data, which is used to extract the cavity- ringdown decay rate ⁇ "1 is indicated by a double-headed arrow for each of the three signal profiles.
  • the incident SF laser radiation then moves out of resonance and oscillations set in with their period and depth of modulation decreasing as time delay increases.
  • the modulations are due to optical interferences between the portion of the instantaneous incident SF laser field that enters the optical cavity and the cavity-ringdown field that is built up and stored inside the cavity.
  • the ringdown decay envelope depends on the energy loss-rate of the optical cavity and can be used for CRD measurements.
  • the optical-heterodyne-detected (OHD) cavity-ringdown (CRD) signal (Figure 2(b)) contains information about the amplitudes and relative phase of the optical fields E] and E B exp(- t/2 ⁇ ) that are monitored by photodetector 36 (PDl in Figure 1) at the SF-induced difference frequency.
  • Another efficient method employs a demodulating logarithmic amplifier to convert the envelope of the single-exponential ringdown decay curve to a smooth, straight line from which the ringdown time ⁇ can be rapidly and accurately derived.
  • the slope of the straight line is proportional to the exponential decay rate of the signal received by the demodulating logarithmic amplifier and to the characteristic logarithmic slope value of the demodulating logarithmic amplifier.
  • the characteristic logarithmic slope value of the demodulating logarithmic amplifier is usually known or can be predetermined by measuring its outputs with given test input signals.
  • Suitable demodulating logarithmic amplifiers are available commercially, for example, Analog Devices model AD8307 (bandwidth DC- 500- MHz, linearity ⁇ 1 dB, dynamic range 92 dB).
  • AD8307 bandwidth DC- 500- MHz, linearity ⁇ 1 dB, dynamic range 92 dB.
  • the latter approach directly converts the exponentially decaying full-wave envelope of optical-heterodyne oscillations, as depicted in Figure 2(b), into a smooth linear decay, the slope of which provides a means of measuring the ringdown time ( ⁇ ).
  • optical-heterodyne detection and demodulating logarithmic amplifier advantageously preserves the two-fold slower cavity-ringdown decay rate (2 ⁇ ) " ' of the full-wave envelope in Figures 2(b), relative to the ringdown decay rate ⁇ '1 of the directly transmitted ringdown signal of Figure 2(a).
  • the double-headed arrows in Figures 2(a), 2(b) and 2(c) indicate qualitatively the time ranges over which reliable values of ringdown time can be measured. Extracting the decay rate of an exponential profile or the slope of a straight line is general knowledge in the field of data modelling and numerical analysis. More information on this topic could be found in the book: W. H. Press, S. A. Teukolsky, W. T. Vetterling, and B. P. Flannery,
  • SF swept-frequency
  • CEAS swept-frequency cavity-enhanced abso ⁇ tion spectroscopy
  • Detection of the peak amplitude of transmitted CRD signals or peak-to-peak amplitude of OHD CRD signals may be performed by conventional analog or digital circuitry in the processing unit 40 or in the data-acquisition unit 42 or in the processing- and-control unit 44 shown in Figure 1.
  • the integrated signal profile for build-up and decay of the transmitted CRD signal in Figure 2(a) may be used optionally for swept-frequency (SF) integrated cavity output spectroscopy (ICOS), given that this integrated signal profile decreases as optical abso ⁇ tion by the sample in the cavity increases and/or the frequency-sweep rate increases.
  • SF swept-frequency
  • the rectified and integrated signal profile for build-up and decay of the full-wave envelope of the OHD CRD signal in Figure 2(b) may also be used optionally for SF ICOS measurements, given that this integrated quantity decreases as optical abso ⁇ tion by the sample in the cavity increases and/or the frequency-sweep rate increases.
  • the integrated signal profile for the demodulated logarithm of build-up and decay of the full-wave envelope of the OHD CRD signal in Figure 2(c) may also be used optionally for SF ICOS measurements, given that this integrated quantity decreases as optical abso ⁇ tion by the sample in the cavity increases and/or the frequency-sweep rate increases.
  • Integration of transmitted CRD signals or rectification and integration of OHD CRD signals, as mentioned above, may be performed by conventional analog or digital circuitry in the processing unit 40 or in the data-acquisition unit 42 or in the processing-and-control unit 44 shown in Figure 1.
  • SF CEAS and SF ICOS may be useful for the pu ⁇ ose of recording survey spectra over a broad range of coherent-light-source/laser wavelengths. They may efficiently generate a characteristic spectroscopic signature for a given molecule or mixture of molecules for the sample contained within the optical cavity.
  • Figures 3(a) to 3(g) show the various steps in a method of obtaining a cavity ringdown spectrum by means of the system depicted in Figure 1.
  • the abscissas of Figures 3(a) to 3(g) are correlated to each other, as indicated by the elapsed times in Figure 3(g).
  • Figure 3(a) the electronically detected profile of an optical signal reflected back from the optical cavity 18 is shown.
  • the signal represents the reflection that may be obtained during a single SF cycle.
  • the amplitude of the signal increases at discrete resonance frequencies with the optical cavity 18, as the tunable SF coherent-light-source/laser 12 conducts a frequency sweep.
  • the amplitude increases each time the optical cavity 18 resonates with incident light from the cw tunable SF coherent-light-source/laser 12 when its optical cavity length is an integer multiple of the wavelength (inversely proportional to frequency) of the input light.
  • Vi N ⁇ c / (2 n D), (2) where Ni is an integer number (Ni, N 2 , N 3 , 7), c is the speed of light in vacuum, D is the distance between the two cavity mirrors, and n is the refractive index of the sample inside the cavity.
  • the distance between successive resonance frequencies is called the free-spectral range
  • FSR c / (2 n D)
  • FWHM full width at half maximum
  • Some SF coherent-light-source/laser output light of frequency around vl ⁇ FWHM may be coupled into the optical cavity 18 by means of constructive-interference interaction within the cavity 18.
  • the decaying cavity-ringdown light of fixed frequency vl is combined, in the backward-propagating direction during the period immediately after time tl, with a portion of incident SF coherent-light-source/laser light that is reflected collinearly off the front mirror 16.1 of the cavity.
  • the frequency of the reflected SF coherent-light-source/laser light which varies continuously during the period immediately after time tl, beats against the fixed frequency vl of the cavity-ringdown light.
  • Combination of these two backward-propagating light waves generates the optical-heterodyne-detected (OHD) signal having a continuously varying difference (beat) frequency that is determined by the sweep rate of the SF coherent-light- source/laser frequency.
  • OHD signal generation is to regard the entire optical cavity 18 as an active reflector that interacts and responds dynamically when the frequency of the incident light sweeps across its resonance frequency.
  • the ringdown decay of the OHD signal amplitude gives a measure of the ringdown decay characteristics of the cavity.
  • the intensity of the decaying cavity-ringdown light of frequency vl decays to a negligibly weak level in a time interval usually shorter than the time interval that the cw tunable SF coherent-light-source/laser 12 takes to sweep from one cavity resonance frequency (e.g., vl at time tl) to the next (e.g., v2 at time t2).
  • FIG. 3(b) the demodulated logarithm of the envelope of the signal reflected back from the optical cavity 18 is shown.
  • Figure 3(c) the profile of the optical signal transmitted through the optical cavity 18, as detected by the photodetector 38, is shown.
  • Figures 3(a), 3(b) and 3(c) contain the same cavity-ringdown information. Each of the build-up and ringdown-decay events may be analysed, as discussed in the context of Figure 2, to derive the cavity-ringdown time information.
  • the ringdown times of some of the ringdown events, at different resonance frequencies vO, vl, v2 and so forth, are labelled in Figure 3(c) as ⁇ O, ⁇ l, ⁇ 2, and so forth, respectively.
  • Figure 3(d) there is shown a frequency sweep of the SF coherent-light- source/laser 12. Its frequency sweep starts at v-start and changes monotonically across the cavity resonance frequencies at vO, vl, and so forth, as indicated with black dots.
  • the intensity of the SF coherent-light-source/laser 12 preferably needs to be stable as its optical frequency is swept. A stability of better than 90% is preferred, and a stability of better than 99% is most preferred.
  • Figure 3(e) is a control signal indicating the frequency sweep status. This control signal is derived from the control electronics (not shown) of the coherent-light-source/laser 12 which is used for the operation of its output frequency. The data acquisition is synchronised by this control signal.
  • Figure 3(f) depicts a typical cumulative number of data points (0 - 3000 data points) recorded by the data-acquisition module 42 (as shown in Figure 1).
  • Figure 3(b) and/or Figure 3(c) are usually recorded by the data-acquisition module 42 (as shown in Figure 1) for further analysis of cavity-ringdown decay rates at each of the build-up and ringdown-decay events, as discussed in the context of Figure 2, to derive the cavity-ringdown time information.
  • data points 0 to 499 contain the details for the first build-up and ringdown-decay event at the cavity resonance frequency vO;
  • data points 500 to 999 contain the details for the second build-up and ringdown-decay event at the cavity resonance frequency vl;
  • data points 1000 to 1499 contain the details for the third build-up and ringdown-decay event at the cavity resonance frequency v2; and so forth.
  • Each of these data sections represents a build-up and ringdown-decay event.
  • Each of these build-up and ringdown-decay events can be analysed, as discussed in the context of Figure 2, to derive the cavity-ringdown time information ⁇ O, ⁇ l, ⁇ 2 and so forth, at their corresponding cavity frequencies vO, vl, v2 and so forth, respectively.
  • Figure 3(g) shows a typical elapsed time scale of the SF cycle (0- 180 ⁇ s).
  • a fast data sampling rate and a large record memory size of data- acquisition module 42 are advantageous to record detailed build-up and ringdown-decay events in large numbers in a single SF cycle.
  • the frequency range of an SF cycle may conveniently be set to match either the frequency range of interest or the recording capacity of the data-acquisition module 42, or the frequency tuning ability of the SF coherent-light-source/laser 12.
  • Alternative data acquisition modes may be employed to record data.
  • a data acquisition module may be set firstly to a "ready" mode by the Frequency Sweep Status signal ( Figure 3(e)), and again each time when a start of the build-up event is detected by monitoring the outputs of photodetector 36 or 38.
  • the data acquisition module 42 will then record only for a preset period of time. In this way, much of the time interval of the empty signal is omitted.
  • Figure 4 depicts a cavity-ringdown spectrum obtained by using the embodiment of the system shown in Figure 1. It may also be obtained by using any of the embodiments shown in any one of Figures 6, 8, 9, 10 and 1 1, applying the method as described with reference to Figure 2 and Figure 3.
  • the black dots in Figure 4 show the cavity-ringdown decay rate of the cavity 18 (with rate being inversely proportional to time) at discrete cavity resonance-frequency values while the dashed line indicates the underlying continuous spectrum profile.
  • SF coherent-light-source/laser 12 to conduct a SF cycle spanning a broad frequency range over many free spectral ranges of the ringdown cavity, as shown in Figure 3, and to generate and analyse the cavity-ringdown (CRD) profiles detected in optical-heterodyne- detected (OHD) mode by the photodetector 36 (PDl) or (alternatively but less advantageously in non-OHD mode) by the photodetector 38 (PD2), usually while holding the optical distance between the mirrors 16.1 and 16.2 constant during a frequency sweep over the abso ⁇ tion spectrum of the sample 24.
  • CCD cavity-ringdown
  • Each of the build-up and ringdown-decay events may be analysed, as described in the context of Figure 2, to derive the cavity-ringdown time information.
  • This generates a plot of cavity-ringdown decay rate ( ⁇ "1 ) against optical resonance frequency of the cavity, as in Figure 4, where a series of data points with cavity-ringdown decay rates 1/ ⁇ l , l/ ⁇ 2, l/ ⁇ 3 and so forth are plotted at their corresponding optical frequencies vl, v2, v3 and so forth, respectively, during the SF cycle.
  • Such a plot comprises a coarsely resolved optical abso ⁇ tion spectrum, which is 'an optical characteristic of the sample 24 in the ringdown cavity, the amplitude of the spectrum being dependent on the concentration of a substance in the sample 24.
  • the optical frequency corresponding to each build-up and ringdown-decay event can be derived from the start frequency, v-start, that has been set for the SF cycle, the frequency change rate or the characteristics of the sweep, and the elapsed time or corresponding recorded data point position since the start of the SF cycle. If necessary, the sensitivity in determining the ringdown-time values may be improved by signal-averaging over successive frequency-sweeps. In addition or alternatively, the spectrum range of the recorded spectrum may be extended by repeating the measurement for additional wavelength ranges.
  • Figure 5 shows a combination of three sets of coarsely resolved CRD spectra to improve the frequency resolution. This comprises three series of data points, one with ringdown times ⁇ l, ⁇ 2, ⁇ 3 and so forth for one cavity length and another with ringdown times ⁇ 'l, ⁇ '2, ⁇ '3 and so forth for another cavity length and another with ringdown times ⁇ "l , ⁇ "2, ⁇ "3 and so forth for another cavity length.
  • Similar outcomes may be achieved by continuously scanning the cavity length, using the SSCL procedure as described above.
  • One set of data points from the spectrum, displayed in Figure 5(a) as black dots, is measured at one cavity length;
  • the second set of data points from the spectrum, displayed in Figure 5(b) as open squares, is measured at an altered cavity length;
  • a third set of data points from the spectrum, displayed in Figure 5(c) as open circles, is measured at another altered cavity length.
  • the frequency points of measured coarsely resolved CRD spectra are determined at the cavity resonance frequencies, which in turn depend on the length of the optical cavity 18 (or distance between the mirrors 16.1, 16.2).
  • FIG. 5(d) shows a composite spectrum, comprising the two spectra as shown in Figure 5(a) and Figure 5(b).
  • Figure 5(e) shows a composite spectrum, comprising the three spectra as shown in Figure 5(a), Figure 5(b) and Figure 5(c).
  • the change in the distance between the mirrors may be made in accordance with the incrementally (or decrementally) stepped cavity-length (ISCL) procedure that has been described in the "Summary of the Invention” above.
  • ISCL incrementally (or decrementally) stepped cavity-length
  • the distance between the mirrors may be continuously scanned, using the SSCL procedure that has also been described in the "Summary of the Invention” above.
  • the effect of a change in the distance between the two mirrors can be derived by considering Equations (2) and (3).
  • the resonance frequencies of the cavity are periodic and the separation between successive resonance frequencies corresponds to FSR, as in Equation (3).
  • a small change in cavity length shifts the structure of resonance frequencies of the cavity.
  • a change of the cavity length by one half of the wavelength of the coherent-light-source/laser light shifts the resonance frequency by one FSR interval and reproduces the same structure of resonance frequencies of the cavity.
  • the cavity length change needs only be of the order of magnitude of the wavelength of the light used.
  • a two-mirror cavity with an optical separation of 50 cm has an FSR of about 300 MHz.
  • a cavity length change of 155 nm i.e., a tenth of the wavelength of 1550 nm, will shift each of the resonance frequencies of the cavity by about 60 MHz.
  • a cavity length change of 310 nm will shift each of the resonance frequencies of the cavity by about 120 MHz.
  • FIG. 6 shows an embodiment of a system 610 in accordance with the invention, as an alternative to that depicted in Figure 1.
  • Like reference numerals are used to designate like components or features. Two ways in which the embodiment of the system shown in Figure 6 differs from that shown in Figure 1 are that it does not include an optical circulator 32 or optical fibres 28, 30.
  • the optical detection system 610 comprises a coherent light source in the form of cw tunable swept-frequency (SF) coherent-light-source/laser 612 that emits coherent light of which the optical frequency can be varied over a range from about 1 MHz to many GHz or THz.
  • the cw tunable SF coherent-light-source/laser 612 is optically coupled to an optical resonator system in the form of a ringdown-cavity measuring cell 614.
  • the embodiment of the system shown in Figure 6 also differs from the system shown in Figure 1 in that the optical cavity 618 of the system shown in Figure 6 is formed by four mirrors 616.1, 616.2, 616.3 and 616.4 aligned with respect to an optical axis 620 in a bow-tie or ring configuration.
  • Comparable ring-cavity designs using three e.g., such as adopted in US Patent 5,912,790
  • five or more mirrors are also feasible, and it is to be understood that such cavity designs are included in the scope of this invention.
  • the surfaces of the mirrors 616.1, 616.2, 616.3 and 616.4 that face one another are highly reflective and are concave, each mirror 16.1, 16.2, 616.3 and 616.4 typically having a radius of curvature of 1 m.
  • the distance between the mirrors 16.1 and 16.4, along the optical path 620 is initially set at about 0.5 m, but it may be varied by an optical-length adjuster in the form of a piezoelectric translator (PZT) 622, that is attached to the mirror 612.3, for moving the mirror 616.3, relative to the mirrors 616.1, 616.2 and 616.4.
  • PZT piezoelectric translator
  • the four cavity mirrors 616.1 - 616.4 are arranged in such a way that the incident light is close to normal incidence, in order to minimise any polarisation dependence of the mirror reflectivity.
  • the ringdown-cavity measuring cell 614 and optical cavity 618 are designed to operate according to the cavity-ringdown principle.
  • a gaseous sample 624 is introduced into the ringdown-cavity measuring cell 614 by means of a sample-handling system 626, so that it constitutes an optical medium in the optical cavity 618.
  • the optical system 610 is designed to detect the presence of trace chemical species in the sample 624.
  • the sample 624 is directed into the ringdown-cavity measuring cell 614 by sample handling means 626.
  • Two transparent windows 614.1 and 614.2 are provided in opposite ends of the wall of the ringdown-cavity measuring cell 614, so as to allow coherent light to enter and leave the measuring cell 614.
  • the ringdown-cavity measuring cell 614 is optically coupled to the cw tunable SF coherent- light-source/laser 612 via a set of mirrors 629.1, 629.2.
  • a lens 634 is provided in the optical path of the light being emitted by the cw tunable SF coherent-light-source/laser 612 towards the ringdown-cavity measuring cell 614, to collect and direct such light into the optical cavity 618 of the ringdown-cavity measuring cell 614.
  • Residual backward-propagating light emerging from the ringdown-cavity measuring cell towards the cw tunable SF coherent-light-source/laser 612 is blocked by the optical isolator 631, which serves to protect the SF coherent-light-source/laser 612 from interruption or damage by such returned light.
  • An additional optical isolator (not shown in Figure 6) may be integrated into the SF coherent-light-source/laser 612, to provide further suppression of back-reflected light reaching the SF coherent-light-source/laser 612.
  • Another optical detector in the form of a photodetector 638 (PD2) is provided on the remote side of the ringdown-cavity measuring cell 614, to detect light that has passed through the optical cavity 618, the mirror 616.2 and the window 614.2.
  • the photodetectors 636 (PDl) and 638 (PD2) detect light by converting it into corresponding electrical signals.
  • the photodetector 636 (PDl) converts the combined beam from the optical circulator 632 into a' corresponding electrical signal. This beam has oscillatory characteristics suitable for optical-heterodyne detection.
  • PDl is processed by an electronic circuit 667 to obtain the demodulated logarithm of the envelope of the oscillatory signal.
  • An electronic data-acquisition module 642 is provided to collect and store the output signal of the electronic circuit 667 and of the photodetector 638 (PD2).
  • a suitable data-acquisition module 642 may be a digital oscilloscope or an analog-to-digital converter.
  • a data-processing and computer control unit 644.2 is coupled to the data-acquisition module 642 (not shown separately).
  • the control electronics 640 are connected, via a connection 668, to the piezoelectric translator 622 to enable controlled incremental or continuous movement of the mirror 616.3.
  • the processing-and-control unit 644.2 is also coupled to the SF coherent-light-source/laser 612, as is shown by line 670, to control the frequency of the light emitted by the cw tunable SF coherent-light-source/laser 612 and to analyse the data detected by photodetectors 636 (PDl) and 638 (PD2) that are recorded by the data-acquisition module 642.
  • the embodiment of the system shown in Figure 6 also differs from the embodiment of the system shown in Figure 1 in that a reference etalon 650 and a reference gas cell 652 are used to calibrate the output frequency of the cw tunable SF coherent-light-source/laser 612.
  • the cavity mirrors 616.1 , 616.2, 616.3 and 616.4 are highly reflective dielectric concave mirrors and each has a radius of curvature of 1 m.
  • the optical path-length of the cavity and therefore its resonance frequencies can be adjusted by the piezoelectrically controlled mirror
  • the resonance frequencies of the cavity may be altered by varying the cavity length either in controlled discrete steps or by continuous scanning.
  • the sample 624 is directed into the chamber 614 by a sample handling unit 626.
  • Two transparent windows 614.1 and 614-.2 allow light to enter and leave the ringdown-cavity measuring cell 614.
  • the output of the cw tunable SF 612 coherent-light-source/laser passes through an optical isolator 631, a beam splitter 633, a pair of steering mirrors 629.1, 629.2 to change the direction of the light emitted by the coherent-light-source/laser 612, a focusing lens (or lens system) 634 and the window 614.1 before entering the ringdown-cavity measuring cell 614.
  • the light reflected by the mirror 616.1 and the cavity-ringdown light emitted through the same mirror 616.1 is directed by mirrors 635 and 637, and focused by a lens 639 to the photodetector 636 (PDl).
  • the output of the photodetector 636 (PDl) is processed by an electronic circuit 640 to obtain the demodulated logarithm of the envelope of the oscillating signal.
  • Another optical detector in the form of photodetector 638 (PD2) is provided. It is in optical communication with light emitted through the mirror 616.2 and the window 614.2, such light being reflected by a mirror 641 through a further focusing lens 643 on to the photodetector 638.
  • a portion of the output of the SF coherent-light-source/laser 612 is taken from the beam splitter 633 and directed into another beam 645 by a mirror 647.
  • a portion of the beam 645 is taken from a second beam splitter 649 and is coupled by a lens 651 to the reference etalon 650.
  • Light transmitted through the etalon is collected by a lens 653 and is detected by a photodetector 654 (PD3).
  • PD3 photodetector
  • the etalon assembly may be rigidly constructed of materials with low coefficient of thermal expansion; if necessary, the etalon unit may also be pressure- and temperature-stabilised and actively locked to a stabilised reference laser.
  • the remaining part of the beam 645 passing through the second beam splitter 649 is coupled by a mirror 656 and a lens 658 through the reference gas cell 652.
  • Light transmitted through the reference gas cell 652 is collected by a lens 660 and is detected by a photodetector
  • the reference gas cell 652 contains a suitable gas or a gas mixture that has optical abso ⁇ tion features at known wavelengths of light, for calibration pu ⁇ oses.
  • a module 664 comprises the control electronics 640, the electronic data acquisition module 642, the data processing control electronics 644.1 and the processing-and-control unit 644.2.
  • the electronic data acquisition module 642 collects and processes the output signals from photodetector 636 (PDl) via a logarithmic amplifier 667 (which converts the envelope of the oscillatory output signal from the photodetector 636 into a demodulated logarithmic signal), from photodetector 638 (PD2), from photodetector 654 (PD3), and from photodetector 662 (PD4).
  • the processing-and-control unit 644.2 is coupled to the data-acquisition module 642 and the control electronics 640 to enable controlled incremental or continuous movement of the mirror 616.3 as shown by line 668.
  • the module 664 is also coupled to the SF coherent-light-source/laser 2 as shown by line 670 to control the optical frequency or wavelength of the light emitted by the cw tunable SF coherent-light-source/laser 612.
  • Figure 7 depicts the various signals and sequences associated with the embodiment of the system as shown in Figure 6.
  • Figure 7(h) depicting a signal derived from the reference etalon 650 as detected in, or derived from, the signal detected in the photodetector 654 (PD3) and Figure 7(i) depicting a signal derived from the reference gas cell 652 as detected in or derived from the photodetector 662 (PD4).
  • PD3 photodetector 654
  • Figure 7(i) depicting a signal derived from the reference gas cell 652 as detected in or derived from the photodetector 662 (PD4).
  • similar traces are assigned the same reference numerals.
  • the abscissas of Figures 7(a) to 7(i) are correlated to each other, as indicated by the elapsed times in Figure
  • the cw tunable SF coherent-light-source/laser 612 emits light over a range of optical frequencies during a swept-frequency (SF) cycle.
  • the intensity of the SF coherent-light- source/laser 612 preferably needs to be stable as its optical frequency is swept. A stability of better than 90% is preferred, and a stability of better than 99% is most preferred.
  • Radiation in the cavity 618 builds up and decays at successive resonance frequencies as indicated in Figures 7(a), 7(b) and 7(c). The speed of the frequency sweep is chosen so that the decay of each resonant ringdown profile is complete before the SF coherent-light-source/laser frequency has advanced to the next resonant frequency.
  • the cw tunable SF coherent-light-source/laser 612 emits light over a continuously varying range of optical frequencies during each swept-frequency (SF) cycle, as is indicated in Figure 7(d).
  • the resonance frequencies vl, v2, v3, v4, v5, etc., are also indicated by arrows and black dots (•) in Figure 7(d).
  • Open circles (o) also show the position of calibration frequency markers, e.g., as generated by the reference etalon 650 and photodetector 654 (PD4) and depicted in Figure 7(h).
  • the status of the start and end of a sweep cycle is indicated by an electronic logic signal as shown in Figure 7(e).
  • This frequency sweep status signal may be provided by control electronics associated with the cw tunable SF coherent-light-source/laser 612 or it may be derived from a parameter associated with its operation.
  • Figure 7(f) depicts a typical cumulative number of data points (0 - 3000 data points) recorded by the data-acquisition module 642 (as shown in Figure 6).
  • Figure 7(c), Figure 7(h), and Figure 7(i) are usually recorded by the data-acquisition module 642 (as shown in Figure 6) for further analysis of cavity-ringdown-decay rates at each of the build-up and ringdown-decay events, as discussed in the context of Figure 2, to derive the cavity-ringdown- time information.
  • data points 200 to 699 contain the details for the build-up and ringdown-decay event at the cavity resonance frequency vl; data points 700 to 1199 contain the details for the next build-up and ringdown-decay event at the cavity resonance frequency v2; data points 1200 to 1699 contain the details for the build-up and ringdown-decay event at the cavity resonance frequency v3; and so forth.
  • Each of these data sections represents a build-up and ringdown-decay event.
  • FIG. 7(f) uses black dots (•) and open circles (o) to show data points recorded at the corresponding cavity resonance frequencies and frequency-marker positions of the reference etalon, respectively, as previously depicted in Figure 7(d). The elapsed time is shown in Figure
  • FIG. 7(g)- Figure 7(h) shows the frequency markers of the reference etalon, used for frequency calibration pu ⁇ oses.
  • the separation between the frequencies of successive frequency markers is chosen to be larger than the frequency setting uncertainty of the SF coherent-light-source/laser 612.
  • the start frequency v-start of the SF coherent-light-source/laser 612 is usually set in the middle between two frequency markers of the reference etalon 650, in such a way that the first frequency marker generated after the start of the sweep is uniquely defined.
  • the frequency markers of the reference etalon 650 provide the information needed to calibrate the frequency change of the output of the SF coherent-light-source/laser 612.
  • Figure 7(i) shows the transmission of radiation from the SF coherent-light-source/laser 612 through the reference gas cell 652, as registered by photodetector 662 (PD4).
  • the known abso ⁇ tion features of the reference gas provide the absolute frequency of the SF coherent-light- source/laser output at the position where such abso ⁇ tion occurs.
  • Absolute frequency information for the coherent-light-source/laser from Figure 7(h) and relative frequency change information from Figure 7(i) together provide an independent frequency calibration for the frequency of the SF coherent-light-source/laser output.
  • the start of the data acquisition can be triggered by the frequency sweep status signal in Figure 7(e) at the beginning of the SF sweep cycle, or by a frequency marker after the start of the SF sweep, or it can be based on a transmission feature of the reference gas cell 652.
  • the data acquisition electronics unit 642 comprises a digital oscilloscope (not shown explicitly in Figure 6), to determine the average of the signal so as to improve the signal-to-noise ratio and, therefore, the detection sensitivity of the system 610.
  • the fluctuation of the preferred trigger event from sweep-to-sweep conveniently does not exceed a few MHz. It preferably does not exceed a few hundred kHz.
  • Figure 7(f) shows how the first frequency marker after the start of the SF sweep may be used as a trigger event for data acquisition.
  • the cumulative number of recorded data points as indicated in Figure 7(f) may vary.
  • the elapsed time as indicated in Figure 7(g) may also vary.
  • the data point positions when the SF coherent-light-source/laser moves into resonance with the cavity are indicated by black dots (•) in Figure 7(f).
  • the data point positions when the SF coherent-light-source/laser moves into resonance with the reference etalon are indicated by open circles (o) in Figure 7(f).
  • a cavity ringdown (CRD) spectrum may be obtained in OHD mode, the spectrum being based on a signal derived from the light detected by the photodetector 636 (PDl).
  • the spectrum may be based on a signal derived from the light detected by the photodetector 638 (PD2).
  • PD2 photodetector 638
  • the wavelength range of a single recorded spectrum may be extended by operating the SF coherent-light-source/laser 612 at different wavelength ranges and by combining the spectra recorded at those different wavelength ranges.
  • the ringdown-cavity resonance frequencies are varied incrementally or decrementally by changing the optical length of the optical cavity 618 of the ringdown-cavity measuring cell 614, namely, by using the PZT device 622 to change the position of the cavity mirror 616.3 by a desired distance.
  • the amount of the frequency shift can be determined by several means, namely: (l)by measuring the change of voltage applied to the PZT device 622 and converting such voltage to displacement using calibration data; or (2) by relating cavity-resonance positions, each associated with a cavity build-up event as shown in Figure 7(c), to reference etalon markers, as in Figure 7(h); or (3) by relating cavity-resonance positions, each associated with a cavity build-up event as shown in Figure 7(c), to recorded data points, as in Figure 7(f).
  • the resonance frequencies of the cavity may be varied by slow, continuous scanning of the cavity length.
  • Figure 8 shows an embodiment 810 of the detection system of the invention, as an alternative to that depicted in Figure 1.
  • the system shown in Figure 8 differs from that shown in Figure 1 in that it does not include an optical circulator 32 or optical-fibre coupling 28, 30.
  • the optical path of the emitted light from the cw tunable SF coherent-light-source/laser 812 (shown in solid lines with arrows) to the ringdown-cavity measuring cell 814 passes through an optical isolator 831, a reflector 829, a polarising beam splitter 870, a polarisation control unit 872, and a lens 834.
  • a reflector 841 directs forward- propagating light transmitted by the ringdown-cavity measuring cell 814 to a photodetector 838 (PD2), which is optional.
  • PD2 photodetector 838
  • a data-acquisition module (not shown separately) is integrated into the control electronics unit 864.
  • An optical isolator 831 is provided, in the optical path of the coherent light emitted from the coherent-light-source/laser 812, to suppress residual back-reflected light from the ringdown-cavity measuring cell 814 that could otherwise interrupt or damage the SF coherent-light-source/laser 812. If the SF coherent-light-source/laser 812 is not sensitive to residual back-reflected light, the optical isolator 831 may be omitted.
  • the orientation of the polarising beam splitter 870 is important.
  • the polarisation control unit 872 is a magneto-optical Faraday rotator that rotates the plane of polarisation of the light passing through it by 45 degrees and enables the backward-propagating beam to be directed efficiently to the photodetector 836 (PDl) via the polarising beam splitter 870.
  • the polarising beam splitter 870 and the polarisation control unit 872 together also serve as an additional optical isolator to protect the SF coherent-light-source/laser 812.
  • FIG. 9 shows an embodiment 910 of the detection system of the invention, as an alternative to that shown in Figure 8. Where appropriate, corresponding reference numerals are used to designate corresponding features or components of the system 910. In the system shown in Figure 9, components corresponding to the polarising beam splitter 870 and the polarisation control unit 872 (which form part of the embodiment of the system shown in Figure 8) are not required.
  • an additional beam splitter 970 directs a portion of light emitted by the SF coherent-light-source/laser 912 along a second optical path 972 to a second beam splitter 974, where it is combined with cavity-ringdown light emerging from the mirror 916.2 and directed to photodetector 938 by the reflector 941.
  • the OHD CRD signal may optionally be detected at any one of several locations by any one of the photodetectors 938, 976 and 978. These photodetectors effectively take the place of photodetectors 36, 636 and 836 (PDl) that appear in Figures 1, 6 and 8, respectively.
  • Photodetectors 978 or 938 may also perform the non-OHD CRD-spectroscopic function of the previously displayed photodetector 38, 638, 838 (PD2) by blocking the light path 972 between the reflectors 970 and 974 by a beam flag (not shown in Figure 9).
  • PD2 non-OHD CRD-spectroscopic function of the previously displayed photodetector 38, 638, 838
  • the embodiment 910 is a suitable choice, owing to its simplicity.
  • Figure 10 shows an integrated system 1010 that comprises a ringdown-cavity module 1011 comprising a ringdown-cavity measuring cell 1014 and an optical transmitter-receiver module 1013 comprising a set of three standard pigtail-coupled diode lasers 1012.1, 1012.2 and 1012.3 and a standard photodetector 1036.
  • the ringdown-cavity module 1011 and the optical transmitter-receiver module 1013 are interconnected by standard optical fibre cables 1030.1 and 1030.2.
  • the ringdown-cavity module 1011 may be situated at a remote location. Alternatively, the system may comprise two or more than three diode lasers, if required.
  • Each of the diode lasers 1012.1, 1012.2 and 1012.3 may have a different wavelength range, or a different frequency stability. Alternatively, one of them may be used as a replacement or standby laser for fault protection in real time.
  • At least one of the diode lasers 1012.1, 1012.2 and 1012.3 is a SF tunable laser of the form used singly in the embodiments of the system shown in Figures 1, 6, 8, 9 and 11. Any cw diode laser 1012.1, 1012.2 or 1012.3 that is not SF-tunable needs to be provided with a means of being shifted in and out of resonance with the ringdown cavity, for instance, by frequency- or amplitude-modulation or by an optoelectronic switch.
  • the transmitter-receiver module 1013 also comprises a wavelength division multiplexer 1080, that is a (N x 1) combiner/multiplexer (with N equal to or greater than the number of diode lasers employed) capable of combining the various cw laser beams of the diode lasers 1012.1, 1012.2 and 1012.3 into one single-mode optical fibre 1028.
  • the transmitter-receiver module 1013 further comprises a computerised control, data processing and analysis unit 1044 for controlling the operation of the various components of the optical transmitter-receiver module 1013 and, if required, those of the ringdown-cavity module 1011.
  • the transmitter-receiver module 1013 further comprises an optical circulator 1032 similar to the optical circulator 32 shown in Figure 1.
  • Additional optical isolators may be integrated into the optical fibre 1028 or may be an integral part of each diode laser 1012.1, 1012.2 and 1012.3, to protect them from unwanted optical feedback. It should be noted that fibre-optic optical circulators are generally insensitive to variations in the polarisation of the light, which is advantageous in this application. Optical circulators are widely used in fibre-optical telecommunications . As mentioned above, the ringdown-cavity module 1011 housing the ringdown-cavity measuring cell 1014 is remotely located relative to the optical transmitter-receiver module 1013. Components corresponding to the PZT device 22 and associated electrical cable 46.1, as shown in Figure 1, to vary the optical path-length of the ringdown cavity 14 are not explicitly shown in Figure 10.
  • an alternative embodiment of the system shown in Figure 10 may comprise a PZT voltage supply close to or in the ringdown-cavity module 1011 responsive to control signals transmitted from the optical-heterodyne transmitter- receiver module 1013 by another optical fibre or by wireless means. Either way, these ⁇ o embodiments yield a single-ended CRD optical detection system, in which the ringdown-cavity section can be remotely located relative to the main instrumental transmitter-receiver module 1013.
  • a second photodetector corresponding to component 38 in Figure 1, component 638 in Figure 6 and component 838 in Figure 8, to monitor the forward-transmitted light field E F is
  • the mirror 1016.2 may be in the form of a total reflector, whereby the amplitude of the backward-propagating optical-heterodyne signal may be enhanced.
  • an optical fibre designed for single-mode operation in the telecommunications C band at a near-infrared wavelength in the range 1.53 - 1.57 ⁇ m can serve as a single-mode fibre at longer wavelengths with acceptable transmission characteristics extending to longer infrared wavelengths (2 ⁇ m and beyond, say).
  • Optical fibres are desirable in the methods and systems of the invention because passing multiple laser beams through a single-mode optical fibre provides a
  • each laser beam is coupled into the single-mode optical fibre 1030.1, 1030.2, by means of either beam-steering reflectors such as mirrors or prisms (not
  • multiple-coherent-light-source/laser embodiments of the invention may comprise means to combine beams of multiple frequencies together in free space, or to replace multi-wavelength optical fibres by suitable beam splitters, dichroic mirrors or other mirrors or reflectors. Diffraction gratings and prisms may also be used.
  • Prisms may, as an alternative, be used as non-dispersive reflectors for guiding one or more light beams into a microscope objective lens and then into an optical fibre, as mentioned above.
  • Each cw diode laser 1012.1, 1012.2 and 1012.3 shown in Figure 10 may be selected to emit light over a particular frequency range. Some of these frequency ranges may be chosen to be resonant with spectroscopic features of gas-phase chemical species that are of particular interest. At least one other frequency range may be chosen such that it is removed from any known spectroscopic features, in order to measure the non-resonant background or baseline. Chemical species that are typically of interest usually have many characteristic optical abso ⁇ tion features within a narrow frequency (wavelength) range.
  • the multiplex control and analysis electronics system 1044 performs a similar function to that in Figures 1 and 6 and, in addition, it controls the concerted operation of the cw tunable diode lasers 1012.1, 1012.2 and 1012.3, such that the optical build-up of each resonance point occurs at a different time in the SF cycle(s) of the two or more lasers that are in operation.
  • a reference cell (not shown) and/or a frequency (wavelength) meter (also not shown) may also be included in the system 1010 to check laser frequency (wavelength).
  • Figure 11 shows yet another alternative embodiment 1110 of the system in accordance with the invention. As in the case of Figure 10, the embodiment shown in Figure 11 is also an integrated system.
  • ringdown-cavity module 1111 comprising a ringdown- cavity measuring cell 1114 and an optical transmitter-receiver module 1113 comprising a standard photodetector 1136.
  • the ringdown-cavity module 1111 may also be located at a remote location.
  • the ringdown-cavity module 1111 and the optical transmitter-receiver module 1113 may be interconnected by a standard optical fibre cable or, alternatively, the tunable cw coherent-light- source/laser 1112 may propagate to the ringdown-cavity module 1111 through free space.
  • a tunable cw coherent-light-source/laser 1112 provides coherent light which is coupled into the ringdown-cavity measuring cell 1114 of the ringdown-cavity module 1111 via the free-space path 1130.
  • An optical shutter 1182 is provided to transmit or block, for suitable periods of time, the beam emitted by the tunable cw coherent-light-source/laser 1 112.
  • the optical shutter 1112 may be an acousto-optic modulator or an electro-optical modulator.
  • the optical shutter may, for example, on the basis that the first-order deflected beam of an acousto- optic modulator is turned on or off by the presence or the absence of a driving acoustic wave.
  • the emitted light from the tunable cw SF coherent-light-source/laser 1112 passes through the optical shutter 1182, a polarising beam splitter 1184 and a polarisation control unit 1186 before being coupled through a focusing lens 1188 to the ringdown-cavity measuring cell 1114.
  • the polarisation control unit 1186 is typically a magneto-optical Faraday rotator that enables the backward-propagating beam to be directed efficiently to the photodetector 1136 (PDl) via the polarising beam splitter 1184.
  • the polarising beam splitter 1184 and the polarisation control unit 1186 also serve as an optical isolator to protect the SF coherent-light- source/laser 1112.
  • the ringdown-cavity module 1111 housing the ringdown-cavity measuring cell 1114 is remotely located relative to a separate transmitter-receiver module 1113 that houses the cw tunable SF coherent-light-source/laser 1112, photodiode 1136 (PDl) and the scan and detection unit 1144 containing computerised control, data-acquisition and analysis electronics.
  • the cw tunable SF coherent-light-source/laser the cw tunable SF coherent-light-source/laser
  • the default status of the optical shutter 1182 is "open", allowing the coherent-light- source/laser beam being coupled to the ringdown cavity 1118.
  • the photodetector 1136 (PDl) receives signals similar to the one as shown in Figure 3(a) when the SF coherent-light-source/laser frequency is swept.
  • An electronic control unit 1190 detects the build-up of coherent-light-source/laser energy into the ringdown cavity 1 118 by monitoring the output of the photodetector 1136 (PDl).
  • the electronic control unit 1190 causes the optical shutter 1182 to block the coupling of the coherent-light-source/laser beam to the ringdown cavity 1118 for a pre-set time period before the shutter 1182 is re-opened.
  • the scan and detection unit 1144 ignores the transient output signal of the photodetector 1136 (PDl) for a short time, usually a few microseconds, whilst the shutter 1182 is opened, to allow the output of the photodetector 1136 to settle down.
  • the time period for the shutter to block the coherent-light-source/laser beam may be set to be several times longer than the longest ringdown time observed.
  • the signal that the photodetector 1136 receives after the closing of the shutter is similar to the directly transmitted signal as shown in Figure 3(a), detected by the photodetector 38 (PD2) in Figure 1.
  • the frequency sweep rate of the SF coherent- light-source/laser 1 112 is not required to be fast to clearly separate the build-up and the following ringdown and to enable the determination of the ringdown time.
  • a slower sweep rate of the SF coherent-light-source/laser frequency across the cavity resonance allows more coherent-light- source/laser energy to be coupled into the cavity 1118 with a consequent higher output from the photodetector 1136 and an improved detection sensitivity of the ringdown time.
  • the intensity profile of the detected signal is accordingly less modulated than the signal as shown in Figure 3(a).
  • the scope of this invention includes instrument designs and measurement strategies involving swept-frequency (SF) variants of cavity-enhanced abso ⁇ tion spectroscopy (CEAS) and integrated cavity output spectroscopy (ICOS), in addition to SF variants of cavity-ringdown (CRD) spectroscopy.
  • SF CEAS and SF ICOS variants are included in the scope of this invention, as well as embodiments depicted in Figures 1 and 2 as discussed above.
  • SF CEAS and SF ICOS approaches may be useful for the pu ⁇ ose of recording survey spectra over a broad range of coherent-light-source/laser wavelengths. They may efficiently generate a characteristic spectroscopic signature for a given molecule or mixture of molecules for the sample contained within the optical cavity. Relative to SF CRD approaches, the SF CEAS and SF ICOS approaches may require fewer data points and less detail of the ringdown-decay profile to be recorded for a given spectral range. However, the SF CEAS and SF ICOS approaches are typically less sensitive than SF CRD approaches.
  • Such a spectrum may be relatively coarsely resolved, but can be sufficiently well defined if the linewidth of features in the spectrum is substantially greater than the FSR of the cavity.
  • the method described in relation to Figures 3 and 4 above is experimentally realised for a single molecular optical abso ⁇ tion feature.
  • SF cw-CRD spectrometer and variants and derivatives thereof are to be understood as to be referring to embodiments of an apparatus or system in accordance with the invention, and vice versa.
  • Components or elements represented by abbreviations in or forming part of the aforementioned expressions, such as “cw”, are not to be taken as referring to essential features of the invention.
  • the expressions “swept-cavity OHD cw-CRD” and “swept- cavity cw-CRD” do not refer to embodiments of an apparatus or system in accordance with the invention, and vice versa.
  • FIG. 12(a) A coarsely resolved SF cw-CRD abso ⁇ tion spectrum of carbon dioxide gas (C0 2 ) is presented in Figure 12(a), with its pressure-broadened linewidth exceeding the FSR by a factor of more than three.
  • Figure 12(a) shows a scan of ⁇ "1 values for 23 cavity resonances, plotted as open circles (o); these span a range of cavity-ringdown rates from 0.1 ⁇ s "1 to 0.7 ⁇ s "1 .
  • FIG. 12(b) shows the corresponding sequence of 23 directly transmitted cw-CRD waveforms recorded in a single rapid sweep of the SF laser frequency, with a sweep rate of 2.4 THz s "1 and a sweep duration of about 3 ms. Each waveform is separated by the 0.33-GHz (0.011-cm "1 ) FSR of the cavity. Ringdown times ⁇ at the centre of the scan are an order of magnitude shorter than those on either side, consistent with the peak absorbance in the centre of the scan.
  • Its monochromatic output power (about 10 mW) can be rapidly swept with an optical frequency (or wavelength) sweep rate ranging over 1.25 - 125 THz s "1 (0.01 - 1.0 ⁇ m s "1 ) and a repetition rate range of 0.125 - 100 Hz.
  • the fibre-coupled output of this MEMS SF laser has an intrinsic optical bandwidth of 125 kHz and a frequency stability of about 1 MHz over about 3.5 ⁇ s.
  • This monochromatic output is continuously tunable and can be rapidly swept mode-hop-free over its entire range of about 8 THz at communications-band wavelengths of 1.51 - 1.57 ⁇ m, spanning the ITU C band and beyond.
  • the compact laser package (50 x 70 x 13 mm) inco ⁇ orates a complete external-cavity diode laser system with MEMS-based Littman-Metcalf architecture, a reference etalon for internal wavelength calibration, and control electronics.
  • a computerised interface to the MEMS cw SF tunable diode laser output provides several electronic output signals that indicate its operational status and help to control and synchronise data collection.
  • Wavelength calibration of recorded SF cw-CRD spectra can be based on the internal wavelength reference in the MEMS SF laser system itself and/or registered by external devices (e.g., a stable reference etalon or a calibrant gas cell to monitor the laser frequency).
  • the ringdown cavity itself can serve incidentally as a convenient, economical reference etalon, eliminating the extra components that are usually required to generate wavelength markers and to calibrate wavelength-scan linearity.
  • Well-characterised and identified spectral features of a recorded CRD spectrum can also be used for wavelength-calibration pu ⁇ oses.
  • the process of spectroscopic sampling by the FSR grid of the ringdown cavity enhances detection stability and enables signal averaging to be insensitive to imperfections in sweep-to-sweep reproducibility of the SF laser frequency.
  • the optical power from the MEMS SF laser was about 2 mW, incident on the cavity.
  • This MEMS SF laser can be swept over a wide frequency range, recording successive SF cw-CRD waveforms that are generated at cavity resonance points separated by the cavity's FSR (i.e., at each point of an effective sampling grid). It is thereby feasible to record an extensive CRD spectrum at a high data-acquisition rate within a single wide- range laser-frequency sweep cycle (typically within about I s).
  • SLM single-longitudinal- mode
  • Its single-longitudinal- mode (SLM) ouput power of about 5 mW over a wavelength range of 1.50 - 1.59 ⁇ m was coarsely tunable by picomotor in steps of about 0.01 nm (about 1.3 GHz) with a repeatability of about 0.1 nm (about 13 GHz) and a short-term (about 50 ms) optical bandwidth of about 150 kHz.
  • the piezoelectrically-controlled frequency-sweep capability of this cw tunable diode laser (tuning range about 30 GHz at a sweep rate of about 6 THz s "1 ) was limited, but nevertheless useful for SF laser frequency sweeps spanning up to about 90 FSR intervals of the ringdown cavity.
  • a second conventional external-cavity cw tunable diode laser (Photonetics Tunics-Plus) has also been used for some experiments with limited frequency-sweep ranges (output power of about 5-mW, 6-GHz PZT-controlled frequency sweep, coarsely tunable in steps of about 0.001 nm over the wavelength range 1.50 - 1.64 ⁇ m).
  • the ringdown cavity 18 used to record the results in Figure 12 was mounted inside an evacuable cell 14 fitted with Brewster-angle silica glass windows, 14.1 and 14.2, and electronic manometers to register the pressure of the gaseous sample 24.
  • the cavity comprised two rigidly mounted high reflectors (Newport model 10CV00SR.70F, typically at least_99.98% reflectivity at about 1.55 ⁇ m, 1-m radius of curvature), separated by 0.453 m; its corresponding FSR was 0.331 GHz (0.0110 cm "1 ). These cavity parameters were chosen here for economy and compactness, rather than for ultimate detection sensitivity which would be favoured by using a longer cavity with ultra-high-reflectivity mirrors.
  • the cw output from the MEMS SF laser 12 used in the context of Figure 12 was coupled to the ringdown cavity measuring cell 14 by single-mode optical fibres 28 and 30 via a three-port fibre-optical circulator 32 (Photonic Technologies HRC-1550-03-1010) or via Faraday-rotator optical isolators (Optics for Research model IO-4-IR2-HP, 60-dB isolation from two units in series).
  • the geometry of the beam emerging from the fibre is matched to the longitudinal mode of the ringdown cavity by a lens system 34 consisting of an aspheric objective and a focusing lens with 50-cm focal length. Coupling to higher-order transverse modes of the cavity is suppressed to less than 1%.
  • Backward-propagating light from the ringdown cavity can be measured by an InGaAs photodetector 36 (also labelled PDl; New Focus model 1617, 800- MHz bandwidth, 0.5-ns rise time) by way of optical fibre 32.1.
  • InGaAs photodetector 36 also labelled PDl; New Focus model 1617, 800- MHz bandwidth, 0.5-ns rise time
  • These components also protect the cw SF tunable diode laser system from interference or damage by backward-propagating light.
  • the back surface of each ringdown-cavity mirror is slightly tilted away from normal to the cavity axis, which is itself displaced by about 5 mm from the midpoint of each of the 25-mm-diameter mirror surfaces; this avoids interference problems caused by reflections from the back surfaces of the mirrors.
  • a second photodetector 38 (labelled PD2; New Focus model 1811, 125-MHz bandwidth, 3-ns rise time) was also provided to detect CRD signals directly transmitted by the optical cavity. This latter, forward-propagating detector was used to record the CRD signals shown in Figure 12(b).
  • the output from photodetector 38 (PD2) was processed by a digital oscilloscope (Tektronix model TDS794D, 4 Gsample s "1 ) and associated electronics that comprise the electronic data-acquisition module 42 of Figure 1. Peak amplitudes of the build-up and ringdown-decay waveforms shown in Figure 12(b) are found to vary erratically.
  • Tektronix TDS794D digitiser 4 Gsample s "1 with 2 interleaved channels, 8-bit resolution) employed.
  • This capability can be enhanced by using a computer-based plug-in digitiser with greater resolution and memory depth (e.g., National Instruments model 5122; 100-MHz bandwidth, 14-bit resolution, 32-MB memory depth).
  • a gated mode of data acquisition can also be used to record the SF cw-CRD signal for a pre-set period of time whenever the start of a build- up event is detected (once per FSR); this effectively bypasses uninformative time intervals between successive build-up and ringdown-decay events, thereby optimising use of the digitiser's memory depth.
  • the range over which CRD spectra can be measured is ultimately determined by the SLM tuning range of the SF laser employed (about 8 THz at 1.51-1.57 ⁇ m in this case, which can be covered by one pair of cavity mirrors).
  • Figure 13 shows two simultaneously recorded sets of signal profiles obtained using an apparatus similar to the one depicted schematically in Figure 1.
  • FIG. 13(a) shows the forward-propagating optical power directly transmitted by the cavity and monitored by photodetector 38 (PD2); each CRD feature decays quasi-exponentially with time constant ⁇ , as explained in the context of Figures 2 and 3.
  • Figure 13(b) depicts the demodulated logarithm of full-wave oscillatory waveforms of corresponding OHD optical power retro-reflected by the cavity, monitored by photodetector 36 (PDl) and processed by a demodulating logarithmic amplifier 40 (Analog Devices model AD8307, bandwidth DC - 500 MHz, linearity 1 dB, dynamic range 92 dB).
  • PDl photodetector 36
  • a demodulating logarithmic amplifier 40 Analog Devices model AD8307, bandwidth DC - 500 MHz, linearity 1 dB, dynamic range 92 dB.
  • This directly converts the exponentially decaying full-wave envelope of the oscillating OHD SF cw-CRD signal, as in Figure 2(b), to a straight line, as in Figure 2(c), from the negative slope of which 2 ⁇ can be conveniently and efficiently derived.
  • FIG. 13(c) shows a spectrum of sample-and-hold output levels proportional to the ringdown time ⁇ , derived by processing OHD signals in real time as will be explained in the context of Figures 15 and 16 below; this spectrum, for which the sweep rate is 900 GHz s "1 , spans a spectral range of 3.6 GHz (0.12 cm "1 ) across a single pressure-broadened abso ⁇ tion peak of C0 2 gas at wavelengths around 1.54- ⁇ m.
  • the SF laser tunable diode laser used for the measurements in Figure 13 was a Photonetics Tunics-Plus laser and the resulting CRD signals were processed by a Tektronix TDS3054B digitiser at 2.5 Msample s "1 .
  • Figure 13 also shows how successive build-up and decay events vary as the laser frequency sweeps over an abso ⁇ tion feature of the sample gas.
  • a piezoelectric translator 22 (PZT; Piezotechnik model HPSt 150/20) attached to a mirror mount 16.2 of the ringdown cavity 18, as shown in Figure 1, allows the options of fine cavity-length adjustment, ICSL-type stepped-cavity or SSCL-type synchronously scanned SF cw-CRD operation, and or parallel swept-cavity and swept-frequency CRD measurements.
  • Figure 14(a) shows a plot of ⁇ "1 values for 23 cavity resonances as in Figure 12(a), plotted as open circles (o) and spanning a range of cavity-ringdown rates from 0.1 ⁇ s "1 to 0.7 ⁇ s "1 .
  • FIG. 14(b) A survey scan of the corresponding directly transmitted CRD waveforms is shown in Figure 14(b), as monitored by photodetector 38 (PD2) in Figure 1.
  • Figure 14(c) depicts an additional set of directly transmitted CRD waveforms, recorded after using the PZT 22 to change the length of the ringdown cavity by an increment of about 0.27 ⁇ m, thereby shifting the cavity-resonance grid by about 0.35 times the FSR.
  • the corresponding additional 23 values of ⁇ "1 derived from Figure 14(c) are plotted as dots (•) in Figure 14(a), filling in gaps in the spectrum between the open circles (o) derived from Figure 14(b).
  • SF cw-CRD spectra arise from a sequence of multiple build-up and ringdown-decay events, either directly transmitted as in Figures 2(a), 3(c), 7(c), 12(b) and 13(a) or OHD as in Figures 2(b, c), 3(a, b), 7(a, b) and 13(b), with the laser frequency rapidly swept.
  • a data point (indexed as j, say) is registered each time the laser frequency passes through successive longitudinal-mode resonance points of the cavity (separated by its FSR); each such frequency vj yields a corresponding ringdown time ⁇ j that varies with absorbance as the laser frequency is swept through the spectrum of the gas.
  • the laser-frequency sweep rate needs to be such that the decay of each CRD event (j) is complete before the next CRD event (j + 1) builds up, as is evident in Figures 3, 12(b), 13(a, b), and 14(b, c).
  • Figure 14(a) comprises two superimposed SF cw-CRD spectra of C0 2 gas at about 1.54 ⁇ m, with the origin of the frequency scale centred on the 6490.06-cm "1 P(16) peak in the (30°1)- (00°0) combination band of C0 2 ; the same peak was recorded in Figures 12(a) and 13(c).
  • a Lorentzian profile with FWHM linewidth of 1.1 GHz (0.037 cm “1 ) provides a satisfactory fit to either set of plotted values of ⁇ "1 in Figure 14(a).
  • Ringdown times ⁇ at the center of the scan are an order of magnitude shorter than those on either edge, consistent with the peak absorbance of about 2 x 10 "5 cm "1 in the center of the scan and a 'zero- absorbance' or 'empty-cell' ringdown time ⁇ of about 10 ⁇ s (consistent with the 99.98 % mirror reflectivity).
  • signal averaging over a number (N) of repetitive laser-frequency sweeps can enhance the signal-to-noise ratio by a factor of N 1/2 , but with a corresponding recording-time penalty.
  • signal averaging by means of this FSR-grid spectroscopic sampling process is not degraded by minor irreproducibility of the cw SF tunable diode laser frequency from sweep to sweep.
  • the pressure-broadened linewidth in the case of Figure 14(a) is sufficiently large for the spectrum to be adequately defined by a single set of data points (represented either by o or by •), recorded with a single fixed ringdown-cavity length.
  • An alternative SSCL-type synchronously scanned approach would be to scan the cavity length continuously and in synchrony with the SF cycle, at a rate that is sufficiently slow to allow many SF sweeps while the cavity length spans the FSR.
  • Another important advantage of this invention is that it offers a convenient way to record wide-ranging SF cw-CRD spectra for rapid 'finge ⁇ rint' sensing of airborne molecules. As illustrated in Figures 12(a), 13(c) and 14(a), such spectra are adequately recorded with a relatively coarse resolution of about 0.5 GHz (about 0.015 cm "1 ) so that it suffices to record satisfactory spectra with L, and hence the FSR grid, held fixed.
  • an analog circuit e.g., a level discriminator and a time-to-amplitude convertor
  • a level discriminator and a time-to-amplitude convertor may be used to measure the time interval for the CRD decay to drop from one preset level to another.
  • An efficient and effective way to record wide-ranging SF CRD spectra employs analog electronic circuits for real-time processing of the OHD signal retro-reflected from the ringdown cavity, that is, as monitored by photodetector 36 (PDl) in Figure 1.
  • This approach employs level discriminator, time-to-amplitude converter, and sample-and-hold circuits to extract a reliable instantaneous measure of the ringdown time ⁇ in a single data point for each build-up and decay event, without needing to record and process details of the CRD waveform at each laser frequency sampled by the FSR-spaced grid of the cavity's resonance frequencies. This imposes much less demand on digitiser speed and memory depth and on data processing.
  • Electronic circuits such as differentiators, may be used to determine signal slopes, including an analog detection scheme (together with a lock-in amplifier) in CRD spectroscopy, as in the paper entitled “A laser-locked cavity ring-down spectrometer employing an analog detection scheme, " published in Review of Scientific Instruments 71, 347 - 353 (2000) by T. G. Spence, C. C. Harb, B. A. Paldus, R. N. Zare, B. Wilke, and R. L. Byer; see also US Patent No 6,532,071 entitled “Analog detection for cavity lifetime spectroscopy” by R N Zare, C C Harb B A Paldus and T G Spence.
  • Figure 15 illustrates an analog detection scheme for processing of the CRD signal.
  • the scheme is based on a pair of level discriminators and a time-to-amplitude converter (TAC).
  • Figure 15(a) shows schematically how the straight-line decay of the demodulated logarithm of OHD CRD signal, e.g., as depicted in Figure 2(c), is sampled by level discriminators at two preset reference levels. As shown in Figure 15(b), this generates a rectangular pulse with its duration ⁇ t corresponding to the level-crossing time interval during which the signal decays from one preset discriminator reference level to the other.
  • the slope of the straight-line decay is proportional to the decay rate ⁇ "1 , so that the time interval ⁇ t is proportional to ringdown time ⁇ .
  • This duration ⁇ t is converted to an analog output voltage V by charging an integrator at a constant current for that period ⁇ t.
  • the outcome is a (slope " ')-to-amplitude converter that generates an analog voltage V with amplitude proportional to ringdown time ⁇ .
  • the conversion factor of the circuit is calibrated by comparing the true exponential decay times ⁇ of directly transmitted CRD waveforms and outputs V of the (slope " ')-to-amplitude converter.
  • Figure 16 shows how a sample-and-hold circuit is combined with the (slope ' ⁇ -to-amplitude converter to enable continuous real-time processing of successive SF cw-CRD build-up and decay events,.
  • the overall data-processing sequence, illustrated in Figure 16, depicts three successive (slope ' ⁇ -to-amplitude conversion processes in Figures 16(a) - 16(c).
  • the output amplitudes V from Figure 16(c) are transferred to a sample-and-hold circuit with control gate, reset logic levels, and output as in Figures 16(d), 16(e), and 16(f), respectively.
  • the (slope ' ⁇ -to-amplitude converter is reset after each ringdown-decay event to await arrival of the next build-up event.
  • the output of the sample-and-hold circuit stays at a constant level before it is refreshed in the next (slope " ')-to-amplitude conversion step.
  • FIG. 13(c) shows sample-and-hold output levels varying (in proportion to ringdown decay time ⁇ ) as the laser frequency traverses the 6490.06-cm "1 (30°l)-(00°0) P(16) abso ⁇ tion line of intracavity C0 2 gas.
  • Figure 17 shows survey spectra of C0 2 gas over a broad wavelength range (1.52 - 1.55 ⁇ m).
  • the SF cw-CRD spectrum in Figure 17(a) comprises a total of 10,000 SF cw-CRD signal build-up and decay events spanned by a 3.3-THz (110-cm "1 ) sweep of the output frequency of the MEMS SF tunable diode laser (iolon model SLE1040). Data from single frequency sweep were recorded by a Tektronix TDS3054B digital oscilloscope within a period of 2 s, at a data rate of 5 Ksample s "1 . Values of ringdown time ⁇ were processed in real time by analog electronic circuits, as explained in the context of Figures 15 and 16.
  • the signal-to-noise ratio of a single-sweep spectrum was further improved by signal averaging at the digitiser.
  • the SF cw-CRD spectrum shown in Figure 17(a) is an average of 128 sweeps, recorded in less than 5 minutes; projected modifications to the analog electronic circuits are expected to minimise the need for such signal averaging and thereby yield high-quality SF cw-CRD spectra more readily.
  • CEAS cavity- enhanced abso ⁇ tion spectroscopy
  • CEAS techniques entail measuring the peak amplitude of build-up and decay waveforms rather than the characteristic exponential decay time ⁇ , as in CRD spectroscopy; CEAS therefore lacks the time-domain discrimination of CRD methods and are more susceptible to uncertainty due to fluctuations in the intensity and frequency sweep rate of the incident laser radiation.
  • Swept-cavity variants of CEAS and the closely related ICOS (integrated cavity output spectroscopy) technique have been outlined above in the "Background of the Invention" section.
  • CEA SF cavity-enhanced abso ⁇ tion
  • the SF CEA spectrum provides a clearly recognizable finge ⁇ rint of gas-phase C0 2 , as shown by reference to the upper portion of Figure 18 which is a corresponding spectrum from the HITRAN'96 spectroscopic database ["The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation): 1996 edition” in Journal of Quantitative Spectroscopy and Radiative Transfer 60 (1998) 665 - 710 by L. S. Rothman et al . This provides convincing proof of the principle that the FSR-spaced grid of resonance frequencies of the ringdown cavity can efficiently sample an abso ⁇ tion spectrum of interest over a wide wavelength range.
  • SF cw-CEA spectra are found to be highly susceptible to minor fluctuations in the incident laser radiation, much more so than corresponding SF cw-CRD spectra, as in Figure 17(a).
  • This shortfall of SF cw-CEA spectroscopy is evident in the erratic peak signal amplitude variations of the successive SF cw-CRD build-up and decay waveforms in Figures 12(b) and 14(b, c), compared to the regular profile of the spectra (plotted as o or as •) in Figures 12(a) and 14(a) on the basis of measurements of ringdown time ⁇ rather than of peak signal amplitude.
  • the scatter of data points relative to the Lorentzian curve of best fit, for the SF CRD spectrum in Figure 12(a) or for either SF CRD spectrum in Figure 14(a), indicates that the minimum detectable abso ⁇ tion loss (MDAL) is about 9 x 10 "8 cm "1 ; the corresponding signal-to- noise ratio at the peak of the spectrum was about 220, which can be substantially enhanced by signal averaging.
  • MDAL minimum detectable abso ⁇ tion loss
  • the FSR-sweep period during which a single build-up and ringdown- decay waveform can be recorded, was about 145 ⁇ s; this yields a data-rate-normalised MDAL of about 1.1 x lO "9 cm "1 Hz "1 2 .
  • the last of these three features is particularly advantageous in that a much greater body of spectroscopic information is available virtually instantaneously, compared to conventional CRD techniques where recording of wide-ranging spectra requires a relatively slow scan of the frequency or wavelength of the laser or coherent light source.
  • the performance of the SF cw-CRD spectrometer described in this invention is already well suited to many practical applications, such as combustion diagnostics or clinical breath testing.
  • the SF cw-CRD spectrometer's detection sensitivity e.g., its attainable MDAL
  • the SF cw-CRD spectrometer's detection sensitivity is capable of improvement by orders of magnitude if necessary, for instance, by signal averaging over a longer detection period, or by increasing the cavity length and/or the mirror reflectivity.
  • the temporal profiles for directly transmitted SF CRD signals and/or backward-propagating OHD SF CRD signals may be used to generate detailed or high-resolution portions of the spectrum of interest from which one or more molecular species of interest may be characterised with high sensitivity and their concentrations determined.
  • the analysis procedure used to assemble detailed or high-resolution portions of the spectrum of interest in such measurements with rapidly swept coherent-light-source/laser frequency has been explained and depicted schematically in the context of Figures 3, 4, 5 and 7, with relatively slow cavity-length adjustment by either stepped (ISCL) or synchronously scanned (SSCL) methods.
  • the frequency or wavelength range of the detailed or high-resolution portion (or portions) of the spectrum of interest may be limited by the memory depth of the computerised electronics used to record the spectrum of interest. This range may be much less than the full range of frequencies or wavelengths over which the coherent radiation can be swept.
  • less detailed features of the temporal profiles for directly transmitted SF CRD signals and/or backward-propagating OHD SF CRD signals may be used to generate less detailed spectra (with reduced sensitivity) but with a digitiser of a given memory depth able to span a much wider range of rapidly swept coherent-light-source/laser frequencies or wavelengths.
  • a 50-cm ringdown cavity has a FSR of 0.01 cm "1 (i.e., 300 MHz) whereas a representative commercially available communications-band swept-frequency laser may be tuned in a single rapid sweep over a wavelength range of 1510 - 1570 nm, corresponding to a frequency-sweep of 250 cm "1 (i.e., 7.5 THz); this means that a full-range sweep of a spectrum of interest will span 25,000 (2.5 x 10 4 ) ringdown-cavity FSRs. Recording such a full-range spectrum at 1 data point for each ringdown-cavity FSR may be achieved with the memory depth of readily available digitisers or digital oscilloscopes.
  • the SF CEAS and SF ICOS approaches will typically be less sensitive (in terms of analytical detection limits) than SF CRD approaches.
  • the recording of spectra across a wide frequency or wavelength range by the optional SF CEAS and/or SF ICOS approaches enables acquisition of survey spectra spanning a broad range of coherent-light-source/laser wavelengths.
  • Such procedures also offer an efficient way to generate a characteristic spectroscopic signature for a given molecule or mixture of molecules for the gas sample contained within the optical cavity.
  • a single data point is recorded at each ringdown-cavity resonance, separated by 1 FSR of the ringdown cavity, with a digitiser of a given memory depth able to span a wide range of rapidly swept coherent-light-source/laser frequencies or wavelengths.
  • analog circuits (as explained in the context of Figures 15 and 16) enable real-time measurement of the ringdown time ⁇ at each ringdown-cavity resonance (as depicted in Figure 13).
  • the abso ⁇ tion lines of any gas-phase spectrum of interest will usually be pressure-broadened such that the profile of each molecular abso ⁇ tion line will be spanned by many ringdown-cavity resonances, since the FSR of the cavity is usually substantially less than the pressure-broadened line-width of molecular abso ⁇ tion lines or spectroscopic features.
  • the pressure-broadened linewidth for near-infrared abso ⁇ tion spectra of carbon dioxide (C0 2 ) molecules diluted in air at a total pressure of 1 bar is about 0.5 cm "1 (i.e., 15 GHz) FWHM, which is 50 times the FSR of a 50-cm ringdown cavity.
  • any of the above-mentioned SF CRD, SF CEAS and SF ICOS approaches may be used to record coarsely resolved molecular spectra that may be sufficiently reliable without necessarily employing a variable-cavity-length approach (either ISCL or SSCL).
  • molecular spectra need to be recorded at a total pressure much less than atmospheric (i.e., much less than 1 bar), either because that is the natural condition of the gas-phase sample or because the sample pressure has been reduced by evacuation to obtain a spectrum that is more finely structured owing to reduced (or eliminated) pressure broadening.
  • each molecular abso ⁇ tion line may be spanned by very few (if any) ringdown-cavity resonances, since the FSR of the cavity may be comparable to (or greater than) the line-width of molecular abso ⁇ tion lines or spectroscopic features.
  • Coarsely resolved spectra recorded with a single ringdown-cavity length by the above-mentioned SF CRD, SF CEAS or SF ICOS approaches may not reveal all features of the spectrum, as abso ⁇ tion lines may coincide with too few ringdown-cavity resonances.
  • variable-cavity-length approach either ISCL or SSCL
  • SF CRD spectroscopic measurements that are sufficiently finely resolved or to record reliable survey spectra with sufficient detail across a wide frequency or wavelength range.
  • condensed-phase samples such as liquids, solutions, aerosols, solids or surface films
  • molecular beams such as liquids, solutions, aerosols, solids or surface films
  • matrix-isolated molecules such as molecules in very cold droplets.
  • SF CRD or SF CEAS approaches to record reliable survey spectra or spectroscopic signatures across a wide frequency or wavelength range for a liquid- or solution-phase sample contained in a suitable ringdown- cavity cell, despite the much higher optical abso ⁇ tion coefficients and correspondingly faster cavity-ringdown decay rates that usually occur in condensed-phase samples.
  • SF CRD, SF CEAS or SF ICOS approaches for CRD-spectroscopic measurements of a sample that is adsorbed as a film on the inner surface of at least one of the mirrors of the ringdown cavity.
  • a lower-gain, low-noise photodetector may be used to take advantage of the full 5-mW power of the tunable diode laser(s) or to use optical-fibre coupling to allow the ringdown cavity to be remotely located relative to the tunable diode laser(s) and ⁇ hotodetector(s) in the OHD transmitter-receiver, thereby facilitating scientific, industrial, medical, agricultural and environmental sensing applications.
  • optical-heterodyne-detected cavity-ringdown apparatus with rapidly swept coherent-light-source/laser frequency is that the signal to be detected returns to the optical transmitter-receiver section of the apparatus by counter-propagating along the same path as that of the incident coherent-light-source/laser beam, thereby enabling single-ended detection.
  • the invention is amenable to embodiments in which optical fibres are used to transmit the light over various portions of the beam path.
  • Standard optical fibres are typically optimised for working in individual wavelength ranges (e.g., 1.5 - 1.8 ⁇ m).
  • An optical fibre optimised for one wavelength may however still be used with single-mode characteristics at longer wavelengths above its cutoff wavelength.
  • optical fibres covering a wide spectral range from visible to the near infrared may be used.
  • An embodiment that entails optical-heterodyne detection with optical-fibre coupling comprises an embodiment in which there is a single, central instrumental control system (including the optical-heterodyne transmitter-receiver section of the apparatus, with at least one SF coherent-light-source/laser and a single photodetector PDl) and numerous spatially distributed ringdown cavities, each coupled by a single-mode optical fibre and optional PZT control link to the central instrumental system.
  • central instrumental control system including the optical-heterodyne transmitter-receiver section of the apparatus, with at least one SF coherent-light-source/laser and a single photodetector PDl
  • numerous spatially distributed ringdown cavities each coupled by a single-mode optical fibre and optional PZT control link to the central instrumental system.
  • a suitable optical-fibre splitter or switch module can be used to distribute the coherent-light-source/laser and return cavity-ringdown light to and from different locations of a site at which the various ringdown cavities are positioned.
  • This approach enables the more expensive, less rugged components of the overall apparatus to be positioned in a central secure location (e.g., in an air-conditioned control room) while less expensive, more robust ringdown cavities are multiply distributed in more hostile and?or less accessible locations (e.g., at various gas effluent sources on an industrial, environmental or agricultural site or in a series of wards in a hospital).
  • a vacuum pump to direct the gas-phase sample through a suitable particle filter into a sealed ringdown cavity where the spectroscopic measurements are made at sub-atmospheric sample pressure to minimise pressure broadening of optical abso ⁇ tion lines and thereby increase the specificity of detection.
  • the system of the present invention has the advantage (relative to prior techniques) that only a single widely-tunable SF coherent-light-source/laser is needed to record wide-ranging spectra rapidly in a single frequency sweep, by sampling at successive ringdown-cavity resonance points.
  • the SF coherent-light-source/laser itself generates build-up and ringdown- decay events directly, without any primary need for a rapid cavity sweep or an optical (AO or EO) switch as in other forms of cw-CRD spectroscopy.
  • PZT control of ringdown cavity length is a secondary requirement in swept-frequency (SF) CRD spectroscopy, necessary only to vary the set of available resonance frequencies that are sampled, thereby refining the resolution relative to the coarsely sampled spectrum that arises in a single SF cycle with fixed cavity length.
  • Such changes of cavity length and resonance frequencies can either be performed in discrete steps, using the ISCL procedure, or by continuous scanning, using the SSCL procedure.
  • optical path-length variations may arise from small changes of ambient temperature or mechanical changes (e.g., from external vibrations or dimensional shifts when the internal pressure in the ringdown-cavity cell is varied or when the external atmospheric pressure changes) or changes of refractive index (e.g., when the composition of the gas contained in the ringdown- cavity cell is altered), they merely cause a given feature in the SF CRD or SF CEAS or SF ICOS spectrum of interest to be registered by a different set of ringdown-cavity resonances, each separated by 1 FSR, as discussed in the context of Figures 4 and 5.
  • any necessary calibration of the frequency or wavelength scale may readily be provided by reference to spectra from a reference gas cell or a temperature-stabilised etalon, as discussed in the context of Figures 6 and 7.
  • the optional SF CEAS and SF ICOS approaches may be useful for the pu ⁇ ose of recording survey spectra over a broad range of coherent-light-source/laser wavelengths. They may efficiently generate a characteristic spectroscopic signature for a given molecule or mixture of molecules for the sample contained within the optical cavity. This has been demonstrated by Figure 18, in the SF CEAS case.
  • SF CEAS and SF ICOS approaches may require fewer data points and less detail of the ringdown-decay profile to be recorded for a given spectral range.
  • the SF CEAS and SF ICOS approaches are typically less sensitive than SF CRD approaches.
  • a further SF cw-CRD spectroscopic embodiment of the invention employs analog circuits for real-time measurement of the ringdown time ⁇ at each ringdown-cavity resonance, as depicted methodologically in Figures 13, 15 and 16. This SF cw-CRD approach is also useful for the pu ⁇ ose of recording survey spectra over a broad range of coherent-light-source/laser wavelengths, as has been demonstrated in Figure 17.
  • the apparatus of the invention is suitable for use in any application where it is desirable to determine whether trace absorbing species are present in a gas-phase sample having an appropriate wavelength for use with a continuous-wave tunable coherent-light-source/laser or where it is desirable to determine an optical abso ⁇ tion spectrum of a known compound at very low concentration or with very low optical abso ⁇ tion coefficient.
  • Suitable trace species which could be detected by the method and apparatus of the invention include, but are not limited to, carbon dioxide (C0 2 ), carbon monoxide (CO), water vapour (H 2 0), nitric oxide (NO), oxygen (0 2 ), hydrogen fluoride (HF), nitrous oxide (N 0), hydrogen cyanide (HCN), methane (CEL t ), light alkanes (C n H 2n+2 ), ethylene (C 2 H 4 ), acetylene (C 2 H 2 ), ethanol (C 2 H 5 OH), acetaldehyde (CH 3 CHO), ketones such as acetone, (CH 3 ) 2 CO, chloroform (CHC1 3 ) and ammonia (NH 3 ), either alone or simultaneously in combination.
  • Suitable specific applications include use with smelters to determine emitted process gas outputs, for measurement of CO/C0 2 ratios to optimise combustion or smelting efficiency, for determining methane gas in mines or near natural gas pipelines, to monitor effluents of various kinds in high-temperature furnaces and other industrial plants, for identifying hydrocarbons in engines, to monitor exhaust emissions from traffic in road tunnels or city streets, and for determining the presence of toxic gases such as HF, HCN, nerve gas and phosgene.
  • Another application comprises monitoring of air quality in closed environments such as the interior of an aircraft or spacecraft.
  • a dust filter is present to remove any dust from a gas-phase sample prior to introduction into the optical cavity.
  • the apparatus of the present invention has the advantage that, because of its ability to make instantaneous measurements, it can anticipate the presence of a hazard or potential contaminant before it becomes a problem and seriously affects the quality of the end product or environmental safety.
  • the apparatus and method also have significant applications to scientific research and measurement technology.
  • the following Table presents a selective survey of representative molecules that are amenable to sensitive, specific detection by optical-heterodyne-detected (OHD) CRD spectroscopy. TABLE: REPRESENTATIVE MOLECULES THAT MAY BE DETECTED BY OPTICAL-HETERODYNE SWEPT-FREQUENCY CAVITY RINGDOWN SPECTROSCOPY IN THE WAVELENGTH RANGE OF 1.25 - 2.5 ⁇ m
  • the overtone optical abso ⁇ tion wavelength ranges listed are accessible in various compilations of gas-phase infrared spectroscopic data, notably the HITRAN database: "The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation): 1996 edition” in Journal of Quantitative Spectroscopy and Radiative Transfer 60 (1998) 665 - 710 by L. S. Rothman et al.
  • the tabulated survey is confined to the coherent-light-source/laser wavelength range of 1.25 - 2.5 ⁇ m, where it is feasible to take advantage of available diode lasers, fibre optics and other telecommunications and photonics components.
  • single-mode optical fibres and related optical components are selected with design characteristics to optimise the transmission and processing of the set of coherent-light-source/laser wavelengths employed within this wavelength range.
  • Table Also summarised in the Table is a very wide range of potential industrial, environmental, agricultural and medical diagnostic applications for which the invention in its various embodiments offers practical, cost-effective solutions.
  • the optional SF CEAS and SF ICOS approaches, that are included in the scope of this invention, may be useful for the pu ⁇ ose of recording survey spectra over a broad range of coherent-light-source/laser wavelengths.
  • this SF cw-CRD spectroscopic approach offers a particularly efficient way to generate a characteristic spectroscopic signature for a given molecule or mixture of molecules for the sample contained within the optical cavity, even if spectra of the molecular species of interest are not necessarily known from an existing data base.
  • Different optical components are suitably used for different wavelength ranges. Where a single set of optical components used cannot simultaneously cover the characteristic wavelength range for all species present or suspected to be present in the sample, it is within the scope of this invention to combine the apparatus with a second apparatus so as to detect these species.

Abstract

In a cavity ringdown spectrometer, a laser beam (12) is rapidly swept in frequency. When the frequency matches a resonant frequency of an optical cavity (20) formed between two highly reflective mirrors (16.1, 16.2) it is coupled into the cavity (20). Cavity-ringdown light is generated each time the frequency reaches a value that is resonant in the cavity. As the frequency reaches the value, the ringdown light builds up. As the frequency moves away from the value the ringdown light decays. A portion of the ringdown light is detected (38) and the rate of light decay is measured from the ringdown light signal. The variation of the rate of decay is attributable to a property of a sample that is in the cavity (20) between the mirrors (16.1; 16.2). A method of processing the ringdown light signal is also described. The arrangement allows rapid determination of the rates of decay of ringdown light at many frequencies for a single sample.

Description

Cavity Ringdown Spectroscopy with Swept-Frequency Laser Technical Field The present invention relates to optical absorption spectroscopy and in particular is directed to a method and apparatus for optical detection using an optical ringdown-cavity cell. Background of the Invention The detection of trace and weakly absorbing gas-phase species is of importance in scientific, industrial, medical, agricultural and environmental spectroscopic sensing applications. In recent years optical cavity ringdown (CRD) laser absorption spectroscopy has become a new analytical technique for deteπnination of such trace concentrations. The technique is simple, quick, versatile and an accurate way to acquire weak optical absorption spectra, the method typically being able to make optical absorption measurements with sensitivities of the order of 10" 7 per cm of sample. General information on CRD spectroscopy is obtainable for example from: US Patent 5,528,040 by K. K. Lehmann; "Cavity Ringdown Laser Absorption Spectroscopy: History, Development and Application to Pulsed Molecular Beams" in Chemical Reviews 97 (1997) 25 - 51 by J. J. Scherer, J. B. Paul, A. O'Keefe, and R. J. Saykally; "Cavity-Ringdown Spectroscopy - An Ultratrace-absorption Measurement Technique", edited by K. W. Busch and M. A. Busch, ACS Symposium Series (1999) No. 720, ISBN 0-8412-3600-3; and "Cavity ring- down spectroscopy: Experimental schemes and applications", International Reviews of Physical Chemistry 19 (2000) 565 - 607 by G. Berden, R. Peeters, and G. Meijer. CRD spectroscopy involves injecting coherent light into an optical cavity acting as a high- finesse stable optical resonator (e.g., a Fabry-Perot optical cavity) formed by at least two highly reflecting input and output mirrors. A portion of the light incident on one of the mirrors enters the optical cavity and is multiply reflected inside the cavity. When no sample is present in the optical cavity, the intensity of radiant energy injected into the resonator decreases in time following an exponential decay with a ringdown time (which depends on the reflectivity of the mirrors, their separation and the speed of light in the optical cavity). When a sample is present in the optical cavity, the radiant energy decrease is accelerated at those wavelengths where optical absorption by the sample occurs; this results in a shorter ringdown time (at such wavelengths). An optical absorption spectrum for the sample gas is obtained by placing a photodetector after the output mirror to detect light emerging from the optical cavity and plotting the energy loss rate τ"1 versus the wavelength (or, alternatively, frequency) of the cavity-ringdown light and subtracting the corresponding spectrum of the empty optical cavity. This energy loss rate τ"1 is also known as the cavity-ringdown decay rate, which is the reciprocal of the ringdown time τ (obtained from the time profile of the intensity of light emerging from the optical cavity). The shape or profile of the resulting absorption spectrum changes if the composition of the absorbing species present in the optical cavity changes. For sufficiently weak optical absorption, the cavity-ringdown decay rate τ"1 increases linearly as optical absorbance or optical absorption coefficient of the sample. CRD spectroscopy has the advantage that it increases the effective optical absorption path- length by many orders of magnitude relative to the physical length of the cavity, owing to multiple reflections by the highly reflective cavity mirrors and optical resonance in that cavity. Additionally, CRD spectroscopy is highly sensitive because it measures time decay and not amplitude of light, and is therefore not affected by fluctuations in amplitude (optical intensity) of the light generated by the coherent light source. As indicated above, CRD spectroscopy is generally capable of detecting optical absorption of a sample with very high sensitivity, routinely (in typical instruments) of the order of 10"7 - 10"9 per cm. In certain circumstances, particularly for detecting very low concentrations of absorbing species in gases, it is desirable to achieve greater sensitivities. Detection at greater sensitivities has been reported by using instruments with more specialised optical and electronic components but is not readily attainable with typical current CRD spectroscopic systems, owing to problems such as optical feedback and noise present in detection electronics. Other problems that may be encountered in high-sensitivity CRD spectroscopy are that the frequency of the coherent light must be highly stable during detection and a substantial period of time may be required to conduct species detection if a broad range of the CRD spectrum in a sample needs to be analysed. Furthermore, many prior-art forms of apparatus require the use of large pulsed lasers which are not readily portable and are therefore unsuitable in many applications. CRD spectroscopy performed with a continuous-wave (cw) laser has some advantages relative to its pulsed-laser counterpart, in that higher spectroscopic resolution can be attained and in that inexpensive, compact, single-mode tunable semiconductor diode laser sources may be used. Compared to a typical pulsed laser, the spatial beam profile of such a laser is generally superior, which helps to couple the laser light into the ringdown cavity. However, it is necessary in the cw CRD approach to interrupt the passage of light through the ringdown cavity. This generally entails at least one of the following three operational strategies: (a) by means of a fast optical switch, e.g., acousto-optic (AO) or electro-optic (EO); (b) by rapid electronic modulation of either the frequency or amplitude of the cw light; (c) by varying the length, and hence optical resonance frequency, of the ringdown cavity. In this context, the descriptions "continuous-wave" or "cw" pertain to any laser or other coherent light source that continuously delivers optical power without interruption, including any such laser or coherent light source with an optical frequency or amplitude that is varied in time, in a controlled fashion (e.g., by frequency modulation or amplitude modulation or frequency sweeping). Various implementations of cw-CRD spectroscopy have depended primarily on strategy (a), combined in some cases with (b) and (c) to ensure that the optical cavity and the light are in resonance. However, AO and EO switches tend to be expensive, cumbersome, and limited in their spectral range, so that there is some practical advantage in eliminating them in applications where compactness, portability, cost, and wavelength range are considered. With regard to strategy (b), it has not always been straightforward to achieve reliable, reproducible electronic control of laser amplitude and frequency, as required for high-resolution cw-CRD-spectroscopic applications. A recently preferred approach in some laboratories has therefore been to implement strategy (c), thereby avoiding the inherent complexity and expense of an AO or EO switch. This approach (c) has been described in detail by Y. He and B. J. Orr: "Rapidly swept, continuous- wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi- wavelength sensing of gases", Applied Physics B 75 (2002) 267 - 280. The swept-cavity OHD cw-CRD spectroscopic technique is also described in International Patent Publication No. WO 02/04903 Al. Another approach, cavity-enhanced absorption spectroscopy (CEAS), has also been used with a rapidly swept ringdown cavity and reported by Y. He and B. J. Orr: "Ring-down and cavity-enhanced absorption spectroscopy using a continuous-wave tunable diode laser and a rapidly swept optical cavity", Chemical Physics Letters 319 (2000) 131 - 137. In the CEAS technique, the peak amplitude for build-up and decay of the transmitted CRD signal is measured, rather than the cavity-ringdown-decay rate. Another related approach, continuous-wave integrated cavity output spectroscopy (cw- ICOS), has been used with a rapidly modulated ringdown cavity and reported by A. O'Keefe, J. J. Scherer and J. B. Paul: "cw Integrated cavity output spectroscopy", Chemical Physics Letters 307 (1999) 343 - 349. In the cw-ICOS technique, integration of the signal profile for build-up and decay of the transmitted CRD signal is measured, rather than the cavity-ringdown-decay rate or the peak CRD signal amplitude. In swept-cavity cw-CRD spectroscopic, CEAS or cw-ICOS approaches, a piezoelectric translator (PZT) is used to rapidly vary the length of the optical cavity through resonance with narrowband cw laser radiation. This proves to be a straightforward way to facilitate growth and subsequent ringdown decay of optical energy in the cavity and yields absorption spectra with high sensitivity and with minimal instrumental complexity and cost. A further distinctive refinement of the rapidly-swept cw-CRD spectroscopic technique reported by He and Orr (op. cit.) comprises an optical-heterodyne-detected (OHD) approach that is intrinsically simpler than other OHD variants of cw-CRD spectroscopy employing at least one active EO or AO modulator to generate OHD sidebands. In the swept-cavity OHD cw-CRD spectroscopic technique, the backward- propagating light actively reflected off the rapidly swept cavity may be envisaged as a heterodyne beat signal between the directly reflected part of the incident laser light field and the backward- propagating ringdown light field, given that the moving mirror of the rapidly swept cavity Doppler-shifts the frequency of the intracavity light. One of the principal advantages of the swept-cavity OHD cw-CRD technique is that it enables "single-ended" detection with both the optical transmitter and receiver naturally collocated in a single console that can be widely separated from the rapidly swept ringdown cavity module. It also avoids the need for fine control and stabilisation of the ringdown cavity in order to optimise the build-up and ringdown decay of intracavity light as the laser frequency is tuned. This in turn makes the swept-cavity OHD cw-CRD technique amenable to multi- wavelength operation, using a set of fixed-wavelength lasers (each tuned to a different on- or off- resonance wavelength for the species of interest) that undergo build-up and ringdown decay at separate points in the cavity-sweep cycle. This multi-wavelength swept-cavity OHD cw-CRD approach, which is intrinsically and uniquely reliant on a rapidly swept ringdown cavity and a single-ended OHD transmitter-receiver configuration, offers a substantial multiplex advantage for multi-species spectroscopic sensing applications. A disadvantage of the swept-cavity OHD cw-CRD spectroscopic technique is that it relies on either a slow, continuous scan of a single laser wavelength or on the availability of several suitable fixed-wavelength lasers for multiplex detection. Object of the invention It is an object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages or at least provide a suitable alternative. Summary of the Invention According to a first aspect of the present invention, there is provided a method for optical detection comprising: - generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - coupling, into an optical cavity defined by at least two reflectors and containing between those reflectors a sample, at least a portion of the beam of coherent light, such that cavity- ringdown light is generated by the optical cavity each time the frequency of the coherent light reaches a value that is resonant in the optical cavity, the intensity of the cavity-ringdown light being associated with build-up and ringdown decay of intracavity radiation caused when the frequency of the coherent light respectively reaches said value and departs therefrom; - decoupling at least a portion of the cavity-ringdown light from the optical cavity; and - detecting, during a period of time, a signal attributable to a property of the sample and derived from a variation in intensity of the cavity-ringdown light decoupled from the optical cavity during the period of time,. The exponential ringdown decay during this period of time may be characterised by a ringdown time τ. A cavity-ringdown decay rate may be calculated from the ringdown time. The cavity-ringdown decay rate may be expressed as τ"1. The phrase, "a signal ... derived from a variation in intensity of the cavity-ringdown light," as used in this specification, shall be taken to include any coherent combination of the cavity- ringdown light with any part of the aforesaid beam of coherent light. One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time. Optionally, either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a variation in intensity, rather than the cavity-ringdown decay rate. Optionally, either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a variation in intensity, rather than the cavity-ringdown decay rate. The variation in intensity of the decoupled cavity-ringdown light and/or the frequency thereof may be associated with a variation in a parameter of the sample. The parameter of the sample may be the concentration of a constituent thereof or a pollutant therein. The frequency or a series of resonant frequencies may be characteristic of a specific chemical compound whereas the rate at which the intensity of the decoupled light decays may be associated with its concentration. The variation in intensity of the cavity-ringdown light typically decays exponentially and may have superimposed oscillations owing to optical interference effects. This may be measured by a photodetector and recorded as the amplitude of an output signal obtained from the photodetector. The decay period may vary from tens of nanoseconds to hundreds of micro-seconds. A convenient decay period has been found to be about 5 μs, preferably from about 1 μs to about 100 μs. Generally speaking, a longer decay period will yield a higher sensitivity, but may result in a slower rate at which data can be recorded. The beam of coherent light may be generated by a laser, a nonlinear-optical frequency converter such as a difference-frequency generator, a sum-frequency generator, or an optical parametric generator. The beam of coherent light may be a single-longitudinal-mode frequency output such as that obtainable from an external grating cavity semiconductor diode laser. The range of optical frequencies over which the frequency of the coherent light may be swept may typically be over two or more successive resonance frequencies of the optical cavity. The interval of successive resonance frequencies of the optical cavity equals the speed of light divided by the round-trip cavity length. Thus, for a 50-cm long two-mirror cavity, for example, the interval of successive resonance frequencies of the optical cavity is about 300 MHz. Therefore, the range of frequencies, over which the frequency of the coherent light may be swept to cover two cavity resonance frequencies, may be from about 600 MHz to about 900 MHz. The range of frequency sweep may be up to many GHz or THz within the tunability of the light source to cover a larger frequency range. The range of frequency sweep may be a few MHz and may be swept over just one single cavity resonance frequency. By "rapidly swept" is meant that the amount by which the frequency of the coherent light is changed during the ringdown (decay) time τ of the optical cavity exceeds 1 / (2 πτ). However, such amount should preferably be less than the frequency interval between successive resonance frequencies of the optical cavity. The cavity-ringdown light decoupled from the optical cavity may be combined, usually outside of the optical cavity, with a portion of the beam from the swept-frequency coherent light source, and may be split therefrom. The combination may form a coherent light beam with oscillatory characteristics suitable for optical-heterodyne detection. The method according to this aspect of the invention may further include the step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means. The coherent swept-frequency light may be interrupted, deflected or terminated after it has been coupled into the optical cavity, and it may be restored again after a second period of time. The second period of time is preferably several times the cavity-ringdown time τ (typically tens of microseconds), in order to allow each cavity-ringdown decay process to be completed. The electronic means may include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source. For instance, a suitable acousto-optic modulator can typically deflect or switch a light beam within about 100 ns. According to a second aspect of the present invention there is provided a method for detecting one or more parameters of a sample located within an optical cavity defined by at least two reflectors, the method comprising: - generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - coupling into the optical cavity at least a portion of the beam of coherent light, such that cavity-ringdown light is generated when the frequency of the coherent light reaches a value that is resonant in the optical cavity; - decoupling at least a portion of the cavity-ringdown light from the optical cavity; - detecting, during a period of time and for at least some of the resonant frequencies of the range of optical frequencies of the beam of coherent light, a signal attributable to a property of the sample and derived from the temporal variation in intensity of cavity-ringdown light decoupled from the optical cavity, associated with build-up and ringdown decay of intracavity radiation; and - determining a value of one or more parameters of the sample, from the temporal variation in intensity of the cavity-ringdown light decoupled from the optical cavity. The exponential ringdown decay during this period of time may be characterised by a ringdown time τ. A cavity-ringdown decay rate may be calculated from the ringdown time. The cavity-ringdown decay rate may be expressed as τ"1. The phrase, "a signal ... derived from the temporal variation in intensity of cavity- ringdown light," as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light. One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time. Optionally, either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. Optionally, either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. As in the case of the first aspect of the invention, the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of the beam from the swept-frequency light source, which may be split therefrom. The combination may form a coherent light beam with oscillatory characteristics suitable for optical-heterodyne detection. The method according to this aspect of the invention may also include the further step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means immediately after it has been coupled into the optical cavity and by restoring it when the cavity-ringdown process has been concluded. The electronic means may include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source. According to a third aspect of the present invention, there is provided a method for identifying and/or quantifying chemical species in a sample located within an optical cavity defined by at least two reflectors, the method comprising: - generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - coupling into the optical cavity at least a portion of the beam of coherent light, such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - decoupling at least a portion of the cavity-ringdown light from the optical cavity; - detecting, during a period of time, a signal attributable to a property of the chemical species and derived from the temporal variation in intensity of the cavity-ringdown light decoupled from the cavity, associated with build-up and ringdown decay of intracavity radiation; - converting the temporal variation in intensity of cavity-ringdown light decoupled from the cavity into a cavity-ringdown decay rate; and - identifying and/or quantifying at least one chemical species in the sample from the cavity- ringdown signal or a series of cavity-ringdown signals, at one or more suitable wavelengths or frequencies. The exponential ringdown decay during this period of time may be characterised by a ringdown time τ. The cavity-ringdown decay rate may be expressed as τ"1. The phrase, "a signal ... derived from the temporal variation in intensity of the cavity- ringdown light," as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light. One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time. Optionally, either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. Optionally, either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. As in the case of previously mentioned aspects of the invention, the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of a beam from the swept-frequency light source. The combination may form a coherent light beam with oscillatory characteristics suitable for optical-heterodyne detection. The method according to this aspect of the invention may also include the further step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means by interrupting, deflecting or terminating the coherent swept-frequency light immediately after at least one instant when it is coupled into the optical cavity. The electronic means may, as before, include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source. According to a fourth aspect of the present invention there is provided a method for measuring spectroscopic properties of a sample located within an optical cavity defined by at least two reflectors, separated from one another by a cavity length, the method comprising: (a) generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; (b) coupling into the optical cavity at least a portion of the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; (c) decoupling at least a portion of the cavity-ringdown light from the optical cavity; (d) measuring a signal attributable to a property of the sample and derived from the temporal variation in intensity of such cavity-ringdown light during a period of time, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation; (e) converting the temporal variation in intensity of coherent light decoupled from the cavity into a cavity-ringdown decay rate; and (f) varying the cavity length so as to vary the frequencies at which the optical cavity becomes resonant. The exponential ringdown decay during this period of time may be characterised by a ringdown time τ. The cavity-ringdown decay rate may be expressed as τ"1. The phrase, "a signal ... derived from the temporal variation in intensity of such cavity- ringdown light," as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light. One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time. Optionally, either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. Optionally, either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backwardπpropagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. The variation of the cavity length may be performed either in controlled discrete steps or by slow, continuous scanning. The method according to this aspect of the invention may further include a step (g) wherein steps (b) to (f) are repeated to electronically sample coarsely resolved discrete points in the absorption spectrum of the sample. By "coarsely resolved" is meant, for a given cavity length, the frequency interval of the data points of the spectrum is set by the interval of the successive cavity-resonance frequencies of the optical cavity. The method according to this aspect of the invention may also include a step (h) wherein steps (b) to (g) are repeated many times to assemble a more extensive, but still coarsely resolved, absorption spectrum of the sample. The method may be repeated to electronically sample finely resolved discrete points in the absorption spectrum of the sample. By "finely resolved" is meant, for a given cavity length, the frequency interval of the data points of the spectrum is less than the interval of the successive cavity-resonance frequencies of the optical cavity. As in the case of previously mentioned aspects of the invention, the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of a beam from the swept-frequency light source so as to form a combined beam with oscillatory characteristics suitable for optical-heterodyne detection. The method according to this aspect of the invention may also include the further step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means after at least one instant when it is coupled into the optical cavity. The coherent light beam may be restored when the cavity- ringdown process has been concluded. The electronic means may, as before, include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source. According to a fifth aspect of the present invention there is provided a method for measuring spectroscopic properties of a sample located within an optical cavity defined by at least two reflectors separated from one another by a cavity length, the method comprising: (a) generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies using a swept-frequency coherent light source that is tunable over at least one and optionally over at least two optical frequency ranges; (b) coupling into the optical cavity at least a portion of the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; (c) decoupling at least a portion of the cavity-ringdown light from the optical cavity; (d) measuring, during a period of time, a signal attributable to a property of the sample and derived from the temporal variation in intensity of such cavity-ringdown light, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation; (e) converting the temporal variation in intensity of coherent light decoupled from the cavity into a cavity-ringdown decay rate; (f) varying the cavity length so as to vary the frequencies at which the optical cavity becomes resonant; and (g) repeating steps (b) to (f) at least once to electronically sample coarsely resolved discrete points in the absorption spectrum of the sample. The exponential ringdown decay during this period of time may be characterised by a ringdown time τ. The cavity-ringdown decay rate may be expressed as τ"1. The phrase, "a signal ... derived from the temporal variation in intensity of such cavity- ringdown light," as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light. One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time. Optionally, either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. Optionally, either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. The method according to this aspect of the invention may further include a step (h) wherein the cavity length of the optical cavity is adjusted before measuring another coarsely resolved set of discrete points in a different portion of the sample's absoφtion spectrum, corresponding to a different portion of the SF cycle. This variation of the cavity length may be performed either in controlled discrete steps or by slow, continuous scanning. The method according to this aspect of the invention may optionally include a further step (i) in which steps (b) to (h) are repeated many times to assemble a finely resolved absoφtion spectrum of the sample. The absoφtion spectrum may conveniently be representative of the parameter of the sample. The swept-frequency coherent light source may be tunable over at least one and optionally over two or more optical frequency ranges. A single range may be from about 1 MHz to many GHz or THz. Different ranges may be separated or may have overlapping optical frequencies (or wavelengths). As in the case of previously mentioned aspects of the invention, the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of a beam from the swept-frequency light source. The combination may form a coherent light beam with oscillatory characteristics suitable for optical-heterodyne detection. The method according to this aspect of the invention may also include the further step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means by interrupting, deflecting or terminating the coherent swept-frequency light immediately after at least one instant when it is coupled into the optical cavity. The electronic means may, as before, include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source. According to a sixth aspect of this invention there is provided an optical detection system comprising: - a coherent light source capable of generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - an optical cavity, defined by at least two reflectors and adapted to, in use, contain between those reflectors a sample, the optical cavity being optically coupled, in use, to at least a portion of the beam of coherent light, such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler to decouple at least a portion of the cavity-ringdown light from the optical cavity; and - a photodetector to detect, during a period of time, a signal attributable to a property of the sample and derived from a temporal variation in intensity of such cavity-ringdown light, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation for at least one cavity resonance within the optical frequency range of the sweep. The exponential ringdown decay during this period of time may be characterised by a ringdown time τ. The phrase, "a signal ... derived from a temporal variation in intensity of such cavity- ringdown light," as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light. One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time. Optionally, either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. Optionally, either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. The optical detection apparatus or system may further include an electronic processor to convert an output from the photodetector to a cavity-ringdown decay rate. The cavity-ringdown decay rate may be expressed as τ"1. As in the case of previously mentioned aspects of the invention, the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of the beam from the swept-frequency light source. The combination may form a coherent light beam with oscillatory characteristics suitable for optical-heterodyne detection. The method according to this aspect of the invention may also include the further step of controlling the coherent light beam thus formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means by interrupting, deflecting or terminating the coherent swept-frequency light immediately after at least one instant when it is coupled into the optical cavity. The electronic means may, as before, include or comprise, for example, an amplitude modulator or a fast optical switch that may form part of the coherent light source itself or that may, alternatively, be external to the coherent light source. According to a seventh aspect of the present invention there is provided an optical detection system for detecting a parameter of a sample, comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the frequency of the coherent light reaches such values, a signal attributable to the parameter of the sample and derived from the temporal variation in intensity of such cavity-ringdown light decoupled from the cavity, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation; and - an electronic processor to convert photodetector output to a cavity-ringdown decay rate and to process the cavity-ringdown signal for determining one or more parameters of the sample. The exponential ringdown decay during this period of time may be characterised by a ringdown time τ. From the ringdown time τ, a cavity-ringdown decay rate may be calculated. The cavity-ringdown decay rate may be expressed as τ"1. The phrase, "a signal ... derived from the temporal variation in intensity of such cavity- ringdown light," as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light. One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time. Optionally, either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. Optionally, either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. As in the case of previously mentioned aspects of the invention, the cavity-ringdown light decoupled from the optical cavity may be combined, outside of the optical cavity, with a portion of the beam from the swept-frequency light source so as to form a coherent light beam with oscillatory characteristics enabling optical-heterodyne detection. According to an eighth aspect of this invention there is provided an optical system for identifying and/or quantifying at least one chemical species in a sample comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of optical frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the frequency of the coherent light reaches such values, a signal attributable to a property of the chemical species and derived from the temporal variation in intensity of such cavity-ringdown light decoupled from the cavity, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation; and - an electronic processor to convert photodetector output to a cavity-ringdown decay rate and to process the cavity-ringdown signal for determining one or more parameters of the sample. The exponential ringdown decay during this period of time may be characterised by a ringdown time. The ringdown time may be expressed as τ. From the ringdown time τ, a cavity- ringdown decay rate may be calculated. The cavity-ringdown decay rate may be expressed as τ"1. The phrase, "a signal ... derived from the temporal variation in intensity of such cavity- ringdown light," as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light. One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time. Optionally, either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. Optionally, either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. According to a ninth aspect of the present invention, there is provided an optical system for spectroscopically measuring a parameter or a property of a sample, the method comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of optical frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the frequency of the coherent light reaches such values, a signal attributable to the parameter or the property and derived from the temporal variation in intensity of such cavity-ringdown light decoupled from the cavity, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation; and - an electronic processor to convert photodetector output to a cavity-ringdown decay rate and to process the cavity-ringdown signal for determining one or more parameters of the sample. The exponential ringdown decay during this period of time may be characterised by a ringdown time. The ringdown time may be expressed as τ. From the ringdown time τ, a cavity- ringdown decay rate may be calculated. The cavity-ringdown decay rate may be expressed as τ~\ The phrase, "a signal ... derived from the temporal variation in intensity of such cavity- ringdown light," as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light. One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time. Optionally, either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. Optionally, either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. According to a tenth aspect of this invention there is provided an optical system for measuring spectroscopic properties of a sample comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of optical frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the frequency of the coherent light reaches such values, a signal attributable to a property of the sample and derived from the temporal variation in intensity of such cavity-ringdown light decoupled from the cavity, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation; and - an electronic processor to convert photodetector output to a cavity-ringdown decay rate and to process the cavity-ringdown signal for determining one or more parameters of the sample; - an electronic processor to convert photodetector output to a cavity-ringdown signal, to control the frequency of the beam of coherent light, and to adjust the position of at least one of the cavity reflectors so as to vary the resonance frequencies of the cavity, wherein in use, the electronic processor conducts at least two frequency sweeps in which the frequency of the beam of coherent light is varied over a range of frequencies, so as to electronically sample at least two different coarsely resolved data sets from the absoφtion spectrum of the sample, at least two of such data sets corresponding to different resonance frequencies of the cavity. The exponential ringdown decay during this period of time may be characterised by a ringdown time. The ringdown time may be expressed as τ. From the ringdown time τ, a cavity- ringdown decay rate may be calculated. The cavity-ringdown decay rate may be expressed as τ"\ The phrase, "a signal ... derived from the temporal variation in intensity of such cavity- ringdown light," as used in this specification shall be taken to include any coherent combination of the cavity-ringdown light with any part of the aforesaid beam of coherent light. One, two or even more such signals may be detected, and they may be detected simultaneously or at different times or during different periods of time. Optionally, either the peak amplitude for build-up and decay of transmitted cavity- ringdown light or the peak-to-peak amplitude for build-up and decay of backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. Optionally, either the integrated signal profile for build-up and decay of transmitted cavity- ringdown light or the rectified and integrated full-wave signal profile for backward-propagating optical-heterodyne-detected light may be measured as a temporal variation in intensity, rather than the cavity-ringdown decay rate. The resonance frequencies of the cavity may be altered by varying the cavity length either in controlled discrete steps or by continuous scanning, which may be slow. The coherent light may be generated by a coherent light source selected from a continuous- wave (cw) coherent light source (i.e., with amplitude or intensity that does not vary rapidly in time) and a quasi-continuous-wave (quasi-cw) or long-pulse coherent light source, that is able to generate coherent light over a range of optical frequencies. Other suitable coherent light sources include lasers, continuous and quasi-continuous light sources such as nonlinear-optical devices of various forms, including laser-pumped wavelength converters, optical parametric oscillators (OPOs), optical parametric generators (OPGs), optical parametric amplifiers (OP As); second- harmonic generators (SHGs); third-harmonic generators (THGs); sum-frequency generators (SFGs); difference-frequency generators (DFGs); stimulated Raman scattering (SRS) devices, and the like. For the puφose of this invention, the descriptions "continuous-wave" or "cw" are taken to apply to any laser or other coherent light source that continuously delivers optical power without interruption, including any such laser or coherent light source with an optical frequency or amplitude that is varied in time, in a controlled fashion (e.g., by frequency modulation or amplitude modulation or frequency sweeping). The laser may be selected from infrared lasers, diode lasers, and single-mode tunable continuous-wave (cw) dye lasers. Suitable lasers also include distributed-feedback lasers, external optical-cavity lasers and optical-fibre lasers. The laser can be monochromatic or wide- spectrum, provided that means are provided to control or select its wavelength, which control or selection may be part of the laser itself or of an external component situated elsewhere in the system. Conveniently, the laser has an optical bandwidth of less than 10 MHz. One suitable form of a quasi-cw coherent light source is a long-pulse laser. Its optical power needs to be established at a constant plateau level well in advance of the point in the frequency-sweep cycle at which the optical cavity comes into resonance with the light, while optical power builds up in the cavity. To observe readily inteφreted CRD signals, the duration of the plateau value of incident optical power preferably substantially exceeds (e.g., by a factor of 10) the sum of build-up time and ringdown decay time, so that the incident optical power is effectively constant during the build-up and decay cycle. The coherent light source may be a continuous-wave (cw) tunable laser, which may be a semiconductor diode laser that is able to generate continuous-wave (cw) coherent light over a range of optical frequencies during a swept-frequency (SF) cycle. The range of optical frequencies that is generated during a broad SF cycle may be from about 1 MHz to about 10 THz. A single coherent light source may be used, although in some embodiments more than one coherent light source may be desirable (e.g., two, three or more lasers). In such events, each coherent light source may have a different wavelength (or set of SF-cycled wavelengths) to advantageously cover a combined wavelength range which is larger than can be obtained when only one laser is used. By using a plurality of coherent light sources, the performance advantages of one light source (i.e., such as high frequency stability) may be combined with a wide output frequency range of a second light source, or alternatively, to confirm the presence of a parameter. Diode lasers are typically small and inexpensive, enabling an apparatus to be designed for portability and compactness. Continuous-wave (cw) tunable lasers are generally superior to pulsed lasers due to the narrower optical bandwidth of the cw tunable laser output and the narrowband filtering role of the ringdown optical cavity. The laser suitably generates light comprising a wavelength corresponding to an optical absoφtion region of interest in a sample, preferably a gaseous sample. Preferably, the continuous-wave (cw) tunable laser used emits infrared, ultraviolet, or visible wavelengths, typically infrared. Accordingly, the term light as used in the description of the invention is intended to include light or radiation at these wavelengths. The term light is to be inteφreted broadly and may include light within other wavelengths of the wavelength spectrum. The ringdown-cavity cell may be a linear or folded optical cavity or a ring resonator such as adopted in US Patent 5,912,790 by way of reference. The ringdown-cavity cell is preferably a linear cell of two reflectors, suitably two highly reflective dielectrically-coated input and output mirrors having a concave curvature designed to retain light in the optical cavity. Suitable radii of curvature of the mirrors are in a range of 0.01 to 10 metres, typically about 1 metre. An optically stable, low-loss optical cavity is formed from the two mirrors being aligned with respect to the optical axis of the cell, the mirror separation typically being less than twice the radius of curvature. Suitable distances between the optical-cavity mirrors are of the order of 10 to 1000 mm. The distance may be adjusted to optimise separation of low-order transverse optical-cavity modes to the longitudinal cavity modes. This description of the invention is concerned primarily with the longitudinal modes of the optical ringdown cavity, but the transverse-mode structure of the cavity is also within the scope of this invention. The coupling of input coherent-light-source/laser radiation into the lowest-order transverse modes (which correspond to the longitudinal or axial modes) of the cavity may be preferred and may be achieved by using a suitable combination of lenses or mirrors to match the input-beam geometry to the cavity-mode geometry. However, it is also feasible to perform CRD measurements if the input beam is coupled into higher-order transverse modes of the cavity, as well as or instead of the lowest-order transverse modes (i.e., longitudinal or axial modes) of the cavity. Suitable mirrors for use in the invention are well known in the art. They may be suitable for use over a wide range of the visible, ultraviolet and infrared spectrum. Suitable mirrors typically have reflectivity greater than 99.95% and up to 99.999% to enable most radiation entering the optical cavity to be reflected over path-lengths of the order of about 104 times the physical length of the cell, thereby providing a long effective optical absoφtion path-length within the optical cavity and providing a ringdown decay time of a few microseconds. A sample may be provided in the cell. The sample is preferably a gas. It may be introduced into the cell in the vicinity of the optical axis of the ringdown-cavity cell. Suitable gases which can be detected by use of the method and apparatus of the invention include carbon dioxide (C02), carbon monoxide (CO), water vapour (H20), nitric oxide (NO), oxygen (02), hydrogen fluoride (HF), nitrous oxide (N20), hydrogen cyanide (HCN), methane (CH4), light alkanes (CnH2n+2), ethylene (C2H4), acetylene (C2H2), ethanol (C2H50H), acetaldehyde (CH3CHO), ketones such as acetone, (CH3)2CO, chloroform (CHC13) and ammonia (NH3). Other gas-phase chemical agents such as nerve gas, phosgene, and vapour associated with common explosives can be detected for security puφoses if suitable mid-infrared tunable lasers are available. Combinations of gases can also be detected and, if necessary in order to do so, more than one coherent light source, which may be continuous-wave sources, may be used. These sources may have wavelengths corresponding to wavelengths of the optical absoφtion regions of the gases of interest. The ringdown-cavity cell can be formulated as a separate module from the remaining apparatus so that it can be used in remote sensing applications. Certain properties of solid and liquid samples can also be measured using CRD spectroscopy by depositing small amounts of a sample to the coating surface of at least one ringdown-cavity mirror/reflector. Alternatively, an extra environmentally sensitive mirror (e.g., by evanescent-wave coupling) may be contained in the optical cavity so as to form a triangular optical cavity able to detect environmentally sensitive parameters. The means for directing a portion of said coherent light/radiation emitted from the continuous-wave coherent-light-source/laser to the ringdown-cavity cell may ensure that the light/radiation emitted from the coherent-light-source/laser follows an optical path into the ringdown-cavity cell. In one embodiment, suitable means for directing a portion of said light/radiation is provided by an optical circulator which has the advantage of being able to route one portion of the light/radiation by optical fibre from the coherent-light-source/laser to the ringdown-cavity cell and to route the light/radiation back-reflected by the optical cavity by optical fibre to a photodetector, thereby preventing the light/radiation back-reflected by the optical cavity returning to the coherent-light-source/laser. The optical circulator, may be of a type widely used in fibre-optical telecommunication networks. It may direct at least 50%, more preferably at least 90%, most preferably at least 99% of the emitted light/radiation from the coherent-light- source/laser to the input of the ringdown-cavity cell. The light/radiation is preferably coupled to the input of the ringdown-cavity cell by means of a combination of any one of at least one reflector, a polarisation-control means, an optical circulator means, an optical isolator means and a lens system (suitably a 5-cm to 5-m focal-length lens, either single or compound, more typically a 40-cm focal-length lens) which may collect and may direct the light/radiation into the optical cavity. The polarisation-control optics means may function to direct backward-propagating light to the detection means (suitably a photodetector) and to control the polarisation of light incident on the detecting means and/or on the input of the ringdown-cavity cell. The polarisation-control optics means may comprise a polarising prism/beam splitter, waveplate, optical circulator or Faraday rotator. Typically, the polarisation-control optic means comprises a polarising beam splitter and a Faraday rotator, or a polarising beam splitter and a quarter-wave plate or an optical circulator. The rotation angle of a Faraday rotator and thickness of a quarter-wave plate as well as the performance of the optical circulator are wavelength dependent. Accordingly they may be designed for working in a narrow wavelength region. They may be specified by their designed centre wavelengths. Accordingly, it is desirable to select them on the basis of the wavelengths of interest, because operating at wavelengths far away from the designed wavelengths will result in a reduction of coupling efficiency of the backward-propagated light to the detecting means and an increasing amount of the backward-propagated light may reach the coherent-light-source/laser causing disruption or damage. In embodiments where more than one ringdown-cavity cell is used and/or more than one coherent light source is used, it may be desirable to include in the optical path a combiner/multiplexer, an optical switch, a switch module or an optical-fibre splitter so as to distribute the coherent-light-source/laser radiation and to return cavity-ringdown light or to switch between the various wavelengths of multiple coherent light sources or lasers. As the frequency of the coherent radiation during a SF cycle is varied, optical energy is built up and stored in the optical cavity when the frequency moves into the vicinity of a resonance frequency of the cavity. The ringdown decay of light energy that is built up and stored in the optical cavity during the short resonance interval is observable after the frequency of the coherent radiation has moved off resonance, because the input light is then effectively blocked, owing to the absence of resonant interaction within the cavity, by the highly reflective optical cavity reflectors/mirrors during the (relatively long) off-resonance interval. The decay of light energy is gradual with a ringdown time constant ( that depends on the reflector/mirror reflectivity and the absoφtion of the optical medium in the optical cavity. During a SF cycle, this build-up and ringdown-decay event repeats each time during a SF cycle that the frequency of the coherent radiation moves across a cavity resonance frequency. As will be explained further below, the length of the optical cavity may be varied incrementally or decrementally by an optical-length adjuster, in order to obtain additional ringdown-time data at different cavity-resonance frequencies after the coherent-light-source/laser has emitted light/radiation at a range of optical frequencies during a SF cycle. This incrementally (or decrementally) stepped cavity-length (ISCL) adjustment may occur between successive SF cycles. An alternative approach is to vary the length of the optical cavity by continuous scanning,of an analog control input to the optical-length adjuster, in a manner that is synchronised with successive SF cycles and is sufficiently slow that many SF cycles (e.g., at least 10) occur before the cavity length traverses one free spectral range (FSR, corresponding to half the wavelength of the light). This slow, synchronously scanned cavity-length (SSCL) adjustment may either occur continuously during and between successive SF cycles or it may be controlled to occur only during portions of the SF cycle when cavity-ringdown signals are recorded. Either form of optical-length adjustment (ISCL or SSCL) may be provided by a piezoelectric translator (PZT) operated to provide movement of at least one of the reflectors/mirrors constituting the optical-cavity, so as to vary the length thereof. The reflector/mirror may conveniently be mounted on a cylindrical PZT device controlled by an electronic control circuit, so that the reflector/mirror can be moved either in a prescribed incremental fashion between one SF cycle and the next, using the ISCL procedure, or by slow, continuous scanning using the SSCL procedure. Suitable optical-cavity length changes in the ISCL procedure are of the order of from about 1 nm to about 10 μm. The maximum useful cavity-length scan range in the SSCL procedure is one free spectral range (FSR) of the cavity (i.e., half the wavelength of the light). The optical-cavity resonance frequency changes as one of the optical-cavity mirrors is incrementally moved between SF cycles, using the ISCL procedure, or continuously scanned, using the SSCL procedure. Depending on the distance of mirror movement, the amount of frequency shift is typically in the range of 10 kHz to 100 MHz. It should be noted that, by using a SF cycle in which the frequency of emitted coherent light is altered while holding the length of the optical cavity constant (i.e., by maintaining the distance between the two or more reflectors constant), a ringdown signal may be generated while still using light/radiation that is continuous-wave (i.e., with amplitude or intensity that does not vary rapidly, apart from its SF characteristics). In this embodiment, the need to cut off or modulate the intensity of the coherent light may be avoided by using a fast optical switch, such as an electro- optic modulator, a Pockels cell, an electro-optic Kerr cell or an acousto-optic modulator. Further, by the exclusion of fast optical switches, the system of the invention may be made more compact, more robust and less expensive. Moreover, in this embodiment it is not necessary to lock the optical-cavity length and the coherent-light-source/laser wavelength to each other. It will be appreciated that the means for directing the coherent light and the means for combining the coherent light and the cavity-ringdown light may be the same. During each SF cycle, the frequency of coherent light emitted from the swept-frequency coherent light source changes monotonically and may match many successive discrete resonance frequencies of the ringdown cavity. This may create many sequential build-up and ringdown- decay events. Each discrete matching frequency defines the frequency of the cavity-ringdown light/radiation for each corresponding build-up and ringdown-decay event. By combining cavity-ringdown light/radiation with intense light/radiation having different frequencies, an intense detectable signal (a beat signal) may be obtained which is the product of two fields containing a strong radiation component corresponding to the intense light/radiation coupled with a weak build-up and ringdown-decay component of light/radiation emitted by the ringdown cavity. The coherent light that may be used as the intense light/radiation for combining with the cavity-ringdown light/radiation is preferably coherent light emitted from the coherent light source which either. (a) has not passed through the cavity cell but has been back-reflected off the cell and/or (b) coherent light that has puφosely been redirected (e.g., by at least one beam splitter). The cavity-ringdown light which is combined is either cavity-ringdown light emerging from the input mirror or reflector and/or cavity-ringdown light emerging from the output mirror or reflector. The method of the invention enables optical signals related to the ringdown time τ to be detected at a high modulation frequency, determined by the SF sweep rate, rather than at the relatively low frequency of the intrinsic ringdown decay signal, thereby enabling more sensitive detection. The cavity-ringdown light or radiation which is combined is either cavity-ringdown light/radiation emerging from the input mirror or reflector and/or cavity-ringdown light or radiation emerging from the output mirror or reflector. During a given build-up and decay event, the cavity-ringdown light or radiation is fixed at a cavity-resonance frequency, while the much more intense emitted SF light or radiation has a continuously shifting frequency. By combining these two optical fields, an intense detectable wave may be obtained, which is the product of weak cavity-ringdown light/radiation field coupled with a strong SF coherent-light-source/laser light or radiation field which varies in accordance with the swept frequency shift. The method of the invention thereby enables optical signals related to the ringdown time τ to be detected at a high modulation frequency, determined by the SF sweep rate, rather than at the relatively low frequency of the intrinsic ringdown decay signal to enable more sensitive detection. The preferred mode of detection is described in common parlance as optical-heterodyne detection. The Macquarie Dictionary (2nd Edition, The Macquarie Library Pty Ltd) defines heterodyne as denoting or pertaining to a method of receiving continuous-wave radiotelegraph signals by impressing upon the continuous radiofrequency oscillations another set of radiofrequency oscillations of a slightly different frequency, the interference resulting in fluctuations or beats of audiofrequency. The Macquarie Dictionary (2nd Edition, The Macquarie Library Pty Ltd) defines homodyne as a radio receiver which demodulates an amplitude- modulated signal by the process of mixing the carrier signal with the sidebands. Experts are divided as to whether the detection mode used is true heterodyne (i.e., detected at the difference frequency between signal and local oscillator waves) or whether it is actually homodyne (i.e., detected in the frequency domain shared by the signal and local oscillator waves). The descriptions 'heterodyne' and 'homodyne' are therefore used interchangeably in this context. The means for optical-heterodyne detection of the combined light/radiation may conveniently comprise at least one photodetector, typically a square-law photodetector. In some embodiments at least two photodetectors may be required. The photodetector(s) may be coupled to a preamplifier (suitably with a bandwidth of up to 500 MHz, more preferably 10 - 100 MHz) and a data-acquisition device such as an A/D converter, a digital oscilloscope or a boxcar integrator system. The photodetector detects the combined light/radiation measuring the interaction of a sample contained within the optical cavity with intracavity light/radiation. The combined light/radiation is converted by the photodetector into a corresponding signal. Suitable photodetectors include photomultiplier tubes and semiconductor photodiodes. Avalanche, Si, Ge, AIGaAs, InGaAs and HgCdTe photodiodes are suitable. The photodetector(s) may be located before and/or after the ringdown-cavity cell. In one embodiment, the photodetector is located before the ringdown-cavity cell and is preferably located along an optical light path different to that of the incoming light/radiation. This may be achieved by use of a polarising beam splitter and a polarisation controller or by use of an optical circulator. In another embodiment the photodetector is located after the ringdown-cavity cell, prior to reaching the photodetector, a portion of the emitted light/radiation is suitably directed by means of a beam splitter and a number of reflectors prior to combination with cavity-ringdown light/radiation emerging from the output of the ringdown-cavity cell, suitably by means of beam splitters. In another embodiment two photodetectors are used, one located before the ringdown-cavity cell and one after the ringdown- cavity cell. In such an embodiment the photodetector located before the ringdown-cavity cell suitably detects the combined light/radiation as above, and the second photodetector detects cavity-ringdown light/radiation emerging from the output of the ringdown-cavity cell. Means for determining a parameter of a sample is suitably a data analysis and recording device such as a computer. Various parameters of interest which can be determined from the photodetector signals include cavity-ringdown decay rates and the time dependence of the light intensity. The ringdown-decay rate of the combined light/radiation is suitably determined by amplifying, rectifying and digitising the electronic signal and fitting the waveform to an envelope with an exponential decay, using a suitable algorithm. Alternatively, the output of the combined light/radiation may be suitably processed by a demodulating logarithmic amplifier (e.g., Analog Devices, model AD8307) and subsequently fit the waveform to a straight line, using a suitable algorithm. The ringdown decay curve and or the slope of its demodulated logarithm are indicative of the optical absoφtion of a sample. By comparison with an empty-cell ringdown decay rate, the level of trace species in a sample or the optical absoφtion spectra of a known composition, may be determined. The ringdown decay curve can also be correlated to other parameters aside from absoφtion, including light scattering, reflectivity and dielectric relaxation or any other parameter that causes an energy change as a result of the interaction between the intracavity light/radiation and the sample. By using a coherent light source with swept frequency (or wavelength) during a SF cycle, multiple cavity build-up and ringdown decay occurs each time that the frequency of the coherent radiation moves through a cavity resonance frequency. The cavity-ringdown decay rate at multiple discrete cavity resonance frequencies of the optical cavity may be determined to obtain a coarsely resolved spectrum that is electronically sampled in a single SF cycle over the output wavelength range of the coherent light source; this can typically be accomplished in less than 1 second, including time for signal-averaging to enhance the signal-to-noise ratio. The method of the invention may also be used to determine the presence of more than one trace species in a sample (each trace species having different characteristic optical absoφtion wavelengths) or alternatively, to confirm the presence of a trace species having multiple characteristic optical absoφtion wavelengths. Improvement of frequency resolution in the absoφtion spectrum of the sample may be achieved by using the ISCL procedure to record multiple coarsely resolved CRD spectra at successive incrementally or decrementally shifted resonance frequencies of the optical cavity and by combining in a single more finely resolved plot all the cavity-ringdown decay rates and their corresponding frequencies. As noted above, the resonance frequencies of the optical cavity may be shifted incrementally or decrementally by changing the cavity length such as by displacing a cavity mirror by a piezoelectric translator. Likewise, similar outcomes may be achieved by slow, continuous scanning using the SSCL procedure. By use of multiple swept-frequency coherent light sources, a large overall wavelength range may be covered, to combine the performance advantages such as high frequency stability of one light source with a wide output-frequency range of a second light source, or alternatively to confirm the presence of a parameter. In one embodiment of the invention, a triangular (or ring) ringdown-cavity is assembled by introducing a third mirror, the reflectivity of which is designed to be sensitive to its environment, such as temperature, pressure, humidity, external electric or magnetic fields or pressure/concentration of a particular chemical species (e.g., one that adsorbs on the surface of the third mirror). The intracavity optical field may be coupled via an evanescent wave to a medium or environmental conditions that are external to the optical cavity itself. This approach is similar to certain forms of fibre-optic sensor, but CRD spectroscopy offers the prospect of much higher sensitivity. Suitably in this embodiment the angle between the incident and reflected beams on the mirror/reflector connected to the PZT device is small, to minimise displacement of the beam path when PZT-induced adjustments occur. In another embodiment of the invention, the optical cavity may be formed by four or more mirrors. In another typical embodiment of the invention, optical fibres are used to transmit the light/radiation over various portions of the beam path, for example, between the coherent-light- source/laser and the ringdown-cavity cell. Hence in one embodiment, the means for directing a portion of said light/radiation comprises an optical fibre and/or the means for combining a portion of said light/radiation emitted with cavity-ringdown light/radiation comprises an optical fibre. By use of optical fibres, the optical-cavity cell may be remotely isolated from the light source and detection system which may be provided in a single transmitter-receiver unit, thereby facilitating scientific, environmental, agricultural, medical and industrial monitoring applications. In such embodiments the PZT device may be controlled by a separate voltage source, by electrical connection to the control means or by wireless means. Suitable optical fibres include silica fibres (which have a cut-off wavelength of about 1.8 μm) and fluoride glass fibres (which can operate up to 3-μm wavelengths). By use of the apparatus and method of the invention all available coherent-light- source/laser output energy may be used efficiently, thereby enhancing the detection sensitivity of cavity ringdown spectroscopy. A continuous-wave (cw) tunable coherent-light-source/laser that is rapidly frequency-swept may be used in the cavity ringdown (CRD) spectroscopy method and apparatus according to the invention. A complete coarsely resolved CRD spectrum may be obtained at each SF cycle, with the possibility of repetitive cycles for signal-averaging to enhance the signal-to-noise ratio. Furthermore, the ringdown-cavity resonance characteristics of the cavity cell may be varied by incrementally or decrementally altering the optical path-length at the end of a SF cycle and then applying another SF cycle (or repetitive SF cycles, to enable signal-averaging) to the cavity cell, using the ISCL procedure. This advantageously enables successive CRD measurements to be sampled at multiple sets of cavity-resonance frequencies so that the coarsely resolved spectroscopic data from these frequency sweeps can be combined to obtain more finely resolved CRD-spectroscopic results. Likewise, similar outcomes may be achieved by slow, continuous scanning using the SSCL procedure. As the light/radiation from the coherent-light-source/laser changes frequency during the SF cycle, a portion of the radiation builds up firstly into the optical cavity and then decays away each time that the frequency of the radiation moves across a resonance frequency of the optical ringdown cavity. This cavity-ringdown light/radiation of the cavity resonance frequency is then suitably combined efficiently with the original swept-frequency light/radiation enabling generation of an optical-heterodyne signal. Measurement of CRD signals with a swept-frequency cw coherent-light-source/laser by this optical-heterodyne-detected (OHD) approach enhances the detection sensitivity. The resonance properties of a frequency-swept coherent-light-source/laser and optical cavity length that is fixed (or incrementally varied, using the ISCL procedure, or continuously scanned, using the SSCL procedure) also simplify CRD spectroscopy with a swept-frequency cw coherent-light- source/laser in that they eliminate the need for a fast optical switch and avoid locking of the optical-cavity length and coherent-light-source/laser wavelength to each other. Nevertheless, one or more optical switches may still be advantageous in some applications. By use of the apparatus and methods of the invention ultra-sensitive, high-resolution, accurate CRD spectroscopy is obtained with relatively simple, inexpensive, compact apparatus suitable for use in the field or at industrial sites or in clinical situations. Brief Description of the Drawings A preferred form of the present invention will now be described, by way of example, with reference to the accompanying drawings wherein: Figure 1 is a schematic drawing of an optical detection system according to ^ one embodiment of the present invention; Figures 2 (a) to 2(c) are graphs of simultaneously recorded signals measured with a swept- frequency coherent-light-source/laser and an optical cavity of fixed optical cavity length, Figure 2(a) being a representation of a build-up and ringdown signal detected in the forward-propagating or transmitting direction of the optical cavity; Figure 2(b) is a representation of an optical- heterodyne signal prepared by combining a build-up and ringdown signal propagating in the backward direction from the optical cavity, with swept-frequency coherent-light-source/laser light reflected from the front mirror of the optical cavity; and Figure 2(c) represents the demodulated logarithm of the optical-heterodyne signal of Figure 2(b); Figures 3(a) to 3(g) depict a sequence of signals obtained or derived from a single frequency sweep or cycle of the optical apparatus of Figure 1, wherein Figure 3(a) shows a schematic build-up and ringdown-decay profile of the signal for backward-propagating light actively reflected from the optical cavity (effectively a supeφosition of cavity-ringdown light emitted from the cavity combined with swept-frequency coherent-light-source/laser light reflected off the front mirror of the optical cavity); Figure 3(b) shows the demodulated logarithm of the signal envelope shown in Figure 3(a); Figure 3(c) shows the profile of a build-up and ringdown- decay signal transmitted (forward-propagating) through the optical cavity; Figure 3(d) shows a coherent-light-source/laser-frequency sweep graph, with the frequencies at which the optical cavity resonates marked thereon; Figure 3(e) shows the status of the sweeping function of the swept-frequency coherent-light-source/laser; Figure 3(f) shows the cumulative number of recorded data points; and Figure 3(g) shows the elapsed time; Figure 4 is a cavity ringdown spectrum, comprising a plot of cavity-ringdown decay rate (τ" ') versus optical frequency, sampled during a single SF cycle by using the embodiment of the optical detection system of Figure 1 and applying the method as described with reference to Figures 3(a) to 3(g); Figure 5 shows a method including a schematic depiction of the cavity-ringdown decay rate (τ"1) versus optical frequency for three successively assembled frequency sweeps, between each of which the length of the optical cavity of the system shown in Figure 1 has been changed by a single increment or decrement; Figure 6 is a schematic drawing of an optical detection system according to another embodiment of the present invention, comprising a four-mirror ringdown cavity in the form of a bow-tie ring, a reference etalon and a reference gas cell, but excluding optical fibres and an optical circulator; Figures 7(a) to 7(i) respectively depict signals obtained or derived from a single SF cycle of the optical detection system of Figure 6, wherein Figure 7(a) schematically shows build-up and ringdown-decay profiles of the reflected (backward-propagating) signal; Figure 7(b) shows the demodulated logarithm of reflected signal profile envelopes; Figure 7(c) shows build-up and ringdown-decay profiles of the transmitted (forward-propagating) signal; Figure 7(d) shows a coherent-light-source/laser-frequency sweep with etalon- and cavity-resonance frequencies marked; Figure 7(e) shows the frequency-sweep status; Figure 7(f) shows a number of recorded data points; Figure 7(g) shows elapsed time; Figure 7(h) shows frequency markers of a reference etalon; and Figure 7(i) shows a signal derived from the transmission of a reference gas cell; Figures 8 to 11 are schematic diagrams of optical detection systems according to further embodiments of the present invention; Figure 12 depicts an experimental realisation of the method described with reference to Figures 3(a) to 3(g) and Figure 4; Figure 13 depicts a further experimental realisation of the method described with reference to Figures 3(a) to 3(g) and Figure 4; Figure 14 depicts an experimental realisation of the method described with reference to Figure 5; Figures 15 and 16 schematically depict an electronic data-processing method for efficient extraction of ringdown-time data points; Figure 17 depicts absoφtion spectra of carbon dioxide (C02) gas to demonstrate the efficiency with which wide-ranging swept-frequency cavity ringdown spectra can be rapidly recorded using the method of Figures 15 and 16; and Figure 18 depicts an experimental realisation of cavity-enhanced absoφtion spectroscopy (CEAS) obtained by using an embodiment of a system in accordance with the invention in which signal amplitude, rather than ringdown time, is used to record a wide-ranging absoφtion spectra of carbon dioxide (C02) gas. Detailed Description of the Preferred Embodiments In the description that follows of the various drawings and diagrams of different embodiments of the invention, like or corresponding reference numerals indicate like or corresponding components, parts or features. Referring to Figure 1, there is shown a schematic diagram of one embodiment of an optical detection system 10 in accordance with the invention, for measuring a spectroscopic property of a sample. The optical detection system 10 comprises a coherent light source in the form of continuous-wave (cw) tunable swept-frequency (SF) coherent-light-source/laser 12 that emits coherent light of which the optical frequency can be varied over a range from about 1 MHz to many GHz or THz. The cw tunable SF coherent-light-source/laser 12 is optically coupled to an optical resonator in the form of a ringdown-cavity measuring cell 14. The ringdown-cavity measuring cell 14 is provided with a pair of highly reflective mirrors 16.1, 16.2 facing one another and defining an optical cavity 18 between them. Suitable mirrors typically have a reflectivity greater than 99.95% and up to 99.999%. The surfaces of the mirrors 16.1, 16.2 that face one another are concave, each mirror 16.1, 16.2 having a radius of curvature of about 1 m. The mirrors 16.1, 16.2 are aligned with respect to an optical axis 20. The distance between the mirrors 16.1, 16.2 is initially set at about 0.5 m, but this distance can be varied by an optical-length adjuster in the form of a piezoelectric translator (PZT) 22, that is attached to the mirror 16.2, for moving the mirror 16.2 incrementally or continuously along the optical axis 20, relative to the mirror 16.1. The ringdown-cavity measuring cell 14 is designed to operate according to the cavity- ringdown principle, as described in the following review articles: "Cavity-Ringdown Spectroscopy - An Ultratrace-absoφtion Measurement Technique", edited by K. W. Busch and M. A. Busch, ACS Symposium Series (1999) No. 720, ISBN 0-8412-3600-3; and "Cavity ring- down spectroscopy: Experimental schemes and applications", International Reviews of Physical Chemistry 19 (2000) 565 - 607 by G. Berden, R. Peeters, and G. Meijer. In use, a gaseous sample 24 is introduced into the ringdown-cavity measuring cell 14, so that it constitutes an optical medium in the optical cavity 18. The optical system 10 is designed to detect the presence of trace chemical species in the sample 24. The sample 24 is directed into the ringdown-cavity measuring cell 14 by sample handling means 26. Two transparent windows 14.1 and 14.2 are provided in opposite ends of the wall of the ringdown-cavity measuring cell 14, so as to allow coherent light to enter and leave the ringdown- cavity measuring cell 14. The ringdown-cavity measuring cell 14 is optically coupled to the cw tunable SF coherent- light-source/laser 12 by single-mode optical fibres 28, 30 and an optical coupler in the form of an optical circulator 32. The ends (not shown) of the optical fibres 28, 30 are preferably cut at an angle (typically 8 degrees, as widely used in fibre-optic technology) to avoid interference caused by the returning reflection from the end facets of the fibres. A lens (or system of lenses) 34 is provided in the optical path of the light being emitted by the cw tunable SF coherent-light-source/laser 12 towards the ringdown-cavity measuring cell 14, to collect and direct such light into the optical cavity 18 of the ringdown-cavity measuring cell 14. In this embodiment of the invention, where light is transmitted to and from the ringdown-cavity measuring cell 14 by single-mode optical fibre, the lens system 34 comprises a microscope objective in a suitable optical-fibre micropositioner. The lens system 34 and optical fibre 30 are also able to collect and transmit both backward-propagating intracavity light and reflected incident SF coherent-light-source/laser light from the mirror 16.1 of the ringdown-cavity measuring cell 14. The optical circulator 32 directs backward-propagating light from the ringdown-cavity measuring cell 14 towards a photodetector 36 (also designated as PD1 in Figure 1), via a line 32.1 (that may for example be an optical fibre or a freely propagating light beam). Moreover, the optical circulator 32 also suppresses backward-propagating light from returning to the cw tunable SF coherent-light-source/laser 12, and protects the SF coherent-light-source/laser 12 from interruption or damage by the returned light. An additional optical isolator (not shown in Figure 1) may also be integrated into the SF coherent-light-source/laser 12, and/or into the optical fibre 28 and/or be inserted between the SF coherent-light-source/laser 12 and the optical circulator 32, to provide further suppression of back-reflected light reaching the SF coherent-light-source/laser 12. Another optical detector in the form of a photodetector 38 (PD2) is provided on the remote side of the ringdown-cavity measuring cell 14, to detect light that has passed through the optical cavity 18, the mirror 16.2 and the window 14.2. A further optical fibre may be provided for transmitting light emerging from the window 14.2 to the photodetector 38 (PD2). The photodetectors 36 (PD1) and 38 (PD2) detect light by converting it into corresponding electrical signals. The photodetector 36 (PD1) converts the combined beam 32.1 from the optical circulator 32 into a corresponding electrical signal. This beam has oscillatory characteristics suitable for optical-heterodyne detection, as explained above under "Summary of the Invention". The manner in which an optical-heterodyne beat signal is generated in the swept-frequency case here is subtly different to that in the swept-cavity OHD cw-CRD spectroscopic technique, as outlined above in the "Background of the Invention" section and explained in the following article: "Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical-heterodyne detection: single- and multi-wavelength sensing of gases", Applied Physics B 75 (2002) 267 - 280 by Y. He and B. J. Orr. The optical-heterodyne beat signal in the previous swept-cavity case is generated by interference of Doppler-shifted cavity-ringdown light emitted from the cavity with fixed-frequency light reflected from the front mirror of the cavity, whereas the optical-heterodyne beat signal in the swept-frequency case here may be generated by interference of fixed-frequency cavity-ringdown light emitted from the cavity with swept-frequency light reflected from the front mirror of the cavity. The signal from photodetectors 36 (PD1) or 38 (PD2) may also be used optionally for swept-frequency (SF) cavity-enhanced absoφtion spectroscopy (CEAS), in which either the peak amplitude for build-up and decay of the transmitted CRD signal or the peak-to-peak amplitude of the OHD CRD signal is measured, rather than the cavity-ringdown decay rate. These SF CEAS approaches are different from the swept-cavity CEAS technique, as outlined above in the "Background of the Invention" section and explained in the following article by Y. He and B. J. Orr: "Ringdown and cavity-enhanced absoφtion spectroscopy using a continuous-wave tunable diode laser and a rapidly swept optical cavity", Chemical Physics Letters 319 (2000) 131 - 137. The CEAS signal in the previous swept-cavity case is generated by build-up and decay of light with fixed laser frequency and swept optical-cavity length, whereas the SF CEAS signal in the swept-frequency cases here may be generated by build-up and decay of light with fixed optical- cavity length and swept coherent-light-source/laser frequency. A swept-frequency (SF) variant of integrated cavity output spectroscopy (ICOS) may also be used optionally, either by integrating the signal profile for build-up and decay of the transmitted CRD signal or by rectifying and integrating the full-wave OHD CRD signal, rather than measuring the cavity-ringdown-decay rate or the SF CEAS signal. These SF ICOS approaches are different from the modulated-cavity cw-ICOS technique, as outlined above in the "Background of the Invention" section and explained in the following article by A. O'Keefe, J. J. Scherer and J. B. Paul: "cw Integrated cavity output spectroscopy", Chemical Physics Letters 307 (1999) 343 - 349. The cw-ICOS signal in the previous swept-cavity case is generated by build-up and decay of light with fixed laser frequency and modulated optical-cavity length, whereas the SF ICOS signal in the swept-frequency cases here may be generated by build-up and decay of light with fixed optical-cavity length and swept coherent-light-source/laser frequency. The output of the photodetector 36 (PD1) has the profile as is shown in Figure 3(a). It is sent to an electronic unit 40 via a line 36.1 and is there processed to obtain the demodulated envelope of the oscillatory signal as is shown in Figure 3(b). An electronic data-acquisition module 42 is provided to collect and store the output signal, as shown in Figure 3(b), of the electronic circuit 40 via a line 40.1. The same data-acquisition module 42 is also provided to collect and store the output signal, as shown in Figure 3(c), of the photodetector 38 (PD2) via a line 46.4. A suitable data-acquisition module 42 may be a digital oscilloscope or an analog-to-digital converter. The module 42 is in communication with the coherent-light-source/laser 12 via a line 46.3. A signal representing the status of the coherent- light-source/laser 12, as is shown in Figure 3(e) is transmitted along this line 46.3. A data- processing and computer .control unit 44 including control electronics (not shown separately) is coupled to the data-acquisition module 42 by means of a line 46.6. The control electronics of the processing-and-control unit 44 are connected, via a connection 46.1, to the piezoelectric translator 22, to enable controlled incremental, decremental or continuous movement of the mirror 16.2. The control electronics of the processing-and-control unit 44 are also connected to the sample handling means 26, via a line 46.5. The processing-and-control unit 44 is also coupled to the SF coherent-light-source/laser 12, as is shown by line 46.2, to control the frequency of the light emitted by the cw tunable SF coherent-light-source/laser 12 and to analyse the data detected by photodetectors 36 (PD1) and 38 (PD2) that are recorded by the data-acquisition module 42. In use, the cw tunable SF coherent-light-source/laser 12 emits light of which the frequency varies over a range of optical frequencies during each swept-frequency (SF) cycle. The light from the cw tunable SF coherent-light-source/laser 12, which has a frequency that varies continuously, is transmitted via the optical fibres 28 and 30 and the optical circulator 32 to the ringdown-cavity measuring cell 14 via the focusing lens system 34 and the inlet window 14.1. A portion of the emitted SF coherent-light-source/laser light then enters the optical cavity 18 where it is reflected between the highly reflective mirrors 16.1 and 16.2 a great number of times. The distance between the two mirrors 16.1 and 16.2 may be held constant while the SF coherent-light- source/laser 12 emits light of varying frequency during each SF cycle. Alternatively, it may be varied continuously. Each time in a SF cycle that the SF coherent-light-source/laser 12 emits light having a frequency or wavelength that is resonant within the optical cavity, cavity-ringdown light/radiation builds up rapidly inside the optical cavity 18 and then (as the frequency or wavelength is swept out of resonance with the cavity) decays away relatively slowly. Cavity- ringdown light emerges through the mirrors 14.1 and 14.2. The characteristic ringdown time τ observed when a chemical species is present within the sample 24 varies relative to the ringdown time for an empty ringdown-cavity measuring cell 14, because intracavity light of a particular frequency or wavelength is absorbed by the chemical species. The forward-propagating cavity- ringdown light emerging through the mirror 14.2 is detected by the photodetector 38 (PD2). This light has the profile of the signal shown in Figure 2(a). Cavity-ringdown light also emerges in the backward-propagating direction through the mirror 14.1 to the optical circulator 32, where it is combined with incident SF coherent-light- source/laser light that has been back-reflected off the mirror 16.1, towards the optical circulator 32. The optical circulator 32 transmits the combined light via path 32.1 to the photodetector 36 (PD1). These two backward co-propagating beams form a combined beam with oscillatory characteristics suitable for optical-heterodyne detection. This combined beam emerging from the optical circulator 32 via path 32.1 is detected by a square-law photodetector 36 (PD1) to yield an optical-heterodyne-detected (OHD) signal as is shown in Figure 2(b). An optical electric field may be associated with each of the various light paths in Figure 1 , at various points in the system. Thus, a forward-propagating optical electric field EL may be associated with the coherent light emitted, in use, by the cw tunable SF coherent-light-source/laser 12 through the optical fibres 28 and 30, the optical circulator 32 and the lens system 34, to the ringdown-cavity measuring cell 14. Also, a forward-propagating cavity-ringdown optical electric field EFexp(-t/2τ) maybe associated with the light which, in use, is transmitted through the mirror 16.2 to the photodetector 38 (PD2). Similarly, an optical electric field Ei may be associated with the light which constitutes the back-reflected portion of the field E which, in use, returns from the mirror 16.1 and propagates in a backward direction. The field Ei propagates together with a backward-propagating cavity-ringdown optical electric field EBexp(-t/2τ) via the focusing lens system 34, the optical fibre 30 and the optical circulator 32, via path 32.1 to the photodetector 36 (PD2). The optical electric fields Ei and EBexp(-t/2τ) are detected by photodetector 36 (PD1). The optical fields represented by Ei and EBexp(-t/2τ) vary rapidly with time at their respective optical frequencies. The field EBexp(-t/2τ) decays relatively slowly due to the cavity- ringdown effect, as is shown explicitly. The former (rapid) time-dependences lead to oscillatory interference effects between Ei and EBexp(-t/2τ) as their frequency difference varies; such effects are not shown explicitly in Equation (1) below, but they are evident in Figure 2(b). The latter (slow) time-dependence of EBexp(-t/2τ) leads to ringdown decay, as is shown explicitly in Equation (1) below. The signal (S) of interest is therefore of the form: <S> oc I Ei + EB exp(-t/2τ) |2 o |E,|2 + |EB|2 exp(-t/τ) + 2 Re (E,*.EB) exp(-t/2τ) (1) The first line of Equation (1) indicates that the signal (S) is related to the square of the combined electric fields of the light propagating backwards from the ringdown cavity, which offers the advantage that the signal arising from the combined fields is of a much higher intensity than the electric light field EBexp(-t/2τ) of the backward-propagating cavity-ringdown light, because the amplitude of Ei is much greater than that of EB. The signal term |EB|2exp(-t/τ) is equivalent to the forward-propagating CRD signal term
|EF|2exp(-t/τ) that is detected by the photodetector 38 (PD2). However, the optical-heterodyne- detected signal detected by the photodetector 36 (PD1) is much stronger than the forward- propagating CRD signal that is detected by the photodetector 38 (PD2), because the field Ei is much stronger than EB or EF, and because of the fact, as mentioned above, that the signal (S) is related to the square of the combined electric fields of the light propagating backwards from the ringdown cavity. This causes a significant amplification factor. Moreover, the optical-heterodyne-detected signal detected by the photodetector 36 (PD1) decays much slower (twice as slowly) than the forward-propagating ringdown signal due to EF that is detected by the photodetector 38 (PD2). Because the invention relies on the use of cavity- ringdown times, this increased decay time further enhances detection sensitivity. Furthermore, because of the higher frequency domain of the optical-heterodyne-detected signal, low-frequency technical noise can be removed more efficiently by means of electronic high-pass filtering, should it be considered necessary to do so. The frequencies of the optical fields Ei and EBexp(-t/2τ) differ because the former varies continuously as the optical frequency of the coherent-light-source/laser 12 is swept, whereas the optical frequency of EBexp(-t/2τ) remains at the frequency at which the optical cavity resonates. This optical frequency difference between E] and EBexp(-t/2τ) allows their cross term (Eι*.EB) exp(-t/2τ) in Equation (1) to appear as an optical-heterodyne signal containing both a slowly varying exponential ringdown decay factor exp(-t/2τ) that depends on the ringdown time τ of the optical cavity 18, and more rapidly varying factor related to oscillations associated with the different optical frequencies of Ε\ and EBexp(-t/2τ). As explained above, the optical frequencies of E] and EBexp(-t/2τ) are not shown explicitly in Equation (1). The operation of the embodiment of the system shown in Figure 1 is now described further with reference to Figures 2 and 3. Figures 2(a) to 2(c) show a comparison of three simultaneously recorded signal profiles derived from a system as shown in Figure 1. Figure 2(a) represents the build-up and ringdown- decay signal of the transmitted light in the optical cavity 18, as detected by the photodetector 38 (PD2). Figure 2(b) represents the backward-propagating optical-heterodyne cavity-ringdown signal as detected by the photodetector 36 (PD1). Figure 2(c) represents the demodulated logarithm of the envelope of optical-heterodyne detected cavity-ringdown signal processed by the electronic processing unit 40. The preferred section of data, which is used to extract the cavity- ringdown decay rate τ"1, is indicated by a double-headed arrow for each of the three signal profiles. Signal profiles outside the indicated ranges are either too complicated or dominated by the electronic noise of the detection and processing electronics. The key conditions for the indicative recordings shown in Figures 2(a) to 2(c) are: measured ringdown time τ = 10 μs; cavity transmission bandwidth FWHM (1 / 2πτ) = 16 kHz; frequency sweep rate of the SF laser = 0.15 THz s"1 (or 150 kHz μs"1); laser power incident on the ringdown cavity = 120 μW. The forward-transmitted signal (Figure 2(a)) builds up and peaks just after the exact resonance point (at t = 0 on the abscissa, determined by model simulations) between the optical cavity and the SF laser wavelength. The incident SF laser radiation then moves out of resonance and oscillations set in with their period and depth of modulation decreasing as time delay increases. The modulations are due to optical interferences between the portion of the instantaneous incident SF laser field that enters the optical cavity and the cavity-ringdown field that is built up and stored inside the cavity. The ringdown decay envelope depends on the energy loss-rate of the optical cavity and can be used for CRD measurements. The optical-heterodyne-detected (OHD) cavity-ringdown (CRD) signal (Figure 2(b)) contains information about the amplitudes and relative phase of the optical fields E] and EBexp(- t/2τ) that are monitored by photodetector 36 (PDl in Figure 1) at the SF-induced difference frequency. For useful OHD CRD measurements it is desirable to extract the cavity-ringdown decay rate τ"1 from the signal of photodetector 36 (PDl). In one alternative approach features of optical-heterodyne-detected waveforms such as shown in Figure 2(b) may be fitted to a model- derived function. A more efficient method, capable of implementation in real time while an optical absoφtion spectrum is being recorded, entails pre-processing signals from photodetector 36 (PDl) by electronic circuits that rectify and smooth the oscillatory part of the ringdown decay using a multiplier and low-pass filter combination. Another efficient method employs a demodulating logarithmic amplifier to convert the envelope of the single-exponential ringdown decay curve to a smooth, straight line from which the ringdown time τ can be rapidly and accurately derived. The slope of the straight line is proportional to the exponential decay rate of the signal received by the demodulating logarithmic amplifier and to the characteristic logarithmic slope value of the demodulating logarithmic amplifier. The characteristic logarithmic slope value of the demodulating logarithmic amplifier is usually known or can be predetermined by measuring its outputs with given test input signals. Suitable demodulating logarithmic amplifiers are available commercially, for example, Analog Devices model AD8307 (bandwidth DC- 500- MHz, linearity ±1 dB, dynamic range 92 dB). The latter approach directly converts the exponentially decaying full-wave envelope of optical-heterodyne oscillations, as depicted in Figure 2(b), into a smooth linear decay, the slope of which provides a means of measuring the ringdown time (τ). Moreover, the use of optical-heterodyne detection and demodulating logarithmic amplifier advantageously preserves the two-fold slower cavity-ringdown decay rate (2τ)"' of the full-wave envelope in Figures 2(b), relative to the ringdown decay rate τ'1 of the directly transmitted ringdown signal of Figure 2(a). The double-headed arrows in Figures 2(a), 2(b) and 2(c) indicate qualitatively the time ranges over which reliable values of ringdown time can be measured. Extracting the decay rate of an exponential profile or the slope of a straight line is general knowledge in the field of data modelling and numerical analysis. More information on this topic could be found in the book: W. H. Press, S. A. Teukolsky, W. T. Vetterling, and B. P. Flannery,
Numerical Recipes in Fortran 77: The Art of Scientific Computing (Cambridge University Press,
2nd edition, 1992). The peak amplitude of the transmitted CRD signal in Figure 2(a), measured just after t = 0 on the abscissa, may be used optionally for swept-frequency (SF) cavity-enhanced absoφtion spectroscopy (CEAS), given that the peak amplitude decreases as optical absoφtion by the sample in the cavity increases and/or the frequency-sweep rate increases. The peak-to-peak amplitude of the OHD CRD signal in Figure 2(b), measured just after t = 0 on the abscissa, may also be used optionally for SF CEAS measurements, given that this peak-to-peak amplitude decreases as optical absoφtion by the sample in the cavity increases and/or the frequency-sweep rate increases. Detection of the peak amplitude of transmitted CRD signals or peak-to-peak amplitude of OHD CRD signals, as mentioned above, may be performed by conventional analog or digital circuitry in the processing unit 40 or in the data-acquisition unit 42 or in the processing- and-control unit 44 shown in Figure 1. The integrated signal profile for build-up and decay of the transmitted CRD signal in Figure 2(a) may be used optionally for swept-frequency (SF) integrated cavity output spectroscopy (ICOS), given that this integrated signal profile decreases as optical absoφtion by the sample in the cavity increases and/or the frequency-sweep rate increases. The rectified and integrated signal profile for build-up and decay of the full-wave envelope of the OHD CRD signal in Figure 2(b) may also be used optionally for SF ICOS measurements, given that this integrated quantity decreases as optical absoφtion by the sample in the cavity increases and/or the frequency-sweep rate increases. The integrated signal profile for the demodulated logarithm of build-up and decay of the full-wave envelope of the OHD CRD signal in Figure 2(c) may also be used optionally for SF ICOS measurements, given that this integrated quantity decreases as optical absoφtion by the sample in the cavity increases and/or the frequency-sweep rate increases. Integration of transmitted CRD signals or rectification and integration of OHD CRD signals, as mentioned above, may be performed by conventional analog or digital circuitry in the processing unit 40 or in the data-acquisition unit 42 or in the processing-and-control unit 44 shown in Figure 1. The specification of this invention shall be taken to include any quantity or function derived from SF CRD signal profiles, as depicted actually in Figure 2 and schematically in Figures 3(a)- (c) and Figures 7(a)-(c), in addition to those designated above as derived via SF CEAS or SF ICOS. These optional SF CEAS and SF ICOS approaches may be useful for the puφose of recording survey spectra over a broad range of coherent-light-source/laser wavelengths. They may efficiently generate a characteristic spectroscopic signature for a given molecule or mixture of molecules for the sample contained within the optical cavity. Relative to SF CRD approaches, the SF CEAS and SF ICOS approaches may require fewer data points and less detail of the ringdown-decay profile to be recorded for a given spectral range. However, the SF CEAS and SF ICOS approaches are typically less sensitive than SF CRD approaches. Figures 3(a) to 3(g) show the various steps in a method of obtaining a cavity ringdown spectrum by means of the system depicted in Figure 1. The abscissas of Figures 3(a) to 3(g) are correlated to each other, as indicated by the elapsed times in Figure 3(g). In Figure 3(a), the electronically detected profile of an optical signal reflected back from the optical cavity 18 is shown. The signal represents the reflection that may be obtained during a single SF cycle. The amplitude of the signal increases at discrete resonance frequencies with the optical cavity 18, as the tunable SF coherent-light-source/laser 12 conducts a frequency sweep. The amplitude increases each time the optical cavity 18 resonates with incident light from the cw tunable SF coherent-light-source/laser 12 when its optical cavity length is an integer multiple of the wavelength (inversely proportional to frequency) of the input light. The corresponding discrete resonance frequencies of the longitudinal modes of the two-mirror optical cavity are: Vi = Nι c / (2 n D), (2) where Ni is an integer number (Ni, N2, N3, ...), c is the speed of light in vacuum, D is the distance between the two cavity mirrors, and n is the refractive index of the sample inside the cavity. The distance between successive resonance frequencies is called the free-spectral range
(FSR) of the cavity: FSR = c / (2 n D), (3) The frequency bandwidth of the discrete cavity resonance, defined as full width at half maximum (FWHM) of these constructive resonant interferences, is given by: FWHM = 1 / (2 π τ), (4) where π * 3.1415926 and τ is the cavity-ringdown time. The FWHM is typically 1 kHz to 1 MHz. Incident light from the SF coherent-light-source/laser 12 has a frequency, v(t) as a function of time t, that is swept continuously and monotonically, such that it moves into a first resonance with the optical cavity 18 at time tl when its instantaneous frequency is vl , v(tl) = vl = N, [c / (2 n D)] . (5) Some SF coherent-light-source/laser output light of frequency around vl ± FWHM may be coupled into the optical cavity 18 by means of constructive-interference interaction within the cavity 18. As the frequency v(t) of cw tunable SF coherent-light-source/laser 12 is swept away from vl (i.e., with cavity-resonance frequencies numbered from the start of the frequency sweep), this constructive-interference interaction stops and incident light is blocked effectively by the highly-reflective front mirror 16.1 of the optical cavity 18 for a period until the coherent-light- source/laser frequency eventually moves into coincidence with the next cavity-resonance frequency (v2). During the period immediately after time tl, light that has been stored inside the optical cavity 18 during the resonance interaction continues to leak from the optical cavity 18 via both cavity mirrors (16.1, 16.2). Accordingly, light at the fixed cavity-resonant frequency vl decays from the optical cavity 18. The decaying cavity-ringdown light of fixed frequency vl is combined, in the backward-propagating direction during the period immediately after time tl, with a portion of incident SF coherent-light-source/laser light that is reflected collinearly off the front mirror 16.1 of the cavity. The frequency of the reflected SF coherent-light-source/laser light, which varies continuously during the period immediately after time tl, beats against the fixed frequency vl of the cavity-ringdown light. Combination of these two backward-propagating light waves generates the optical-heterodyne-detected (OHD) signal having a continuously varying difference (beat) frequency that is determined by the sweep rate of the SF coherent-light- source/laser frequency. An alternative description of this OHD signal generation is to regard the entire optical cavity 18 as an active reflector that interacts and responds dynamically when the frequency of the incident light sweeps across its resonance frequency. In either of these descriptions, the ringdown decay of the OHD signal amplitude gives a measure of the ringdown decay characteristics of the cavity. The intensity of the decaying cavity-ringdown light of frequency vl decays to a negligibly weak level in a time interval usually shorter than the time interval that the cw tunable SF coherent-light-source/laser 12 takes to sweep from one cavity resonance frequency (e.g., vl at time tl) to the next (e.g., v2 at time t2). This process is then repeated at v2 immediately after t2, then at v3 immediately after t3, and so on. This is effectively a multi-point sampling process. In Figure 3(b), the demodulated logarithm of the envelope of the signal reflected back from the optical cavity 18 is shown. In Figure 3(c), the profile of the optical signal transmitted through the optical cavity 18, as detected by the photodetector 38, is shown. Figures 3(a), 3(b) and 3(c) contain the same cavity-ringdown information. Each of the build-up and ringdown-decay events may be analysed, as discussed in the context of Figure 2, to derive the cavity-ringdown time information. The ringdown times of some of the ringdown events, at different resonance frequencies vO, vl, v2 and so forth, are labelled in Figure 3(c) as τO, τl, τ2, and so forth, respectively. Referring to Figure 3(d), there is shown a frequency sweep of the SF coherent-light- source/laser 12. Its frequency sweep starts at v-start and changes monotonically across the cavity resonance frequencies at vO, vl, and so forth, as indicated with black dots. The intensity of the SF coherent-light-source/laser 12 preferably needs to be stable as its optical frequency is swept. A stability of better than 90% is preferred, and a stability of better than 99% is most preferred. As can be seen in Figure 3(a), 3(b) and 3(c), the decay of each resonant ringdown profile is complete before the SF coherent-light-source/laser frequency has been swept to the next consecutive resonance frequency. Figure 3(e) is a control signal indicating the frequency sweep status. This control signal is derived from the control electronics (not shown) of the coherent-light-source/laser 12 which is used for the operation of its output frequency. The data acquisition is synchronised by this control signal. Figure 3(f) depicts a typical cumulative number of data points (0 - 3000 data points) recorded by the data-acquisition module 42 (as shown in Figure 1). Figure 3(b) and/or Figure 3(c) are usually recorded by the data-acquisition module 42 (as shown in Figure 1) for further analysis of cavity-ringdown decay rates at each of the build-up and ringdown-decay events, as discussed in the context of Figure 2, to derive the cavity-ringdown time information. As in the example shown in Figure 3(f), data points 0 to 499 contain the details for the first build-up and ringdown-decay event at the cavity resonance frequency vO; data points 500 to 999 contain the details for the second build-up and ringdown-decay event at the cavity resonance frequency vl; data points 1000 to 1499 contain the details for the third build-up and ringdown-decay event at the cavity resonance frequency v2; and so forth. Each of these data sections represents a build-up and ringdown-decay event. Each of these build-up and ringdown-decay events can be analysed, as discussed in the context of Figure 2, to derive the cavity-ringdown time information τO, τl, τ2 and so forth, at their corresponding cavity frequencies vO, vl, v2 and so forth, respectively. Figure 3(g) shows a typical elapsed time scale of the SF cycle (0- 180 μs). Generally speaking, a fast data sampling rate and a large record memory size of data- acquisition module 42 are advantageous to record detailed build-up and ringdown-decay events in large numbers in a single SF cycle. The frequency range of an SF cycle may conveniently be set to match either the frequency range of interest or the recording capacity of the data-acquisition module 42, or the frequency tuning ability of the SF coherent-light-source/laser 12. Alternative data acquisition modes may be employed to record data. For example, a data acquisition module may be set firstly to a "ready" mode by the Frequency Sweep Status signal (Figure 3(e)), and again each time when a start of the build-up event is detected by monitoring the outputs of photodetector 36 or 38. The data acquisition module 42 will then record only for a preset period of time. In this way, much of the time interval of the empty signal is omitted. An advantage of this method is that the signal-averaging calculations to be carried out by the data acquisition module 42 will not be affected by a low sweep-to-sweep frequency reproducibility of the SF coherent-light-source/laser 12. Figure 4 depicts a cavity-ringdown spectrum obtained by using the embodiment of the system shown in Figure 1. It may also be obtained by using any of the embodiments shown in any one of Figures 6, 8, 9, 10 and 1 1, applying the method as described with reference to Figure 2 and Figure 3. The black dots in Figure 4 show the cavity-ringdown decay rate of the cavity 18 (with rate being inversely proportional to time) at discrete cavity resonance-frequency values while the dashed line indicates the underlying continuous spectrum profile. This is achieved in practice by arranging for the SF coherent-light-source/laser 12 to conduct a SF cycle spanning a broad frequency range over many free spectral ranges of the ringdown cavity, as shown in Figure 3, and to generate and analyse the cavity-ringdown (CRD) profiles detected in optical-heterodyne- detected (OHD) mode by the photodetector 36 (PDl) or (alternatively but less advantageously in non-OHD mode) by the photodetector 38 (PD2), usually while holding the optical distance between the mirrors 16.1 and 16.2 constant during a frequency sweep over the absoφtion spectrum of the sample 24. Each of the build-up and ringdown-decay events may be analysed, as described in the context of Figure 2, to derive the cavity-ringdown time information. This generates a plot of cavity-ringdown decay rate (τ"1) against optical resonance frequency of the cavity, as in Figure 4, where a series of data points with cavity-ringdown decay rates 1/τl , l/τ2, l/τ3 and so forth are plotted at their corresponding optical frequencies vl, v2, v3 and so forth, respectively, during the SF cycle. Such a plot comprises a coarsely resolved optical absoφtion spectrum, which is 'an optical characteristic of the sample 24 in the ringdown cavity, the amplitude of the spectrum being dependent on the concentration of a substance in the sample 24. The optical frequency corresponding to each build-up and ringdown-decay event can be derived from the start frequency, v-start, that has been set for the SF cycle, the frequency change rate or the characteristics of the sweep, and the elapsed time or corresponding recorded data point position since the start of the SF cycle. If necessary, the sensitivity in determining the ringdown-time values may be improved by signal-averaging over successive frequency-sweeps. In addition or alternatively, the spectrum range of the recorded spectrum may be extended by repeating the measurement for additional wavelength ranges. Where it is desirable to do so, the frequency resolution of the spectrum could be improved by altering the resonance frequencies of the ringdown-cavity resonance frequencies by changing the optical length of the cavity cell, as will be explained below in the context of Figure 5. Figure 5 shows a combination of three sets of coarsely resolved CRD spectra to improve the frequency resolution. This comprises three series of data points, one with ringdown times τl, τ2, τ3 and so forth for one cavity length and another with ringdown times τ'l, τ'2, τ'3 and so forth for another cavity length and another with ringdown times τ"l , τ"2, τ"3 and so forth for another cavity length. These are sampled during each SF cycle at optical frequencies vl , v2, v3 and so forth and v'l, v'2, v'3 and so forth and v"l, v"2, v"3 and so forth, respectively. The resulting optical absoφtion spectrum, which is characteristic of the sample in the ringdown cavity and its concentration, is more finely resolved than the spectrum in Figure 4. For the sake of simplicity, Figure 5 shows only three sets of coarsely resolved CRD data points. Further increases of resolution can be achieved by repeating this process for additional successive incremental or decremental changes of the optical length of the ringdown cavity Additional sets of cavity- ringdown decay rate data points may thus be obtained, at additional sets of cavity-resonant optical frequencies. The ISCL procedure may be used. Similar outcomes may be achieved by continuously scanning the cavity length, using the SSCL procedure as described above. One set of data points from the spectrum, displayed in Figure 5(a) as black dots, is measured at one cavity length; the second set of data points from the spectrum, displayed in Figure 5(b) as open squares, is measured at an altered cavity length; and a third set of data points from the spectrum, displayed in Figure 5(c) as open circles, is measured at another altered cavity length. The frequency points of measured coarsely resolved CRD spectra are determined at the cavity resonance frequencies, which in turn depend on the length of the optical cavity 18 (or distance between the mirrors 16.1, 16.2). Once a set of coarsely resolved CRD spectrum data points is satisfactorily recorded, the ringdown-cavity resonance frequencies are altered incrementally or decrementally by changing the length of the optical cavity 18, namely, by stepping the separation between the mirrors 16.1 , 16.2 using the PZT device 22. By repeating the measurement of coarsely resolved CRD spectra at altered cavity lengths, CRD spectra at more frequency points may be obtained. Figure 5(d) shows a composite spectrum, comprising the two spectra as shown in Figure 5(a) and Figure 5(b). Figure 5(e) shows a composite spectrum, comprising the three spectra as shown in Figure 5(a), Figure 5(b) and Figure 5(c). The change in the distance between the mirrors may be made in accordance with the incrementally (or decrementally) stepped cavity-length (ISCL) procedure that has been described in the "Summary of the Invention" above. Likewise, the distance between the mirrors may be continuously scanned, using the SSCL procedure that has also been described in the "Summary of the Invention" above. The effect of a change in the distance between the two mirrors can be derived by considering Equations (2) and (3). Equation (2) can alternatively be expressed in terms of vacuum wavelength λ (which is inversely proportional to frequency v) as follows: λ, = 2 D / NI ; (6) where, as in Equation (2), N] is an integer number (N1; N2, N3, ...) and D is the distance between the two cavity mirrors. The resonance frequencies of the cavity are periodic and the separation between successive resonance frequencies corresponds to FSR, as in Equation (3). A small change in cavity length shifts the structure of resonance frequencies of the cavity. A change of the cavity length by one half of the wavelength of the coherent-light-source/laser light shifts the resonance frequency by one FSR interval and reproduces the same structure of resonance frequencies of the cavity. Therefore, the cavity length change needs only be of the order of magnitude of the wavelength of the light used. For example, a two-mirror cavity with an optical separation of 50 cm has an FSR of about 300 MHz. When working with a SF coherent-light-source/laser operating around 1550 nm wavelength, a cavity length change of 155 nm, i.e., a tenth of the wavelength of 1550 nm, will shift each of the resonance frequencies of the cavity by about 60 MHz. Likewise, a cavity length change of 310 nm will shift each of the resonance frequencies of the cavity by about 120 MHz. A cavity length change of 775 nm, i.e., half of the wavelength, will shift each of the resonance frequencies of the cavity by 300 MHz, i.e., one FSR; overall, the optical cavity will then produce the same resonance frequencies, with each of them being shifted by the aforementioned amount. Figure 6 shows an embodiment of a system 610 in accordance with the invention, as an alternative to that depicted in Figure 1. Like reference numerals are used to designate like components or features. Two ways in which the embodiment of the system shown in Figure 6 differs from that shown in Figure 1 are that it does not include an optical circulator 32 or optical fibres 28, 30. The optical detection system 610 comprises a coherent light source in the form of cw tunable swept-frequency (SF) coherent-light-source/laser 612 that emits coherent light of which the optical frequency can be varied over a range from about 1 MHz to many GHz or THz. The cw tunable SF coherent-light-source/laser 612 is optically coupled to an optical resonator system in the form of a ringdown-cavity measuring cell 614. The embodiment of the system shown in Figure 6 also differs from the system shown in Figure 1 in that the optical cavity 618 of the system shown in Figure 6 is formed by four mirrors 616.1, 616.2, 616.3 and 616.4 aligned with respect to an optical axis 620 in a bow-tie or ring configuration. Comparable ring-cavity designs using three (e.g., such as adopted in US Patent 5,912,790), five or more mirrors are also feasible, and it is to be understood that such cavity designs are included in the scope of this invention. The surfaces of the mirrors 616.1, 616.2, 616.3 and 616.4 that face one another are highly reflective and are concave, each mirror 16.1, 16.2, 616.3 and 616.4 typically having a radius of curvature of 1 m. The distance between the mirrors 16.1 and 16.4, along the optical path 620 is initially set at about 0.5 m, but it may be varied by an optical-length adjuster in the form of a piezoelectric translator (PZT) 622, that is attached to the mirror 612.3, for moving the mirror 616.3, relative to the mirrors 616.1, 616.2 and 616.4. The four cavity mirrors 616.1 - 616.4 are arranged in such a way that the incident light is close to normal incidence, in order to minimise any polarisation dependence of the mirror reflectivity. The ringdown-cavity measuring cell 614 and optical cavity 618 are designed to operate according to the cavity-ringdown principle. In use, a gaseous sample 624 is introduced into the ringdown-cavity measuring cell 614 by means of a sample-handling system 626, so that it constitutes an optical medium in the optical cavity 618. The optical system 610 is designed to detect the presence of trace chemical species in the sample 624. The sample 624 is directed into the ringdown-cavity measuring cell 614 by sample handling means 626. Two transparent windows 614.1 and 614.2 are provided in opposite ends of the wall of the ringdown-cavity measuring cell 614, so as to allow coherent light to enter and leave the measuring cell 614. The ringdown-cavity measuring cell 614 is optically coupled to the cw tunable SF coherent- light-source/laser 612 via a set of mirrors 629.1, 629.2. A lens 634 is provided in the optical path of the light being emitted by the cw tunable SF coherent-light-source/laser 612 towards the ringdown-cavity measuring cell 614, to collect and direct such light into the optical cavity 618 of the ringdown-cavity measuring cell 614. In view of the bow-tie arrangement of the mirrors, there is no significant backward- propagating light of interest emerging from the ringdown-cavity measuring cell 614. An output from the ringdown-cavity measuring cell is extracted through the mirror 616.1, combined with the reflected incident SF coherent-light-source/laser light from the mirror 616.1 of the ringdown- cavity measuring cell 614 and directed towards a photodetector 636 (also designated as PDl in Figure 6). Residual backward-propagating light emerging from the ringdown-cavity measuring cell towards the cw tunable SF coherent-light-source/laser 612 is blocked by the optical isolator 631, which serves to protect the SF coherent-light-source/laser 612 from interruption or damage by such returned light. An additional optical isolator (not shown in Figure 6) may be integrated into the SF coherent-light-source/laser 612, to provide further suppression of back-reflected light reaching the SF coherent-light-source/laser 612. Another optical detector in the form of a photodetector 638 (PD2) is provided on the remote side of the ringdown-cavity measuring cell 614, to detect light that has passed through the optical cavity 618, the mirror 616.2 and the window 614.2. The photodetectors 636 (PDl) and 638 (PD2) detect light by converting it into corresponding electrical signals. The photodetector 636 (PDl) converts the combined beam from the optical circulator 632 into a' corresponding electrical signal. This beam has oscillatory characteristics suitable for optical-heterodyne detection. The output of the photodetector 636
(PDl) is processed by an electronic circuit 667 to obtain the demodulated logarithm of the envelope of the oscillatory signal. An electronic data-acquisition module 642 is provided to collect and store the output signal of the electronic circuit 667 and of the photodetector 638 (PD2). A suitable data-acquisition module 642 may be a digital oscilloscope or an analog-to-digital converter. A data-processing and computer control unit 644.2 is coupled to the data-acquisition module 642 (not shown separately). The control electronics 640 are connected, via a connection 668, to the piezoelectric translator 622 to enable controlled incremental or continuous movement of the mirror 616.3. The processing-and-control unit 644.2 is also coupled to the SF coherent-light-source/laser 612, as is shown by line 670, to control the frequency of the light emitted by the cw tunable SF coherent-light-source/laser 612 and to analyse the data detected by photodetectors 636 (PDl) and 638 (PD2) that are recorded by the data-acquisition module 642. The embodiment of the system shown in Figure 6 also differs from the embodiment of the system shown in Figure 1 in that a reference etalon 650 and a reference gas cell 652 are used to calibrate the output frequency of the cw tunable SF coherent-light-source/laser 612. The cavity mirrors 616.1 , 616.2, 616.3 and 616.4 are highly reflective dielectric concave mirrors and each has a radius of curvature of 1 m. The optical path-length of the cavity and therefore its resonance frequencies can be adjusted by the piezoelectrically controlled mirror
616.3. The resonance frequencies of the cavity may be altered by varying the cavity length either in controlled discrete steps or by continuous scanning. The sample 624 is directed into the chamber 614 by a sample handling unit 626. Two transparent windows 614.1 and 614-.2 allow light to enter and leave the ringdown-cavity measuring cell 614. • The output of the cw tunable SF 612 coherent-light-source/laser passes through an optical isolator 631, a beam splitter 633, a pair of steering mirrors 629.1, 629.2 to change the direction of the light emitted by the coherent-light-source/laser 612, a focusing lens (or lens system) 634 and the window 614.1 before entering the ringdown-cavity measuring cell 614. The light reflected by the mirror 616.1 and the cavity-ringdown light emitted through the same mirror 616.1 is directed by mirrors 635 and 637, and focused by a lens 639 to the photodetector 636 (PDl). The output of the photodetector 636 (PDl) is processed by an electronic circuit 640 to obtain the demodulated logarithm of the envelope of the oscillating signal. Another optical detector in the form of photodetector 638 (PD2) is provided. It is in optical communication with light emitted through the mirror 616.2 and the window 614.2, such light being reflected by a mirror 641 through a further focusing lens 643 on to the photodetector 638. A portion of the output of the SF coherent-light-source/laser 612 is taken from the beam splitter 633 and directed into another beam 645 by a mirror 647. A portion of the beam 645 is taken from a second beam splitter 649 and is coupled by a lens 651 to the reference etalon 650. Light transmitted through the etalon is collected by a lens 653 and is detected by a photodetector 654 (PD3). To ensure that this etalon provides a reliable, constant reference calibration for the frequency or wavelength of the coherent-light-source/laser, the etalon assembly may be rigidly constructed of materials with low coefficient of thermal expansion; if necessary, the etalon unit may also be pressure- and temperature-stabilised and actively locked to a stabilised reference laser. The remaining part of the beam 645 passing through the second beam splitter 649 is coupled by a mirror 656 and a lens 658 through the reference gas cell 652. Light transmitted through the reference gas cell 652 is collected by a lens 660 and is detected by a photodetector
662 (PD4). The reference gas cell 652 contains a suitable gas or a gas mixture that has optical absoφtion features at known wavelengths of light, for calibration puφoses. A module 664 comprises the control electronics 640, the electronic data acquisition module 642, the data processing control electronics 644.1 and the processing-and-control unit 644.2. The electronic data acquisition module 642 collects and processes the output signals from photodetector 636 (PDl) via a logarithmic amplifier 667 (which converts the envelope of the oscillatory output signal from the photodetector 636 into a demodulated logarithmic signal), from photodetector 638 (PD2), from photodetector 654 (PD3), and from photodetector 662 (PD4). The processing-and-control unit 644.2 is coupled to the data-acquisition module 642 and the control electronics 640 to enable controlled incremental or continuous movement of the mirror 616.3 as shown by line 668. The module 664 is also coupled to the SF coherent-light-source/laser 2 as shown by line 670 to control the optical frequency or wavelength of the light emitted by the cw tunable SF coherent-light-source/laser 612. The operation of this embodiment of the invention is described further below, with reference to Figures 7(a) to 7(i). Figure 7 depicts the various signals and sequences associated with the embodiment of the system as shown in Figure 6. This figure differs from Figure 3 in that it includes two additional traces: Figure 7(h) depicting a signal derived from the reference etalon 650 as detected in, or derived from, the signal detected in the photodetector 654 (PD3) and Figure 7(i) depicting a signal derived from the reference gas cell 652 as detected in or derived from the photodetector 662 (PD4). Where appropriate, similar traces are assigned the same reference numerals. The abscissas of Figures 7(a) to 7(i) are correlated to each other, as indicated by the elapsed times in Figure
7(g). In use, the cw tunable SF coherent-light-source/laser 612 emits light over a range of optical frequencies during a swept-frequency (SF) cycle. The intensity of the SF coherent-light- source/laser 612 preferably needs to be stable as its optical frequency is swept. A stability of better than 90% is preferred, and a stability of better than 99% is most preferred. Radiation in the cavity 618 builds up and decays at successive resonance frequencies as indicated in Figures 7(a), 7(b) and 7(c). The speed of the frequency sweep is chosen so that the decay of each resonant ringdown profile is complete before the SF coherent-light-source/laser frequency has advanced to the next resonant frequency. The cw tunable SF coherent-light-source/laser 612 emits light over a continuously varying range of optical frequencies during each swept-frequency (SF) cycle, as is indicated in Figure 7(d). The resonance frequencies vl, v2, v3, v4, v5, etc., are also indicated by arrows and black dots (•) in Figure 7(d). Open circles (o) also show the position of calibration frequency markers, e.g., as generated by the reference etalon 650 and photodetector 654 (PD4) and depicted in Figure 7(h). The status of the start and end of a sweep cycle is indicated by an electronic logic signal as shown in Figure 7(e). This frequency sweep status signal may be provided by control electronics associated with the cw tunable SF coherent-light-source/laser 612 or it may be derived from a parameter associated with its operation. Figure 7(f) depicts a typical cumulative number of data points (0 - 3000 data points) recorded by the data-acquisition module 642 (as shown in Figure 6). Figure 7(b) and/or Figure
7(c), Figure 7(h), and Figure 7(i) are usually recorded by the data-acquisition module 642 (as shown in Figure 6) for further analysis of cavity-ringdown-decay rates at each of the build-up and ringdown-decay events, as discussed in the context of Figure 2, to derive the cavity-ringdown- time information. As in the example shown in Figure 7(f), data points 200 to 699 contain the details for the build-up and ringdown-decay event at the cavity resonance frequency vl; data points 700 to 1199 contain the details for the next build-up and ringdown-decay event at the cavity resonance frequency v2; data points 1200 to 1699 contain the details for the build-up and ringdown-decay event at the cavity resonance frequency v3; and so forth. Each of these data sections represents a build-up and ringdown-decay event. Each of these build-up and ringdown- decay events can be analysed, as discussed in the context of Figure 2, to derive the cavity- ringdown-time information τl, τ2, τ3 and so forth, at their corresponding cavity frequencies vl, v2, v3 and so forth, respectively. Figure 7(f) uses black dots (•) and open circles (o) to show data points recorded at the corresponding cavity resonance frequencies and frequency-marker positions of the reference etalon, respectively, as previously depicted in Figure 7(d). The elapsed time is shown in Figure
7(g)- Figure 7(h) shows the frequency markers of the reference etalon, used for frequency calibration puφoses. The separation between the frequencies of successive frequency markers is chosen to be larger than the frequency setting uncertainty of the SF coherent-light-source/laser 612. The start frequency v-start of the SF coherent-light-source/laser 612 is usually set in the middle between two frequency markers of the reference etalon 650, in such a way that the first frequency marker generated after the start of the sweep is uniquely defined. The frequency markers of the reference etalon 650 provide the information needed to calibrate the frequency change of the output of the SF coherent-light-source/laser 612. Figure 7(i) shows the transmission of radiation from the SF coherent-light-source/laser 612 through the reference gas cell 652, as registered by photodetector 662 (PD4). The known absoφtion features of the reference gas provide the absolute frequency of the SF coherent-light- source/laser output at the position where such absoφtion occurs. Absolute frequency information for the coherent-light-source/laser from Figure 7(h) and relative frequency change information from Figure 7(i) together provide an independent frequency calibration for the frequency of the SF coherent-light-source/laser output. The start of the data acquisition can be triggered by the frequency sweep status signal in Figure 7(e) at the beginning of the SF sweep cycle, or by a frequency marker after the start of the SF sweep, or it can be based on a transmission feature of the reference gas cell 652. In one embodiment of the system, in which the frequency sweep dynamics of the system are reproducible from sweep to sweep, the data acquisition electronics unit 642 comprises a digital oscilloscope (not shown explicitly in Figure 6), to determine the average of the signal so as to improve the signal-to-noise ratio and, therefore, the detection sensitivity of the system 610. In this embodiment, the fluctuation of the preferred trigger event from sweep-to-sweep conveniently does not exceed a few MHz. It preferably does not exceed a few hundred kHz. Figure 7(f) shows how the first frequency marker after the start of the SF sweep may be used as a trigger event for data acquisition. The cumulative number of recorded data points as indicated in Figure 7(f) may vary. The elapsed time as indicated in Figure 7(g) may also vary. The data point positions when the SF coherent-light-source/laser moves into resonance with the cavity are indicated by black dots (•) in Figure 7(f). The data point positions when the SF coherent-light-source/laser moves into resonance with the reference etalon are indicated by open circles (o) in Figure 7(f). A cavity ringdown (CRD) spectrum may be obtained in OHD mode, the spectrum being based on a signal derived from the light detected by the photodetector 636 (PDl). Alternatively, in non-OHD mode, the spectrum may be based on a signal derived from the light detected by the photodetector 638 (PD2). The wavelength range of a single recorded spectrum may be extended by operating the SF coherent-light-source/laser 612 at different wavelength ranges and by combining the spectra recorded at those different wavelength ranges. Once a given set of coarsely resolved CRD profiles has been satisfactorily recorded, the length of the optical-cavity 618 is varied to record a different set of CRD profiles at shifted cavity-resonance frequency points. This allows a series of such coarsely resolved CRD profiles, each signal being averaged if necessary, to be extracted from successive SF cycles and to be combined to construct a composite, more finely resolved, CRD absoφtion spectrum. The ringdown-cavity resonance frequencies are varied incrementally or decrementally by changing the optical length of the optical cavity 618 of the ringdown-cavity measuring cell 614, namely, by using the PZT device 622 to change the position of the cavity mirror 616.3 by a desired distance. The amount of the frequency shift can be determined by several means, namely: (l)by measuring the change of voltage applied to the PZT device 622 and converting such voltage to displacement using calibration data; or (2) by relating cavity-resonance positions, each associated with a cavity build-up event as shown in Figure 7(c), to reference etalon markers, as in Figure 7(h); or (3) by relating cavity-resonance positions, each associated with a cavity build-up event as shown in Figure 7(c), to recorded data points, as in Figure 7(f). Alternatively, the resonance frequencies of the cavity may be varied by slow, continuous scanning of the cavity length. Figure 8 shows an embodiment 810 of the detection system of the invention, as an alternative to that depicted in Figure 1. The system shown in Figure 8 differs from that shown in Figure 1 in that it does not include an optical circulator 32 or optical-fibre coupling 28, 30. In the embodiment shown in Figure 8, the optical path of the emitted light from the cw tunable SF coherent-light-source/laser 812 (shown in solid lines with arrows) to the ringdown-cavity measuring cell 814 passes through an optical isolator 831, a reflector 829, a polarising beam splitter 870, a polarisation control unit 872, and a lens 834. A reflector 841 directs forward- propagating light transmitted by the ringdown-cavity measuring cell 814 to a photodetector 838 (PD2), which is optional. In this embodiment, a data-acquisition module (not shown separately) is integrated into the control electronics unit 864. An optical isolator 831 is provided, in the optical path of the coherent light emitted from the coherent-light-source/laser 812, to suppress residual back-reflected light from the ringdown-cavity measuring cell 814 that could otherwise interrupt or damage the SF coherent-light-source/laser 812. If the SF coherent-light-source/laser 812 is not sensitive to residual back-reflected light, the optical isolator 831 may be omitted. The orientation of the polarising beam splitter 870 is important. It should be aligned with the polarisation control unit 872 so as to facilitate efficient transmission of the linearly polarised SF coherent-light-source/laser light. The polarisation control unit 872 is a magneto-optical Faraday rotator that rotates the plane of polarisation of the light passing through it by 45 degrees and enables the backward-propagating beam to be directed efficiently to the photodetector 836 (PDl) via the polarising beam splitter 870. The polarising beam splitter 870 and the polarisation control unit 872 together also serve as an additional optical isolator to protect the SF coherent-light-source/laser 812. As an alternative to the beam splitter 870 and polarisation control unit 872, a simple partially reflecting beam splitter may be used, directing a light beam to the photodetector 836 (PDl). This option would be simpler and cheaper, but would make less efficient use of the available emitted SF coherent-light-source/laser light and would therefore provide less optical isolation than the alternative shown in Figure 8. Figure 9 shows an embodiment 910 of the detection system of the invention, as an alternative to that shown in Figure 8. Where appropriate, corresponding reference numerals are used to designate corresponding features or components of the system 910. In the system shown in Figure 9, components corresponding to the polarising beam splitter 870 and the polarisation control unit 872 (which form part of the embodiment of the system shown in Figure 8) are not required. However, an additional beam splitter 970 directs a portion of light emitted by the SF coherent-light-source/laser 912 along a second optical path 972 to a second beam splitter 974, where it is combined with cavity-ringdown light emerging from the mirror 916.2 and directed to photodetector 938 by the reflector 941. The OHD CRD signal may optionally be detected at any one of several locations by any one of the photodetectors 938, 976 and 978. These photodetectors effectively take the place of photodetectors 36, 636 and 836 (PDl) that appear in Figures 1, 6 and 8, respectively. Photodetectors 978 or 938 may also perform the non-OHD CRD-spectroscopic function of the previously displayed photodetector 38, 638, 838 (PD2) by blocking the light path 972 between the reflectors 970 and 974 by a beam flag (not shown in Figure 9). For applications using high-power cw tunable coherent light sources or lasers as the cw coherent-light-source/laser 912, the embodiment 910 is a suitable choice, owing to its simplicity. Figure 10 shows an integrated system 1010 that comprises a ringdown-cavity module 1011 comprising a ringdown-cavity measuring cell 1014 and an optical transmitter-receiver module 1013 comprising a set of three standard pigtail-coupled diode lasers 1012.1, 1012.2 and 1012.3 and a standard photodetector 1036. The ringdown-cavity module 1011 and the optical transmitter-receiver module 1013 are interconnected by standard optical fibre cables 1030.1 and 1030.2. The ringdown-cavity module 1011 may be situated at a remote location. Alternatively, the system may comprise two or more than three diode lasers, if required. Each of the diode lasers 1012.1, 1012.2 and 1012.3 may have a different wavelength range, or a different frequency stability. Alternatively, one of them may be used as a replacement or standby laser for fault protection in real time. At least one of the diode lasers 1012.1, 1012.2 and 1012.3 is a SF tunable laser of the form used singly in the embodiments of the system shown in Figures 1, 6, 8, 9 and 11. Any cw diode laser 1012.1, 1012.2 or 1012.3 that is not SF-tunable needs to be provided with a means of being shifted in and out of resonance with the ringdown cavity, for instance, by frequency- or amplitude-modulation or by an optoelectronic switch. The transmitter-receiver module 1013 also comprises a wavelength division multiplexer 1080, that is a (N x 1) combiner/multiplexer (with N equal to or greater than the number of diode lasers employed) capable of combining the various cw laser beams of the diode lasers 1012.1, 1012.2 and 1012.3 into one single-mode optical fibre 1028. The transmitter-receiver module 1013 further comprises a computerised control, data processing and analysis unit 1044 for controlling the operation of the various components of the optical transmitter-receiver module 1013 and, if required, those of the ringdown-cavity module 1011. The transmitter-receiver module 1013 further comprises an optical circulator 1032 similar to the optical circulator 32 shown in Figure 1. Additional optical isolators (not shown) may be integrated into the optical fibre 1028 or may be an integral part of each diode laser 1012.1, 1012.2 and 1012.3, to protect them from unwanted optical feedback. It should be noted that fibre-optic optical circulators are generally insensitive to variations in the polarisation of the light, which is advantageous in this application. Optical circulators are widely used in fibre-optical telecommunications . As mentioned above, the ringdown-cavity module 1011 housing the ringdown-cavity measuring cell 1014 is remotely located relative to the optical transmitter-receiver module 1013. Components corresponding to the PZT device 22 and associated electrical cable 46.1, as shown in Figure 1, to vary the optical path-length of the ringdown cavity 14 are not explicitly shown in Figure 10. These elements may not be required where the system is used at atmospheric pressure (i.e., about 1 bar), because the resolution of the coarsely resolved CRD spectra is usually higher 5 than the pressure-broadened spectral width of the sample. Where it is necessary to vary the distance between any of the mirrors in the optical cavity, an alternative embodiment of the system shown in Figure 10 may comprise a PZT voltage supply close to or in the ringdown-cavity module 1011 responsive to control signals transmitted from the optical-heterodyne transmitter- receiver module 1013 by another optical fibre or by wireless means. Either way, these ιo embodiments yield a single-ended CRD optical detection system, in which the ringdown-cavity section can be remotely located relative to the main instrumental transmitter-receiver module 1013. A second photodetector corresponding to component 38 in Figure 1, component 638 in Figure 6 and component 838 in Figure 8, to monitor the forward-transmitted light field EF is
15 optional in such a single-ended detection embodiment of the system (not shown in Figure 10). Such a photodetector may, however, be useful for preliminary alignment and optimisation of the system 1010. In cases where a second photodetector is not required to monitor forward- transmitted light, the mirror 1016.2 may be in the form of a total reflector, whereby the amplitude of the backward-propagating optical-heterodyne signal may be enhanced.
20 It is feasible to use standard telecommunication optical fibre that is optimised for the shortest wavelength (highest optical frequency) of the set of cw tunable diode lasers 1012.1 , 1012.2 and 1012.3. The other, longer, wavelengths (lower optical frequencies) of the remainder of the set is then above the cutoff wavelength of the optical fibre, so that the fibre remains single- mode for light from the entire set of cw tunable diode lasers 1012, 1 , 1012.2 and 1012.3. For
25 example, an optical fibre designed for single-mode operation in the telecommunications C band at a near-infrared wavelength in the range 1.53 - 1.57 μm can serve as a single-mode fibre at longer wavelengths with acceptable transmission characteristics extending to longer infrared wavelengths (2 μm and beyond, say). Optical fibres are desirable in the methods and systems of the invention because passing multiple laser beams through a single-mode optical fibre provides a
30. spatial-filtering advantage, such that all of the beams emerging out of the other end of a single- mode optical fibre share a common well-defined beam profile. Co-alignment of the different laser beams is desirable to ensure that all can be coupled efficiently into the one ringdown cavity. Each laser beam is coupled into the single-mode optical fibre 1030.1, 1030.2, by means of either beam-steering reflectors such as mirrors or prisms (not
35 shown) or by suitable beam splitters or preferably, as shown explicitly in Figure 10, by optical fibre-pigtails and a standard N x 1 optical combiner/multiplexer 1080 (with N equal to or greater than the number of diode lasers employed). Other multiple-coherent-light-source/laser embodiments of the invention (not explicitly illustrated here) may comprise means to combine beams of multiple frequencies together in free space, or to replace multi-wavelength optical fibres by suitable beam splitters, dichroic mirrors or other mirrors or reflectors. Diffraction gratings and prisms may also be used. Prisms may, as an alternative, be used as non-dispersive reflectors for guiding one or more light beams into a microscope objective lens and then into an optical fibre, as mentioned above. Each cw diode laser 1012.1, 1012.2 and 1012.3 shown in Figure 10 may be selected to emit light over a particular frequency range. Some of these frequency ranges may be chosen to be resonant with spectroscopic features of gas-phase chemical species that are of particular interest. At least one other frequency range may be chosen such that it is removed from any known spectroscopic features, in order to measure the non-resonant background or baseline. Chemical species that are typically of interest usually have many characteristic optical absoφtion features within a narrow frequency (wavelength) range. The multiplex control and analysis electronics system 1044 performs a similar function to that in Figures 1 and 6 and, in addition, it controls the concerted operation of the cw tunable diode lasers 1012.1, 1012.2 and 1012.3, such that the optical build-up of each resonance point occurs at a different time in the SF cycle(s) of the two or more lasers that are in operation. A reference cell (not shown) and/or a frequency (wavelength) meter (also not shown) may also be included in the system 1010 to check laser frequency (wavelength). Figure 11 shows yet another alternative embodiment 1110 of the system in accordance with the invention. As in the case of Figure 10, the embodiment shown in Figure 11 is also an integrated system. It also comprises a ringdown-cavity module 1111 comprising a ringdown- cavity measuring cell 1114 and an optical transmitter-receiver module 1113 comprising a standard photodetector 1136. The ringdown-cavity module 1111 may also be located at a remote location. The ringdown-cavity module 1111 and the optical transmitter-receiver module 1113 may be interconnected by a standard optical fibre cable or, alternatively, the tunable cw coherent-light- source/laser 1112 may propagate to the ringdown-cavity module 1111 through free space. In this embodiment, a tunable cw coherent-light-source/laser 1112 provides coherent light which is coupled into the ringdown-cavity measuring cell 1114 of the ringdown-cavity module 1111 via the free-space path 1130. An optical shutter 1182 is provided to transmit or block, for suitable periods of time, the beam emitted by the tunable cw coherent-light-source/laser 1 112. The optical shutter 1112 may be an acousto-optic modulator or an electro-optical modulator. The optical shutter may, for example, on the basis that the first-order deflected beam of an acousto- optic modulator is turned on or off by the presence or the absence of a driving acoustic wave. In this embodiment, the emitted light from the tunable cw SF coherent-light-source/laser 1112 passes through the optical shutter 1182, a polarising beam splitter 1184 and a polarisation control unit 1186 before being coupled through a focusing lens 1188 to the ringdown-cavity measuring cell 1114. The polarisation control unit 1186 is typically a magneto-optical Faraday rotator that enables the backward-propagating beam to be directed efficiently to the photodetector 1136 (PDl) via the polarising beam splitter 1184. The polarising beam splitter 1184 and the polarisation control unit 1186 also serve as an optical isolator to protect the SF coherent-light- source/laser 1112. The ringdown-cavity module 1111 housing the ringdown-cavity measuring cell 1114 is remotely located relative to a separate transmitter-receiver module 1113 that houses the cw tunable SF coherent-light-source/laser 1112, photodiode 1136 (PDl) and the scan and detection unit 1144 containing computerised control, data-acquisition and analysis electronics. As stated above, in alternative embodiments of the invention, the cw tunable SF coherent-light-source/laser
1112 propagates between the ringdown-cavity module 1111 and the transmitter-receiver module
1113 either in free space as depicted in the Figure 11 or via an optical fibre. The default status of the optical shutter 1182 is "open", allowing the coherent-light- source/laser beam being coupled to the ringdown cavity 1118. With the optical shutter 1182 open, the photodetector 1136 (PDl) receives signals similar to the one as shown in Figure 3(a) when the SF coherent-light-source/laser frequency is swept. An electronic control unit 1190 detects the build-up of coherent-light-source/laser energy into the ringdown cavity 1 118 by monitoring the output of the photodetector 1136 (PDl). When the output of the photodetector 1136 exceeds a pre-set upper or pre-set lower value, the electronic control unit 1190 causes the optical shutter 1182 to block the coupling of the coherent-light-source/laser beam to the ringdown cavity 1118 for a pre-set time period before the shutter 1182 is re-opened. The scan and detection unit 1144 ignores the transient output signal of the photodetector 1136 (PDl) for a short time, usually a few microseconds, whilst the shutter 1182 is opened, to allow the output of the photodetector 1136 to settle down. The time period for the shutter to block the coherent-light-source/laser beam may be set to be several times longer than the longest ringdown time observed. The signal that the photodetector 1136 receives after the closing of the shutter is similar to the directly transmitted signal as shown in Figure 3(a), detected by the photodetector 38 (PD2) in Figure 1. With the help of this optical shutter 1182, the frequency sweep rate of the SF coherent- light-source/laser 1 112 is not required to be fast to clearly separate the build-up and the following ringdown and to enable the determination of the ringdown time. A slower sweep rate of the SF coherent-light-source/laser frequency across the cavity resonance allows more coherent-light- source/laser energy to be coupled into the cavity 1118 with a consequent higher output from the photodetector 1136 and an improved detection sensitivity of the ringdown time. The intensity profile of the detected signal is accordingly less modulated than the signal as shown in Figure 3(a). It is to be understood that the scope of this invention includes instrument designs and measurement strategies involving swept-frequency (SF) variants of cavity-enhanced absoφtion spectroscopy (CEAS) and integrated cavity output spectroscopy (ICOS), in addition to SF variants of cavity-ringdown (CRD) spectroscopy. For each of the embodiments depicted in Figures 3 - 11, SF CEAS and SF ICOS variants are included in the scope of this invention, as well as embodiments depicted in Figures 1 and 2 as discussed above. These optional SF CEAS and SF ICOS approaches may be useful for the puφose of recording survey spectra over a broad range of coherent-light-source/laser wavelengths. They may efficiently generate a characteristic spectroscopic signature for a given molecule or mixture of molecules for the sample contained within the optical cavity. Relative to SF CRD approaches, the SF CEAS and SF ICOS approaches may require fewer data points and less detail of the ringdown-decay profile to be recorded for a given spectral range. However, the SF CEAS and SF ICOS approaches are typically less sensitive than SF CRD approaches. As explained in the context of Figures 3 and 4, a CRD spectrum can be generated within a single rapid sweep of the widely tunable SF laser frequency by sequentially plotting values of cavity-ringdown rates τj"1 that are sampled at successive frequencies vj (where j = 0, 1, 2, ... and so forth) separated by the FSR of the cavity. Such a spectrum may be relatively coarsely resolved, but can be sufficiently well defined if the linewidth of features in the spectrum is substantially greater than the FSR of the cavity. In Figures 12 and 13, the method described in relation to Figures 3 and 4 above is experimentally realised for a single molecular optical absoφtion feature. In this specification, the expression "SF cw-CRD spectrometer" and variants and derivatives thereof are to be understood as to be referring to embodiments of an apparatus or system in accordance with the invention, and vice versa. Components or elements represented by abbreviations in or forming part of the aforementioned expressions, such as "cw", are not to be taken as referring to essential features of the invention. As used in this specification, the expressions "swept-cavity OHD cw-CRD" and "swept- cavity cw-CRD" do not refer to embodiments of an apparatus or system in accordance with the invention, and vice versa. A coarsely resolved SF cw-CRD absoφtion spectrum of carbon dioxide gas (C02) is presented in Figure 12(a), with its pressure-broadened linewidth exceeding the FSR by a factor of more than three. Figure 12(a) shows a scan of τ"1 values for 23 cavity resonances, plotted as open circles (o); these span a range of cavity-ringdown rates from 0.1 μs"1 to 0.7 μs"1. This is a significant effect because the τ"1 values have been recorded where the SF laser frequency is traversing an absoφtion peak of the sample gas and the optical absorbance, which is itself linearly dependent on (c τ)"1, varies from 0 - 2 x 10"5 cm"1. The origin of the frequency scale is centred on the 6490.06-cm"1 P(16) feature in the (30°l)-(0000) combination band of C02. It is remarkable
■ that such a spectrum can be recorded in a single rapid sweep of the SF laser frequency and within much less than one second, with the possibility of repetitive sweeps to enhance the signal-to-noise ratio by signal averaging. Figure 12(b) shows the corresponding sequence of 23 directly transmitted cw-CRD waveforms recorded in a single rapid sweep of the SF laser frequency, with a sweep rate of 2.4 THz s"1 and a sweep duration of about 3 ms. Each waveform is separated by the 0.33-GHz (0.011-cm"1) FSR of the cavity. Ringdown times τ at the centre of the scan are an order of magnitude shorter than those on either side, consistent with the peak absorbance in the centre of the scan. Pressure broadening is dominant for the gas mixture (11% C02 in N2 with total pressure P = 125 Torr at a temperature T = 300 K), so that a Lorentzian profile with 1.1-GHz (0.037-cm"1) FWHM linewidth provides a satisfactory fit to the plotted values of τ"1 (o) in Figure 12(a). The results shown in Figure 12 were recorded with an apparatus of the type depicted schematically in Figure 1. The cw SF laser 12 employed for the measurements in Figure 12 was a state-of-the-art miniature external-cavity tunable diode laser (model SLE1040, manufactured by iolon, Inc. San Jose, CA, USA) based on micro-electromechanical structures (MEMS) technology. Its monochromatic output power (about 10 mW) can be rapidly swept with an optical frequency (or wavelength) sweep rate ranging over 1.25 - 125 THz s"1 (0.01 - 1.0 μm s"1) and a repetition rate range of 0.125 - 100 Hz. The fibre-coupled output of this MEMS SF laser has an intrinsic optical bandwidth of 125 kHz and a frequency stability of about 1 MHz over about 3.5 μs. This monochromatic output is continuously tunable and can be rapidly swept mode-hop-free over its entire range of about 8 THz at communications-band wavelengths of 1.51 - 1.57 μm, spanning the ITU C band and beyond. This covers overtone absoφtion features of many molecules (such as CO, C02) NH3, and various hydrocarbons) of interest in environmentally relevant gas-sensing applications. The compact laser package (50 x 70 x 13 mm) incoφorates a complete external-cavity diode laser system with MEMS-based Littman-Metcalf architecture, a reference etalon for internal wavelength calibration, and control electronics. A computerised interface to the MEMS cw SF tunable diode laser output provides several electronic output signals that indicate its operational status and help to control and synchronise data collection. Wavelength calibration of recorded SF cw-CRD spectra can be based on the internal wavelength reference in the MEMS SF laser system itself and/or registered by external devices (e.g., a stable reference etalon or a calibrant gas cell to monitor the laser frequency). Moreover, the ringdown cavity itself can serve incidentally as a convenient, economical reference etalon, eliminating the extra components that are usually required to generate wavelength markers and to calibrate wavelength-scan linearity. Well-characterised and identified spectral features of a recorded CRD spectrum can also be used for wavelength-calibration puφoses. Remarkably, the process of spectroscopic sampling by the FSR grid of the ringdown cavity enhances detection stability and enables signal averaging to be insensitive to imperfections in sweep-to-sweep reproducibility of the SF laser frequency. In the measurements reflected in Figure 12, the optical power from the MEMS SF laser was about 2 mW, incident on the cavity. This MEMS SF laser can be swept over a wide frequency range, recording successive SF cw-CRD waveforms that are generated at cavity resonance points separated by the cavity's FSR (i.e., at each point of an effective sampling grid). It is thereby feasible to record an extensive CRD spectrum at a high data-acquisition rate within a single wide- range laser-frequency sweep cycle (typically within about I s). It should be noted that this is a much more efficient procedure than most other approaches to CRD spectroscopy, in which the laser-frequency needs to be scanned slowly. In some of the inventors' early SF cw-CRD experiments, recording signals such as those shown in Figures 2(a), 2(b) and 2(c), were performed using a conventional external-cavity cw tunable diode laser (New Focus model 6262 with model 6200 controller). Its single-longitudinal- mode (SLM) ouput power of about 5 mW over a wavelength range of 1.50 - 1.59 μm was coarsely tunable by picomotor in steps of about 0.01 nm (about 1.3 GHz) with a repeatability of about 0.1 nm (about 13 GHz) and a short-term (about 50 ms) optical bandwidth of about 150 kHz. The piezoelectrically-controlled frequency-sweep capability of this cw tunable diode laser (tuning range about 30 GHz at a sweep rate of about 6 THz s"1) was limited, but nevertheless useful for SF laser frequency sweeps spanning up to about 90 FSR intervals of the ringdown cavity. A second conventional external-cavity cw tunable diode laser (Photonetics Tunics-Plus) has also been used for some experiments with limited frequency-sweep ranges (output power of about 5-mW, 6-GHz PZT-controlled frequency sweep, coarsely tunable in steps of about 0.001 nm over the wavelength range 1.50 - 1.64 μm). The ringdown cavity 18 used to record the results in Figure 12 was mounted inside an evacuable cell 14 fitted with Brewster-angle silica glass windows, 14.1 and 14.2, and electronic manometers to register the pressure of the gaseous sample 24. The cavity comprised two rigidly mounted high reflectors (Newport model 10CV00SR.70F, typically at least_99.98% reflectivity at about 1.55 μm, 1-m radius of curvature), separated by 0.453 m; its corresponding FSR was 0.331 GHz (0.0110 cm"1). These cavity parameters were chosen here for economy and compactness, rather than for ultimate detection sensitivity which would be favoured by using a longer cavity with ultra-high-reflectivity mirrors. As in Figure 1, the cw output from the MEMS SF laser 12 used in the context of Figure 12 was coupled to the ringdown cavity measuring cell 14 by single-mode optical fibres 28 and 30 via a three-port fibre-optical circulator 32 (Photonic Technologies HRC-1550-03-1010) or via Faraday-rotator optical isolators (Optics for Research model IO-4-IR2-HP, 60-dB isolation from two units in series). The geometry of the beam emerging from the fibre is matched to the longitudinal mode of the ringdown cavity by a lens system 34 consisting of an aspheric objective and a focusing lens with 50-cm focal length. Coupling to higher-order transverse modes of the cavity is suppressed to less than 1%. Backward-propagating light from the ringdown cavity can be measured by an InGaAs photodetector 36 (also labelled PDl; New Focus model 1617, 800- MHz bandwidth, 0.5-ns rise time) by way of optical fibre 32.1. These components also protect the cw SF tunable diode laser system from interference or damage by backward-propagating light. The back surface of each ringdown-cavity mirror is slightly tilted away from normal to the cavity axis, which is itself displaced by about 5 mm from the midpoint of each of the 25-mm-diameter mirror surfaces; this avoids interference problems caused by reflections from the back surfaces of the mirrors. A second photodetector 38 (labelled PD2; New Focus model 1811, 125-MHz bandwidth, 3-ns rise time) was also provided to detect CRD signals directly transmitted by the optical cavity. This latter, forward-propagating detector was used to record the CRD signals shown in Figure 12(b). The output from photodetector 38 (PD2) was processed by a digital oscilloscope (Tektronix model TDS794D, 4 Gsample s"1) and associated electronics that comprise the electronic data-acquisition module 42 of Figure 1. Peak amplitudes of the build-up and ringdown-decay waveforms shown in Figure 12(b) are found to vary erratically. This effect is attributed to instantaneous fluctuations in sweep speed and or optical bandwidth of the output from the MEMS SF laser 12 that was employed for these measurements. Nevertheless, it was still feasible in Figure 12(a) to generate a high-quality CRD absoφtion spectrum from the extracted ringdown decay rates τ"1, with an accuracy of ±0.003 μs"1. The extent of Figure 12 was limited by the number of data points actively sampled by the
0.5-MB memory depth of the Tektronix TDS794D digitiser (4 Gsample s"1 with 2 interleaved channels, 8-bit resolution) employed. A second digital oscilloscope Tektronix TDS3054B; 5 Gsample s"1 in 4 simultaneous channels, 9-bit resolution, 104 data points) was available in other measurements. This capability can be enhanced by using a computer-based plug-in digitiser with greater resolution and memory depth (e.g., National Instruments model 5122; 100-MHz bandwidth, 14-bit resolution, 32-MB memory depth). A gated mode of data acquisition can also be used to record the SF cw-CRD signal for a pre-set period of time whenever the start of a build- up event is detected (once per FSR); this effectively bypasses uninformative time intervals between successive build-up and ringdown-decay events, thereby optimising use of the digitiser's memory depth. The range over which CRD spectra can be measured is ultimately determined by the SLM tuning range of the SF laser employed (about 8 THz at 1.51-1.57 μm in this case, which can be covered by one pair of cavity mirrors). Figure 13 shows two simultaneously recorded sets of signal profiles obtained using an apparatus similar to the one depicted schematically in Figure 1. This was achieved by rapidly sweeping the SF laser frequency over 11 successive longitudinal-mode resonance frequencies vm of the cavity, near an absoφtion peak of an intracavity sample gas. Figure 13(a) shows the forward-propagating optical power directly transmitted by the cavity and monitored by photodetector 38 (PD2); each CRD feature decays quasi-exponentially with time constant τ, as explained in the context of Figures 2 and 3. Figure 13(b) depicts the demodulated logarithm of full-wave oscillatory waveforms of corresponding OHD optical power retro-reflected by the cavity, monitored by photodetector 36 (PDl) and processed by a demodulating logarithmic amplifier 40 (Analog Devices model AD8307, bandwidth DC - 500 MHz, linearity 1 dB, dynamic range 92 dB). This directly converts the exponentially decaying full-wave envelope of the oscillating OHD SF cw-CRD signal, as in Figure 2(b), to a straight line, as in Figure 2(c), from the negative slope of which 2τ can be conveniently and efficiently derived. Each waveform is separated by the cavity's 0.33- GHz (0.011-cm"1) FSR. The full-wave oscillatory waveforms of OHD signal itself are not plotted here as they would not be resolved on the time scale of this plot. Figure 13(c) shows a spectrum of sample-and-hold output levels proportional to the ringdown time τ, derived by processing OHD signals in real time as will be explained in the context of Figures 15 and 16 below; this spectrum, for which the sweep rate is 900 GHz s"1, spans a spectral range of 3.6 GHz (0.12 cm"1) across a single pressure-broadened absoφtion peak of C02 gas at wavelengths around 1.54-μm. The optical absoφtion feature examined in Figure 13 is the same pressure-broadened peak as was recorded in Figure 12 above (that is, the P(16) absoφtion peak at 6490.06 cm"1) for a 2% mixture of C02 in N2, pressure-broadened with P = 125 Torr and T = 300 K. The SF laser tunable diode laser used for the measurements in Figure 13 was a Photonetics Tunics-Plus laser and the resulting CRD signals were processed by a Tektronix TDS3054B digitiser at 2.5 Msample s"1. Figure 13 also shows how successive build-up and decay events vary as the laser frequency sweeps over an absoφtion feature of the sample gas. Such variations are evident in the widths of the plotted features, arising from the exponential decay rates of each build-up and decay event in Figure 13(a) or from the negative slopes in Figure 13(b): those at either edge of each plot are much broader than those at the centre of the plot, where ringdown time τ is shorter and optical absoφtion correspondingly stronger. More details are shown elsewhere in this specification. The features shown in Figure 13 are due to the longitudinal modes of the ringdown cavity. As explained above, coupling to the transverse mode of the cavity is suppressed to less than 1% in amplitude, by using lenses 34 to match the beam geometry of the input laser beam to the longitudinal mode of ringdown cavity. Real-time numerical processing of the 11 pairs of build-up and decay events in Figure 13 yields values of τ that vary smoothly from one pair of SF cw-CRD signal profiles to the next, in a manner similar to Figure 12(a). Figure 14 presents results that are comparable to those depicted in Figure 12, but that are more finely resolved by using the incrementally (or decrementally) stepped cavity-length (ISCL) procedure that is outlined in the context of Figure 5 above. A piezoelectric translator 22 (PZT; Piezotechnik model HPSt 150/20) attached to a mirror mount 16.2 of the ringdown cavity 18, as shown in Figure 1, allows the options of fine cavity-length adjustment, ICSL-type stepped-cavity or SSCL-type synchronously scanned SF cw-CRD operation, and or parallel swept-cavity and swept-frequency CRD measurements. Figure 14(a) shows a plot of τ"1 values for 23 cavity resonances as in Figure 12(a), plotted as open circles (o) and spanning a range of cavity-ringdown rates from 0.1 μs"1 to 0.7 μs"1. A survey scan of the corresponding directly transmitted CRD waveforms is shown in Figure 14(b), as monitored by photodetector 38 (PD2) in Figure 1. Figure 14(c) depicts an additional set of directly transmitted CRD waveforms, recorded after using the PZT 22 to change the length of the ringdown cavity by an increment of about 0.27 μm, thereby shifting the cavity-resonance grid by about 0.35 times the FSR. The corresponding additional 23 values of τ"1 derived from Figure 14(c) are plotted as dots (•) in Figure 14(a), filling in gaps in the spectrum between the open circles (o) derived from Figure 14(b). SF cw-CRD spectra, as in Figure 14(a), arise from a sequence of multiple build-up and ringdown-decay events, either directly transmitted as in Figures 2(a), 3(c), 7(c), 12(b) and 13(a) or OHD as in Figures 2(b, c), 3(a, b), 7(a, b) and 13(b), with the laser frequency rapidly swept. A data point (indexed as j, say) is registered each time the laser frequency passes through successive longitudinal-mode resonance points of the cavity (separated by its FSR); each such frequency vj yields a corresponding ringdown time τj that varies with absorbance as the laser frequency is swept through the spectrum of the gas. The laser-frequency sweep rate needs to be such that the decay of each CRD event (j) is complete before the next CRD event (j + 1) builds up, as is evident in Figures 3, 12(b), 13(a, b), and 14(b, c). Figure 14(a) comprises two superimposed SF cw-CRD spectra of C02 gas at about 1.54 μm, with the origin of the frequency scale centred on the 6490.06-cm"1 P(16) peak in the (30°1)- (00°0) combination band of C02; the same peak was recorded in Figures 12(a) and 13(c). As in the case of Figure 12, the sample gas is an 11% mixture of C02 in N2 with P = 125 Torr and T = 300 K; the pressure-broadened linewidth then exceeds the 0.33-GHz (0.011-cm"1) FSR of the cavity by a factor of more than 3. A Lorentzian profile with FWHM linewidth of 1.1 GHz (0.037 cm"1) provides a satisfactory fit to either set of plotted values of τ"1 in Figure 14(a). Ringdown times τ at the center of the scan are an order of magnitude shorter than those on either edge, consistent with the peak absorbance of about 2 x 10"5 cm"1 in the center of the scan and a 'zero- absorbance' or 'empty-cell' ringdown time τ of about 10 μs (consistent with the 99.98 % mirror reflectivity). It is remarkable that each of the two superimposed spectra plotted (either as o or as •) in Figure 14(a) can be recorded in a single rapid sweep of the laser frequency and within much less than one second: to record either of the spectra in Figure 14(a), the cw MEMS SF tunable diode laser (iolon model SLE1040) was operated with a frequency sweep rate of 2.4 THz s"1, taking only about 3 ms to traverse the 22 FSR-grid intervals and register the data electronically by means of a Tektronix TDS794D digitiser at 50 Msample s"1. Moreover, signal averaging over a number (N) of repetitive laser-frequency sweeps (each with fixed L) can enhance the signal-to-noise ratio by a factor of N1/2, but with a corresponding recording-time penalty. As mentioned above in the context of Figure 12, signal averaging by means of this FSR-grid spectroscopic sampling process is not degraded by minor irreproducibility of the cw SF tunable diode laser frequency from sweep to sweep. It should be noted that the pressure-broadened linewidth in the case of Figure 14(a) is sufficiently large for the spectrum to be adequately defined by a single set of data points (represented either by o or by •), recorded with a single fixed ringdown-cavity length. Such SF cw-CRD spectra are relatively coarsely resolved, but this is adequate when the FWHM linewidths of spectral features are markedly greater than the cavity's FSR, as is the case in Figures 12(a), 13(b) and 14(a). However, finer resolution may be needed when spectral linewidths are narrower, for example, if the pressure of the sample gas is reduced. This finer resolution could be achieved by increasing the length L of the ringdown cavity, as its FSR is proportional to L"1. However, it is less intrusive to use the PZT 22 to vary the length L of the ringdown cavity 18 in small stepwise increments (corresponding to a fraction of the laser wavelength, ICSL-style) as successive FSR- sampled sets of τ"1 values are recorded. As demonstrated by the two sets of data points (represented either by o or by •) plotted in Figure 14(a), such measures select a slightly different set of cavity resonance points vj each time the SF laser frequency is swept and progressively fill in the gaps between data points sampled with a single, fixed FSR grid. An alternative SSCL-type synchronously scanned approach would be to scan the cavity length continuously and in synchrony with the SF cycle, at a rate that is sufficiently slow to allow many SF sweeps while the cavity length spans the FSR. Another important advantage of this invention is that it offers a convenient way to record wide-ranging SF cw-CRD spectra for rapid 'fingeφrint' sensing of airborne molecules. As illustrated in Figures 12(a), 13(c) and 14(a), such spectra are adequately recorded with a relatively coarse resolution of about 0.5 GHz (about 0.015 cm"1) so that it suffices to record satisfactory spectra with L, and hence the FSR grid, held fixed. The feasibility of recording a single CRD data point per FSR in real time has been demonstrated in the context of this invention. In order to extract values of τ"1 in real time from the CRD decay signal, an analog circuit (e.g., a level discriminator and a time-to-amplitude convertor) may be used to measure the time interval for the CRD decay to drop from one preset level to another. This enables registration of a single τ"1 data point for each build-up and decay event, without needing to record the full CRD waveform at each laser frequency, and imposes much lower demand on digitiser memory depth and on data processing. Data-acquisition schemes of this type are then available to generate characteristic spectroscopic signatures for key gas-sensing applications. The approaches to processing CRD signals, as in Figures 2, 12(b) and 14(b, c), and converting them into spectroscopic profiles, as in Figures 12(a) and 14(a), entail recording details of decay waveforms followed by numerical analysis. This typically requires a fast digitiser with large memory depth and a prolonged computing time. To efficiently record wide-ranging spectra (e.g., taking full advantage of the wide 8-THz frequency-sweep range of the iolon model SLE1040 MEMS SF tunable diode laser), it is therefore desirable to log only a single data point that registers the ringdown time τ for each build-up and decay waveform. An efficient and effective way to record wide-ranging SF CRD spectra employs analog electronic circuits for real-time processing of the OHD signal retro-reflected from the ringdown cavity, that is, as monitored by photodetector 36 (PDl) in Figure 1. This approach employs level discriminator, time-to-amplitude converter, and sample-and-hold circuits to extract a reliable instantaneous measure of the ringdown time τ in a single data point for each build-up and decay event, without needing to record and process details of the CRD waveform at each laser frequency sampled by the FSR-spaced grid of the cavity's resonance frequencies. This imposes much less demand on digitiser speed and memory depth and on data processing. Such an approach makes it feasible to generate characteristic spectroscopic signatures for key gas-sensing applications, as has already been demonstrated in Figure 13(c) above. It is more convenient for an analog processor to extract a ringdown time τ from the slope of the straight-line decay of the demodulated logarithm of OHD CRD signal as in Figure 2(c) than from the exponential decay of the directly transmitted CRD signal as in Figure 2(a). In particular, the slope of a straight line is insensitive to an offset shift of its absolute zero value. Electronic circuits, such as differentiators, may be used to determine signal slopes, including an analog detection scheme (together with a lock-in amplifier) in CRD spectroscopy, as in the paper entitled "A laser-locked cavity ring-down spectrometer employing an analog detection scheme, " published in Review of Scientific Instruments 71, 347 - 353 (2000) by T. G. Spence, C. C. Harb, B. A. Paldus, R. N. Zare, B. Wilke, and R. L. Byer; see also US Patent No 6,532,071 entitled "Analog detection for cavity lifetime spectroscopy" by R N Zare, C C Harb B A Paldus and T G Spence. However, differentiators are usually very sensitive to high-frequency electronic signal noise, as their gain is proportional to the frequency of the noise. Figure 15 illustrates an analog detection scheme for processing of the CRD signal. The scheme is based on a pair of level discriminators and a time-to-amplitude converter (TAC). Figure 15(a) shows schematically how the straight-line decay of the demodulated logarithm of OHD CRD signal, e.g., as depicted in Figure 2(c), is sampled by level discriminators at two preset reference levels. As shown in Figure 15(b), this generates a rectangular pulse with its duration Δt corresponding to the level-crossing time interval during which the signal decays from one preset discriminator reference level to the other. The slope of the straight-line decay is proportional to the decay rate τ"1, so that the time interval Δt is proportional to ringdown time τ. This duration Δt is converted to an analog output voltage V by charging an integrator at a constant current for that period Δt. The outcome is a (slope"')-to-amplitude converter that generates an analog voltage V with amplitude proportional to ringdown time τ. The conversion factor of the circuit is calibrated by comparing the true exponential decay times τ of directly transmitted CRD waveforms and outputs V of the (slope"')-to-amplitude converter. Compared to a conventional approach, in which waveform details are recorded point-by- point and subsequently analysed numerically, the two level discriminators process only values of CRD decay signal close to the preset reference levels, ignoring other parts of the CRD signal. Improved results can be obtained by low-pass filtering of the CRD signal to reduce the influence of local signal variations and electronic noise. Figure 16 shows how a sample-and-hold circuit is combined with the (slope'^-to-amplitude converter to enable continuous real-time processing of successive SF cw-CRD build-up and decay events,. The overall data-processing sequence, illustrated in Figure 16, depicts three successive (slope'^-to-amplitude conversion processes in Figures 16(a) - 16(c). The output amplitudes V from Figure 16(c) are transferred to a sample-and-hold circuit with control gate, reset logic levels, and output as in Figures 16(d), 16(e), and 16(f), respectively. The (slope'^-to-amplitude converter is reset after each ringdown-decay event to await arrival of the next build-up event. The output of the sample-and-hold circuit stays at a constant level before it is refreshed in the next (slope"')-to-amplitude conversion step. This analog detection approach has already been depicted in Figure 13(c), which shows sample-and-hold output levels varying (in proportion to ringdown decay time τ) as the laser frequency traverses the 6490.06-cm"1 (30°l)-(00°0) P(16) absoφtion line of intracavity C02 gas. Figure 17 shows survey spectra of C02 gas over a broad wavelength range (1.52 - 1.55 μm). The SF cw-CRD spectrum in Figure 17(a) comprises a total of 10,000 SF cw-CRD signal build-up and decay events spanned by a 3.3-THz (110-cm"1) sweep of the output frequency of the MEMS SF tunable diode laser (iolon model SLE1040). Data from single frequency sweep were recorded by a Tektronix TDS3054B digital oscilloscope within a period of 2 s, at a data rate of 5 Ksample s"1. Values of ringdown time τ were processed in real time by analog electronic circuits, as explained in the context of Figures 15 and 16. The signal-to-noise ratio of a single-sweep spectrum was further improved by signal averaging at the digitiser. The SF cw-CRD spectrum shown in Figure 17(a) is an average of 128 sweeps, recorded in less than 5 minutes; projected modifications to the analog electronic circuits are expected to minimise the need for such signal averaging and thereby yield high-quality SF cw-CRD spectra more readily. The spectrum covers the relatively weak (30°l)-(0000) and pl'lHO^O) absoφtion bands of C02 gas (2% mixture of C02 in N2; P.= 125 Torr; T = 300 K). Each feature in the spectrum is pressure-broadened, as in Figures 12(a), 13(c) and 14(a). Extra irregularly spaced absoφtion lines due to residual H20 vapour also appear in the spectrum, particularly at its short-wavelength end. This demonstrates the capability of the method and system according to the invention to detect, identify and quantify mixtures of molecules in the gas phase. A simulated spectrum of C02 from the HITRAN'96 database ["The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation): 1996 edition" in Journal of Quantitative Spectroscopy and Radiative Transfer 60 (1998) 665 - 710 by L. S. Rothman, C. P. Rinsland, A. Goldman, S. T. Massie, D. P. Edwards, J.- M. Flaud, A. Perrin, C. Camy-Peyret, V. Dana, J.-Y. Mandin, J. Schroeder, A. McCann, R. R. Gamache, R. B. Wattson, K. Yoshino, K. V. Chance, K. W. Jucks, L. R. Brown, V. Nemtchinov, and P. Varanasi] is shown in Figure 17(b), confirming the accuracy and reliability of the SF cw- CRD spectrum in Figure 17(a). An alternative way to record a single data point per FSR in real time is to use cavity- enhanced absoφtion spectroscopy (CEAS) rather than genuine CRD spectroscopy. In its simplest forms, CEAS techniques entail measuring the peak amplitude of build-up and decay waveforms rather than the characteristic exponential decay time τ, as in CRD spectroscopy; CEAS therefore lacks the time-domain discrimination of CRD methods and are more susceptible to uncertainty due to fluctuations in the intensity and frequency sweep rate of the incident laser radiation. Swept-cavity variants of CEAS and the closely related ICOS (integrated cavity output spectroscopy) technique have been outlined above in the "Background of the Invention" section. Swept-frequency (SF) variants of CEAS and ICOS have been described as aspects of this invention and in the contexts of Figures 1 and 2 above and are understood to be included in the scope of embodiments depicted in Figures 3 - 11 above. The lower portion of Figure 18 depicts a SF cavity-enhanced absoφtion (CEA) spectrum that has been recorded for the 6503-cm"1 (30°1 - 00°0) rovibrational absoφtion band of C02 gas at P = 0.65 atm and T = 300 K with the frequency of the cw SF tunable diode laser (iolon model SLE1040) swept at a rate of about 3 THz s"1 in less than 1 s over a range of 2.3 THz, from 1.529 μm to 1.547 μm, spanning about 70,000 FSR intervals of the ringdown cavity. The SF CEA spectrum provides a clearly recognizable fingeφrint of gas-phase C02, as shown by reference to the upper portion of Figure 18 which is a corresponding spectrum from the HITRAN'96 spectroscopic database ["The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation): 1996 edition" in Journal of Quantitative Spectroscopy and Radiative Transfer 60 (1998) 665 - 710 by L. S. Rothman et al . This provides convincing proof of the principle that the FSR-spaced grid of resonance frequencies of the ringdown cavity can efficiently sample an absoφtion spectrum of interest over a wide wavelength range. However, SF cw-CEA spectra, as in the lower portion of Figure 18, are found to be highly susceptible to minor fluctuations in the incident laser radiation, much more so than corresponding SF cw-CRD spectra, as in Figure 17(a). This shortfall of SF cw-CEA spectroscopy is evident in the erratic peak signal amplitude variations of the successive SF cw-CRD build-up and decay waveforms in Figures 12(b) and 14(b, c), compared to the regular profile of the spectra (plotted as o or as •) in Figures 12(a) and 14(a) on the basis of measurements of ringdown time τ rather than of peak signal amplitude. The observed SF cw-CRD peak signal amplitude fluctuations in Figures 12(b) and 14(b, c) are primarily attributable to irregularity in the sweep rate and/or optical bandwidth of the cw SF tunable diode laser frequency that cause substantial variations in the build-up time and hence the peak signal amplitude, but not in ringdown decay time τ. Because of its relative immunity to such irregularities in cw SF laser output of the SF cw-CRD spectroscopy, using the analog circuitry described in the contexts of Figures 15 and 16, therefore appears to be a preferable way to record wide-ranging fingeφrint spectra, relative to the SF cw-CEAS approach. The scatter of data points relative to the Lorentzian curve of best fit, for the SF CRD spectrum in Figure 12(a) or for either SF CRD spectrum in Figure 14(a), indicates that the minimum detectable absoφtion loss (MDAL) is about 9 x 10"8 cm"1; the corresponding signal-to- noise ratio at the peak of the spectrum was about 220, which can be substantially enhanced by signal averaging. Here, the FSR-sweep period, during which a single build-up and ringdown- decay waveform can be recorded, was about 145 μs; this yields a data-rate-normalised MDAL of about 1.1 x lO"9 cm"1 Hz"1 2. MDALs up to three orders of magnitude better than this have been attained with more elaborate forms of cw-CRD spectrometer, for example, as reported in the following publications: "Cavity-Ringdown Spectroscopy - An Ultratrace-absoφtion Measurement Technique", edited by K. W. Busch and M. A. Busch, ACS Symposium Series (1999) No. 720, ISBN 0-8412-3600-3; "Cavity ring-down spectroscopy: Experimental schemes and applications", International Reviews of Physical Chemistry 19 (2000) 565 - 607 by G. Berden, R. Peeters, and G. Meijer; "A laser- locked cavity ring-down spectrometer employing an analog detection scheme", Review of Scientific Instruments 71, 347 - 353 (2000) by T. G. Spence, C. C. Harb, B. A. Paldus, R. N. Zare, B. Wilke, and R. L. Byer; "Stable isotope ratios using cavity ring-down optical spectroscopy: determination of 13C/12C for carbon dioxide in human breath" Analytical Chemistry 74, 2003 - 2007 (2002) by E. R. Crosson, K. N. Ricci, B. A. Richman, F. C. Chilese, T. G. Owano, R. A. Provencal, M. W. Todd, J. Glasser, A. A. Kachanov, B. A. Paldus, T. G. Spence, and R. N. Zare; and "Trace moisture detection using continuous-wave cavity ring-down spectroscopy", Analytical Chemistry 75,4599 - 4605 (2004) by J. B. Dudek, P. B. .Tarsa, A. Velasquez, M. Wladyslawski, P. Rabinowitz, and K. K. Lehmann. It should be noted that, compared to the abovementioned high-sensitivity CRD spectrometers, the SF cw-CRD spectrometer according to this invention is relatively simple, easy to operate and able to measure wide-ranging spectra in a relatively short recording time, as is demonstrated in the context of Figure 17. The last of these three features is particularly advantageous in that a much greater body of spectroscopic information is available virtually instantaneously, compared to conventional CRD techniques where recording of wide-ranging spectra requires a relatively slow scan of the frequency or wavelength of the laser or coherent light source. The performance of the SF cw-CRD spectrometer described in this invention is already well suited to many practical applications, such as combustion diagnostics or clinical breath testing. The SF cw-CRD spectrometer's detection sensitivity (e.g., its attainable MDAL) is capable of improvement by orders of magnitude if necessary, for instance, by signal averaging over a longer detection period, or by increasing the cavity length and/or the mirror reflectivity. Examples of Experimental Measurement Procedures In some embodiments of the invention, the temporal profiles for directly transmitted SF CRD signals and/or backward-propagating OHD SF CRD signals, as depicted actually in Figures 2, 12 and 14 and schematically in Figures 3(a)-(c) and Figures 7(a)-(c), may be used to generate detailed or high-resolution portions of the spectrum of interest from which one or more molecular species of interest may be characterised with high sensitivity and their concentrations determined. The analysis procedure used to assemble detailed or high-resolution portions of the spectrum of interest in such measurements with rapidly swept coherent-light-source/laser frequency has been explained and depicted schematically in the context of Figures 3, 4, 5 and 7, with relatively slow cavity-length adjustment by either stepped (ISCL) or synchronously scanned (SSCL) methods. The frequency or wavelength range of the detailed or high-resolution portion (or portions) of the spectrum of interest may be limited by the memory depth of the computerised electronics used to record the spectrum of interest. This range may be much less than the full range of frequencies or wavelengths over which the coherent radiation can be swept. In other embodiments of the invention, less detailed features of the temporal profiles for directly transmitted SF CRD signals and/or backward-propagating OHD SF CRD signals, as depicted actually in Figure 2 and schematically in Figures 3(a)-(c) and Figures 7(a)-(c), may be used to generate less detailed spectra (with reduced sensitivity) but with a digitiser of a given memory depth able to span a much wider range of rapidly swept coherent-light-source/laser frequencies or wavelengths. The simpler analysis procedures used to assemble such spectra are then correspondingly less elaborate than that depicted schematically in the context of Figures 3, 4, 5 and 7, as only a single data point needs to be recorded at each ringdown-cavity resonance, separated by 1 FSR of the ringdown cavity. Such data points may comprise any of the following, each recorded without any ISCL- or SSCL-style cavity-length adjustment: - the peak amplitude of the transmitted CRD signal as in Figure 2(a), measured just after t =
0 on the abscissa, for SF CEAS measurements; - the peak-to-peak amplitude of the OHD CRD signal as in Figure 2(b), measured just after t = 0 on the abscissa, for another form of SF CEAS measurement; - the integrated signal profile for build-up and decay of the transmitted CRD signal as in Figure 2(a), for SF ICOS measurements; - the rectified and integrated signal profile for build-up and decay of the full-wave envelope of the OHD CRD signal as in Figure 2(b), for another form of SF ICOS measurement; - the integrated signal profile for the demodulated logarithm of build-up and decay of the full-wave envelope of the OHD CRD signal as in Figure 2(c), for another form of SF ICOS measurement. The first of these five possible approaches, involving measurement of the peak amplitude of the transmitted CRD signal, has been actually demonstrated in the SF cw-CEA spectrum that is presented in Figure 18. The frequency or wavelength range of the less detailed and/or less sensitive spectrum of interest will usually be much more extensive than in the previous detailed/high-resolution case, as it is less limited by the memory depth of the computerised electronics used to record the spectrum of interest. For example, a 50-cm ringdown cavity has a FSR of 0.01 cm"1 (i.e., 300 MHz) whereas a representative commercially available communications-band swept-frequency laser may be tuned in a single rapid sweep over a wavelength range of 1510 - 1570 nm, corresponding to a frequency-sweep of 250 cm"1 (i.e., 7.5 THz); this means that a full-range sweep of a spectrum of interest will span 25,000 (2.5 x 104) ringdown-cavity FSRs. Recording such a full-range spectrum at 1 data point for each ringdown-cavity FSR may be achieved with the memory depth of readily available digitisers or digital oscilloscopes. However, because fewer data points are recorded for each ringdown-cavity FSR, the SF CEAS and SF ICOS approaches will typically be less sensitive (in terms of analytical detection limits) than SF CRD approaches. The recording of spectra across a wide frequency or wavelength range by the optional SF CEAS and/or SF ICOS approaches enables acquisition of survey spectra spanning a broad range of coherent-light-source/laser wavelengths. Such procedures also offer an efficient way to generate a characteristic spectroscopic signature for a given molecule or mixture of molecules for the gas sample contained within the optical cavity. This in turn enables the accumulation of a computerised library or atlas of SF CEAS and or SF ICOS spectra for molecular species that are likely to be of interest in spectroscopic sensing applications. It is feasible, using standard pattern- recognition and/or chemometric analysis procedures to characterise or identify a sample of gas containing an unknown or suspected molecule or mixture of molecules. A further SF cw-CRD spectroscopic embodiment of the invention has been depicted methodologically in Figures 13, 15 and 16; it is actually demonstrated in Figure 17. As in the above less detailed SF CEAS and SF ICOS embodiments, a single data point is recorded at each ringdown-cavity resonance, separated by 1 FSR of the ringdown cavity, with a digitiser of a given memory depth able to span a wide range of rapidly swept coherent-light-source/laser frequencies or wavelengths. In this case, analog circuits (as explained in the context of Figures 15 and 16) enable real-time measurement of the ringdown time τ at each ringdown-cavity resonance (as depicted in Figure 13). This in turn enables rapid acquisition of a wide-ranging SF cw-CRD spectrum (as depicted in Figure 17) that is sufficiently extensive, precise and detailed to enable fmgeφrinting of a mixture of unknown gas-phase molecules. Another important aspect of measurement procedures used in this invention concerns the total pressure of the sample of interest. In many spectroscopic sensing applications a molecule of interest may be present as a dilute component of ambient air or of another carrier gas, with a total pressure equal to the atmospheric pressure (i.e., about 1 bar). In such cases, the absoφtion lines of any gas-phase spectrum of interest will usually be pressure-broadened such that the profile of each molecular absoφtion line will be spanned by many ringdown-cavity resonances, since the FSR of the cavity is usually substantially less than the pressure-broadened line-width of molecular absoφtion lines or spectroscopic features. For example, the pressure-broadened linewidth for near-infrared absoφtion spectra of carbon dioxide (C02) molecules diluted in air at a total pressure of 1 bar is about 0.5 cm"1 (i.e., 15 GHz) FWHM, which is 50 times the FSR of a 50-cm ringdown cavity. Any of the above-mentioned SF CRD, SF CEAS and SF ICOS approaches may be used to record coarsely resolved molecular spectra that may be sufficiently reliable without necessarily employing a variable-cavity-length approach (either ISCL or SSCL). In some other applications, molecular spectra need to be recorded at a total pressure much less than atmospheric (i.e., much less than 1 bar), either because that is the natural condition of the gas-phase sample or because the sample pressure has been reduced by evacuation to obtain a spectrum that is more finely structured owing to reduced (or eliminated) pressure broadening. In the case of such low- pressure gas samples, the profile of each molecular absoφtion line may be spanned by very few (if any) ringdown-cavity resonances, since the FSR of the cavity may be comparable to (or greater than) the line-width of molecular absoφtion lines or spectroscopic features. Coarsely resolved spectra recorded with a single ringdown-cavity length by the above-mentioned SF CRD, SF CEAS or SF ICOS approaches may not reveal all features of the spectrum, as absoφtion lines may coincide with too few ringdown-cavity resonances. It may then be necessary to employ a variable-cavity-length approach (either ISCL or SSCL) either for SF CRD spectroscopic measurements that are sufficiently finely resolved or to record reliable survey spectra with sufficient detail across a wide frequency or wavelength range. Such approaches have been explained in the context of Figure 5 and actually demonstrated in Figure 14. Although the methods of the invention have been described with reference to CRD- spectroscopic measurements of gas-phase samples, the methods of the invention may be used to analyse condensed-phase samples (such as liquids, solutions, aerosols, solids or surface films), molecular beams, matrix-isolated molecules, or molecules in very cold droplets. For example, it is within the scope of the invention to employ the above-mentioned SF CRD or SF CEAS approaches to record reliable survey spectra or spectroscopic signatures across a wide frequency or wavelength range for a liquid- or solution-phase sample contained in a suitable ringdown- cavity cell, despite the much higher optical absoφtion coefficients and correspondingly faster cavity-ringdown decay rates that usually occur in condensed-phase samples. Likewise, it is within the scope of the invention to employ the above-mentioned SF CRD, SF CEAS or SF ICOS approaches for CRD-spectroscopic measurements of a sample that is adsorbed as a film on the inner surface of at least one of the mirrors of the ringdown cavity. It is also within the scope of the invention to employ the above-mentioned SF CRD, SF CEAS or SF ICOS approaches for CRD-spectroscopic measurements of a sample that is outside the ringdown cavity, but nevertheless in communication with or coupled to that ringdown cavity (e.g., via an evanescent light wave at the outer surface of either at least one of the mirrors of the ringdown cavity or an optical fibre that forms part of the optical system, such outer surfaces being immersed or in contact with the sample of interest and optically sensitive to the environment of the ringdown cavity). Particular Instrumental Advantages It will be appreciated by those skilled in the art that the invention can be embodied in various forms. For example, a lower-gain, low-noise photodetector may be used to take advantage of the full 5-mW power of the tunable diode laser(s) or to use optical-fibre coupling to allow the ringdown cavity to be remotely located relative to the tunable diode laser(s) and ρhotodetector(s) in the OHD transmitter-receiver, thereby facilitating scientific, industrial, medical, agricultural and environmental sensing applications. A major advantage of the optical-heterodyne-detected cavity-ringdown apparatus with rapidly swept coherent-light-source/laser frequency is that the signal to be detected returns to the optical transmitter-receiver section of the apparatus by counter-propagating along the same path as that of the incident coherent-light-source/laser beam, thereby enabling single-ended detection. As indicated above, the invention is amenable to embodiments in which optical fibres are used to transmit the light over various portions of the beam path. Standard optical fibres are typically optimised for working in individual wavelength ranges (e.g., 1.5 - 1.8 μm). An optical fibre optimised for one wavelength may however still be used with single-mode characteristics at longer wavelengths above its cutoff wavelength. Accordingly optical fibres covering a wide spectral range from visible to the near infrared may be used. An embodiment that entails optical-heterodyne detection with optical-fibre coupling comprises an embodiment in which there is a single, central instrumental control system (including the optical-heterodyne transmitter-receiver section of the apparatus, with at least one SF coherent-light-source/laser and a single photodetector PDl) and numerous spatially distributed ringdown cavities, each coupled by a single-mode optical fibre and optional PZT control link to the central instrumental system. A suitable optical-fibre splitter or switch module can be used to distribute the coherent-light-source/laser and return cavity-ringdown light to and from different locations of a site at which the various ringdown cavities are positioned. This approach enables the more expensive, less rugged components of the overall apparatus to be positioned in a central secure location (e.g., in an air-conditioned control room) while less expensive, more robust ringdown cavities are multiply distributed in more hostile and?or less accessible locations (e.g., at various gas effluent sources on an industrial, environmental or agricultural site or in a series of wards in a hospital). In some applications, it is advantageous to use a vacuum pump to direct the gas-phase sample through a suitable particle filter into a sealed ringdown cavity where the spectroscopic measurements are made at sub-atmospheric sample pressure to minimise pressure broadening of optical absoφtion lines and thereby increase the specificity of detection. The system of the present invention has the advantage (relative to prior techniques) that only a single widely-tunable SF coherent-light-source/laser is needed to record wide-ranging spectra rapidly in a single frequency sweep, by sampling at successive ringdown-cavity resonance points. Moreover, the SF coherent-light-source/laser itself generates build-up and ringdown- decay events directly, without any primary need for a rapid cavity sweep or an optical (AO or EO) switch as in other forms of cw-CRD spectroscopy. PZT control of ringdown cavity length is a secondary requirement in swept-frequency (SF) CRD spectroscopy, necessary only to vary the set of available resonance frequencies that are sampled, thereby refining the resolution relative to the coarsely sampled spectrum that arises in a single SF cycle with fixed cavity length. Such changes of cavity length and resonance frequencies can either be performed in discrete steps, using the ISCL procedure, or by continuous scanning, using the SSCL procedure. Another advantage of the present invention is that its data-processing and analysis procedures are insensitive to small variations in the optical path-length in the ringdown cavity. While such optical path-length variations may arise from small changes of ambient temperature or mechanical changes (e.g., from external vibrations or dimensional shifts when the internal pressure in the ringdown-cavity cell is varied or when the external atmospheric pressure changes) or changes of refractive index (e.g., when the composition of the gas contained in the ringdown- cavity cell is altered), they merely cause a given feature in the SF CRD or SF CEAS or SF ICOS spectrum of interest to be registered by a different set of ringdown-cavity resonances, each separated by 1 FSR, as discussed in the context of Figures 4 and 5. Any necessary calibration of the frequency or wavelength scale may readily be provided by reference to spectra from a reference gas cell or a temperature-stabilised etalon, as discussed in the context of Figures 6 and 7. The optional SF CEAS and SF ICOS approaches, that are included in the scope of this invention, may be useful for the puφose of recording survey spectra over a broad range of coherent-light-source/laser wavelengths. They may efficiently generate a characteristic spectroscopic signature for a given molecule or mixture of molecules for the sample contained within the optical cavity. This has been demonstrated by Figure 18, in the SF CEAS case. Relative to SF CRD approaches, the SF CEAS and SF ICOS approaches may require fewer data points and less detail of the ringdown-decay profile to be recorded for a given spectral range. However, the SF CEAS and SF ICOS approaches are typically less sensitive than SF CRD approaches. A further SF cw-CRD spectroscopic embodiment of the invention employs analog circuits for real-time measurement of the ringdown time τ at each ringdown-cavity resonance, as depicted methodologically in Figures 13, 15 and 16. This SF cw-CRD approach is also useful for the puφose of recording survey spectra over a broad range of coherent-light-source/laser wavelengths, as has been demonstrated in Figure 17. Medical, Agricultural, Environmental and Industrial Applications The apparatus of the invention is suitable for use in any application where it is desirable to determine whether trace absorbing species are present in a gas-phase sample having an appropriate wavelength for use with a continuous-wave tunable coherent-light-source/laser or where it is desirable to determine an optical absoφtion spectrum of a known compound at very low concentration or with very low optical absoφtion coefficient. Suitable trace species which could be detected by the method and apparatus of the invention include, but are not limited to, carbon dioxide (C02), carbon monoxide (CO), water vapour (H20), nitric oxide (NO), oxygen (02), hydrogen fluoride (HF), nitrous oxide (N 0), hydrogen cyanide (HCN), methane (CELt), light alkanes (CnH2n+2), ethylene (C2H4), acetylene (C2H2), ethanol (C2H5OH), acetaldehyde (CH3CHO), ketones such as acetone, (CH3)2CO, chloroform (CHC13) and ammonia (NH3), either alone or simultaneously in combination. If necessary, it is feasible to perform quasi-simultaneous multi-wavelength detection by means of more than one coherent-light-source/laser, such as described above. The nature of the trace species that can be detected is limited to those species having wavelengths capable of being generated and/or transmitted by the specific components such as lasers, optical fibres, polarisation control optics used. In many medical, agricultural, environmental and industrial processes the concentration of trace species in flowing gas streams must be measured quickly and accurately. Suitable specific applications include use with smelters to determine emitted process gas outputs, for measurement of CO/C02 ratios to optimise combustion or smelting efficiency, for determining methane gas in mines or near natural gas pipelines, to monitor effluents of various kinds in high-temperature furnaces and other industrial plants, for identifying hydrocarbons in engines, to monitor exhaust emissions from traffic in road tunnels or city streets, and for determining the presence of toxic gases such as HF, HCN, nerve gas and phosgene. Another application comprises monitoring of air quality in closed environments such as the interior of an aircraft or spacecraft. Suitably a dust filter is present to remove any dust from a gas-phase sample prior to introduction into the optical cavity. The apparatus of the present invention has the advantage that, because of its ability to make instantaneous measurements, it can anticipate the presence of a hazard or potential contaminant before it becomes a problem and seriously affects the quality of the end product or environmental safety. The apparatus and method also have significant applications to scientific research and measurement technology. The following Table presents a selective survey of representative molecules that are amenable to sensitive, specific detection by optical-heterodyne-detected (OHD) CRD spectroscopy. TABLE: REPRESENTATIVE MOLECULES THAT MAY BE DETECTED BY OPTICAL-HETERODYNE SWEPT-FREQUENCY CAVITY RINGDOWN SPECTROSCOPY IN THE WAVELENGTH RANGE OF 1.25 - 2.5 μm
Figure imgf000075_0001
Figure imgf000076_0001
The overtone optical absoφtion wavelength ranges listed are accessible in various compilations of gas-phase infrared spectroscopic data, notably the HITRAN database: "The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric Workstation): 1996 edition" in Journal of Quantitative Spectroscopy and Radiative Transfer 60 (1998) 665 - 710 by L. S. Rothman et al. The tabulated survey is confined to the coherent-light-source/laser wavelength range of 1.25 - 2.5 μm, where it is feasible to take advantage of available diode lasers, fibre optics and other telecommunications and photonics components. In cases where the invention is used to facilitate multi-species detection by methods explained above, single-mode optical fibres and related optical components are selected with design characteristics to optimise the transmission and processing of the set of coherent-light-source/laser wavelengths employed within this wavelength range. Also summarised in the Table is a very wide range of potential industrial, environmental, agricultural and medical diagnostic applications for which the invention in its various embodiments offers practical, cost-effective solutions. The optional SF CEAS and SF ICOS approaches, that are included in the scope of this invention, may be useful for the puφose of recording survey spectra over a broad range of coherent-light-source/laser wavelengths. They may efficiently generate a characteristic spectroscopic signature for a given molecule or mixture of molecules for the sample contained within the optical cavity, even if spectra of the molecular species of interest are not necessarily known from an existing data base. This has been demonstrated by Figure 18, in the SF CEAS case. The SF cw-CRD spectroscopic embodiment of the invention, as described in the context of Figures 13, 15 and 16, is also useful for the puφose of recording survey spectra over a broad range of laser wavelengths, by using analog circuits for real-time measurement of the ringdown time τ at each ringdown-cavity resonance. As shown in Figure 17, this SF cw-CRD spectroscopic approach offers a particularly efficient way to generate a characteristic spectroscopic signature for a given molecule or mixture of molecules for the sample contained within the optical cavity, even if spectra of the molecular species of interest are not necessarily known from an existing data base. Different optical components are suitably used for different wavelength ranges. Where a single set of optical components used cannot simultaneously cover the characteristic wavelength range for all species present or suspected to be present in the sample, it is within the scope of this invention to combine the apparatus with a second apparatus so as to detect these species. Although the present invention has been described hereinabove with specific reference to presently preferred configurations and constructions, it will be appreciated that various modification, deletions, additions and alterations may be made to the above-described embodiments without departing from the spirit and scope of the invention.

Claims

CLAIMS 1. A method for optical detection comprising: - generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - coupling, into an optical cavity defined by at least two reflectors and containing between those reflectors a sample, at least a portion of the beam of coherent light, such that cavity- ringdown light is generated by the optical cavity each time the frequency of the coherent light reaches a value that is resonant in the optical cavity, the intensity of the cavity-ringdown light being associated with build-up and ringdown decay of intracavity radiation caused when the frequency of the coherent light respectively reaches said value and departs therefrom; - decoupling at least a portion of the cavity-ringdown light from the optical cavity; and - detecting, during a period of time, a signal attributable to a property of the sample and derived from a variation in intensity of the cavity-ringdown light decoupled from the optical cavity during the period of time.
2. A method for detecting one or more parameters of a sample located within an optical cavity defined by at least two reflectors, the method comprising: - generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - coupling into the optical cavity at least a portion of the beam of coherent light, such that cavity-ringdown light is generated when the frequency of the coherent light reaches a value that is resonant in the optical cavity; - decoupling at least a portion of the cavity-ringdown light from the optical cavity; - detecting, during a period of time and for at least some of the resonant frequencies of the range of optical frequencies of the beam of coherent light, a signal attributable to a property of the sample and derived from a temporal variation in intensity of cavity-ringdown light decoupled from the optical cavity, associated with build-up and ringdown decay of intracavity radiation; and - determining a value of one or more parameters of the sample, from the temporal variation in intensity of the cavity-ringdown light decoupled from the optical cavity.
3. A method for identifying and/or quantifying chemical species in a sample located within an optical cavity defined by at least two reflectors, the method comprising: - generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - coupling into the optical cavity at least a portion of the beam of coherent light, such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - decoupling at least a portion of the cavity-ringdown light from the optical cavity; - detecting, during a period of time, a signal attributable to a property of the chemical species and derived from the temporal variation in intensity of the cavity-ringdown light decoupled from the cavity, associated with build-up and ringdown decay of intracavity radiation; - converting the temporal variation in intensity of cavity-ringdown light decoupled from the cavity into a cavity-ringdown decay rate; and - identifying and/or quantifying at least one chemical species in the sample from the cavity- ringdown signal or a series of cavity-ringdown signals, at one or more suitable wavelengths or frequencies.
4. A method for measuring spectroscopic properties of a sample located within an optical cavity defined by at least two reflectors, separated from one another by a cavity length, the method comprising: (a) generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; (b) coupling into the optical cavity at least a portion of the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; (c) decoupling at least a portion of the cavity-ringdown light from the optical cavity; (d) measuring a signal attributable to a property of the sample and derived from a temporal variation in intensity of such cavity-ringdown light during a period of time, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation; (e) converting the temporal variation in intensity of coherent light decoupled from the cavity into a cavity-ringdown decay rate; and (f) varying the cavity length so as to vary the frequencies at which the optical cavity becomes resonant.
5. A method for measuring spectroscopic properties of a sample located within an optical cavity defined by at least two reflectors separated from one another by a cavity length, the method comprising: (a) generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies using a swept-frequency coherent light source that is tunable over at least one and optionally over at least two optical frequency ranges; (b) coupling into the optical cavity at least a portion of the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; (c) decoupling at least a portion of the cavity-ringdown light from the optical cavity; (d) measuring, during a period of time, a signal attributable to a property of the sample and derived from the temporal variation in intensity of such cavity-ringdown light, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation; (e) converting the temporal variation in intensity of coherent light decoupled from the cavity into a cavity-ringdown decay rate; (f) varying the cavity length so as to vary the frequencies at which the optical cavity becomes resonant; and (g) repeating steps (b) to (f) at least once to electronically sample coarsely resolved discrete points in the absoφtion spectrum of the sample.
6. A method as claimed in any one of claims 1 to 5, wherein the variation in intensity is or is derived from a variation in a peak amplitude of transmitted or backward-propagating optical- heterodyne-detected cavity-ringdown light.
7. A method as claimed in any one of claims 1 to 5, wherein the variation in intensity is or is derived from a variation in a peak-to peak amplitude of transmitted or backward-propagating optical-heterodyne-detected cavity-ringdown light.
8. A method as claimed in any one of claims 1 to 5, wherein the variation in intensity is or is derived from a variation in an integrated signal profile for build-up and decay of transmitted cavity-ringdown light or a rectified and integrated full-wave signal profile for backward- propagating optical-heterodyne-detected light.
9. A method as claimed in any one of claims 1 to 5, wherein the variation in intensity of the decoupled cavity-ringdown light and/or the frequency thereof is associated with a variation in a parameter of a sample.
10. A method as claimed in claim 9, wherein the parameter is the concentration of a constituent thereof or a pollutant therein.
11. A method as claimed in claim 9, wherein the frequency or a series of resonant frequencies is characteristic of a specific chemical compound.
12. A method as claimed in claim 11, wherein the rate at which the intensity of the decoupled light decays is associated with the concentration of the chemical compound.
13. A method as claimed in any one of claims 1 to 5, wherein the decoupled signal decays over a decay period and the decay period varies from about 10 ns to about 100 μs.
14. A method as claimed in any one of claims 1 to 5, wherein the beam of coherent light is generated by a device selected from a laser and a nonlinear-optical frequency converter.
15. A method as claimed in claim 14, wherein the beam of coherent light is a single- longitudinal-mode frequency output.
16. A method as claimed in any one of claims 1 to 5, wherein the range of optical frequencies over which the frequency of the coherent light is swept is at least two successive resonance frequencies of the optical cavity.
17. A method as claimed in any one of claims 1 to 5, wherein the cavity-ringdown light decoupled from the optical cavity is combined with a portion of the beam from the swept- frequency coherent light source.
18. A method as claimed in any one of claims 1 to 5, further including a step of controlling the coherent light beam formed by interrupting, deflecting or terminating the beam of coherent swept-frequency light by electronic means after it has been coupled into the optical cavity, and restoring the coherent light beam again after a second period of time.
19. A method as claimed in claim 18, wherein the electronic means comprises an amplitude modulator or a fast optical switch.
20. An optical detection system comprising: - a coherent light source capable of generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - an optical cavity, defined by at least two reflectors and adapted to, in use, contain between those reflectors a sample, the optical cavity being optically coupled, in use, to at least a portion of the beam of coherent light, such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler to decouple at least a portion of the cavity-ringdown light from the optical cavity; and - a photodetector to detect, during a period of time, a signal attributable to a property of the sample and derived from a temporal variation in intensity of such cavity-ringdown light, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation for at least one cavity resonance within the optical frequency range of the sweep.
21. An optical detection system for detecting a parameter of a sample, comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the frequency of the coherent light reaches such values, a signal attributable to the parameter of the sample and derived from the temporal variation in intensity of such cavity-ringdown light decoupled from the cavity, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation; and - an electronic processor to convert photodetector output to a cavity-ringdown decay rate and to process the cavity-ringdown signal for determining one or more parameters of the sample.
22. An optical system for identifying and/or quantifying at least one chemical species in a sample comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of optical frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the frequency of the coherent light reaches such values, a signal attributable to a property of the chemical species and derived from the temporal variation in intensity of such cavity-ringdown light decoupled from the cavity, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation; and - an electronic processor to convert photodetector output to a cavity-ringdown decay rate and to process the cavity-ringdown signal for determining one or more parameters of the sample.
23. An optical system for spectroscopically measuring a parameter or a property of a sample, the method comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of optical frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the frequency of the coherent light reaches such values, a signal attributable to the parameter or the property and derived from the temporal variation in intensity of such cavity-ringdown' light decoupled from the cavity, such variation in intensity being associated with build-up and ringdown decay of intracavity radiation;- and - an electronic processor to convert photodetector output to a cavity-ringdown decay rate and to process the cavity-ringdown signal for determining one or more parameters of the sample.
24. An optical system for measuring spectroscopic properties of a sample comprising: - a light source for generating a beam of coherent light of which the frequency is rapidly swept over a range of optical frequencies; - an optical cavity, defined by at least two reflectors, the optical cavity being adapted to receive the sample, in use, and to be optically coupled to the beam of coherent light such that cavity-ringdown light is generated each time the frequency of the coherent light reaches a value that is resonant in the optical cavity; - a decoupler for decoupling at least a portion of the cavity-ringdown light from the optical cavity; - a photodetector for detecting, during a period of time and for at least some of the frequencies of the beam of swept-frequency coherent light at which the frequency of the coherent light reaches such values, a signal attributable to a property of the sample and derived from the temporal variation in intensity of such cavity-ringdown light decoupled from the cavity, suchi variation in intensity being associated with build-up and ringdown decay of intracavity radiation; and - an electronic processor to convert photodetector output to a cavity-ringdown decay rate and to process the cavity-ringdown signal for determining one or more parameters of the sample; - an electronic processor to convert photodetector output to a cavity-ringdown signal, to control the frequency of the beam of coherent light, and to adjust the position of at least one of the cavity reflectors so as to vary the resonance frequencies of the cavity, wherein in use, the electronic processor conducts at least two frequency sweeps in which the frequency of the beam of coherent light is varied over a range of frequencies, so as to electronically sample at least two different coarsely resolved data sets from the absoφtion spectrum of the sample, at least two of such data sets corresponding to different resonance frequencies of the cavity.
25. An optical system as claimed in any one of claims 20 to 24, wherein the coherent light is generated by a coherent light source selected from a continuous-wave (cw) coherent light source (i.e., with amplitude or intensity that does not vary rapidly in time), a quasi-continuous- wave (quasi-cw) or long-pulse coherent light source, a continuous light source, a quasi-continuous light source, an optical parametric oscillator (OPO), an optical parametric generator (OPG), an optical parametric amplifier (OP A), a second-harmonic generator (SHG), a third-harmonic generator (THG); a sum-frequency generator (SFG), a difference-frequency generator (DFG) and a stimulated Raman scattering (SRS) device.
26. An optical system as claimed in any one of claims 20 to 24, wherein the laser is selected from an infrared laser, a diode laser, a single-mode continuous-wave (cw) lasers, a distributed-feedback laser, an external optical-cavity laser and an optical-fibre laser.
27. An optical system as claimed in any one of claims 20 to 24, wherein the laser has an optical bandwidth of less than 10 MHz.
28. An optical system as claimed in any one of claims 20 to 24, wherein a range of . optical frequencies that is generated during a broad swept frequency cycle is from about 1 MHz to about 10 THz.
29. An optical system as claimed in any one of claims 20 to 24, wherein the ringdown- cavity cell is selected from a linear optical cavity, a folded optical cavity and a ring.
30. A method of processing a cavity-ringdown signal by an electronic circuit for determining the decay rate of the cavity-ringdown signal, the said method including the steps of: - comparing an amplitude of the cavity-ringdown signal with each of a predetermined first electronic reference level and a predetermined second electronic reference level within an anticipated amplitude range of the cavity-ringdown signal, wherein the predetermined first electronic reference level and the predetermined second electronic reference level are different; - generating a first electronic signal when the cavity-ringdown signal becomes equal to the first predetermined electronic reference level and a second electronic signal when the cavity- ringdown signal becomes equal to the second predetermined electronic reference level; - determining a time interval between a first moment when the first electronic signal is generated and a second moment when the second electronic signal is generated; and - relating the said time interval to a cavity-ringdown decay time.
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