CA1208930A - Frequency modulation-polarization spectroscopy method and device for detecting spectral features - Google Patents
Frequency modulation-polarization spectroscopy method and device for detecting spectral featuresInfo
- Publication number
- CA1208930A CA1208930A CA000457031A CA457031A CA1208930A CA 1208930 A CA1208930 A CA 1208930A CA 000457031 A CA000457031 A CA 000457031A CA 457031 A CA457031 A CA 457031A CA 1208930 A CA1208930 A CA 1208930A
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- Prior art keywords
- polarization
- sidebands
- frequency
- light
- spectral feature
- Prior art date
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- 230000003595 spectral effect Effects 0.000 title claims abstract description 37
- 238000000034 method Methods 0.000 title claims abstract description 27
- 238000001874 polarisation spectroscopy Methods 0.000 title abstract description 9
- 230000010287 polarization Effects 0.000 claims abstract description 49
- 239000000523 sample Substances 0.000 claims abstract description 35
- 230000003287 optical effect Effects 0.000 claims abstract description 24
- 238000001228 spectrum Methods 0.000 claims abstract description 12
- 238000012544 monitoring process Methods 0.000 claims abstract description 4
- 239000013078 crystal Substances 0.000 claims description 5
- 238000004611 spectroscopical analysis Methods 0.000 description 14
- 238000010521 absorption reaction Methods 0.000 description 12
- 238000001514 detection method Methods 0.000 description 4
- 230000001066 destructive effect Effects 0.000 description 3
- 238000010586 diagram Methods 0.000 description 3
- 238000000883 frequency modulation spectroscopy Methods 0.000 description 2
- 239000007789 gas Substances 0.000 description 2
- 230000035945 sensitivity Effects 0.000 description 2
- 239000007787 solid Substances 0.000 description 2
- 230000035559 beat frequency Effects 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 239000000969 carrier Substances 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000005684 electric field Effects 0.000 description 1
- 238000000572 ellipsometry Methods 0.000 description 1
- 238000001307 laser spectroscopy Methods 0.000 description 1
- 230000015654 memory Effects 0.000 description 1
- 238000001451 molecular beam epitaxy Methods 0.000 description 1
- 230000010363 phase shift Effects 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 238000012306 spectroscopic technique Methods 0.000 description 1
Classifications
-
- 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
- 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
- G01J3/447—Polarisation 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
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/04—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by beating two waves of a same source but of different frequency and measuring the phase shift of the lower frequency obtained
-
- 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
- G01N21/19—Dichroism
Abstract
FREQUENCY MODUEATION-POLARIZATION SPECTROSCOPY
METHOD AND DEVICE FOR DETECTING
SPECTRAL FEATURES
Abstract of the Disclosure A method and device for detecting dichroic and/or birefringent narrow spectral features in a sample is described. The method includes the steps of providing a beam of light having an optical frequency bandwidth which is narrow compared to the width of the narrow spectral feature and having a center frequency .omega.c which lies near the narrow spectral feature, polarization phase modulating a beam of light with a single RF frequency to provide a pure FM spectrum having upper and lower sidebands in which either the carrier and sidebands have been polarized differently with respect to one another, exposing the sample containing the narrow spectral feature to the polarized modulated light so that the FM sidebands probe the narrow spectral feature, polarization analyzing and then photodetecting the light emerging from the sample to detect a RF beat at the specific RF frequency used for the polarization phase modulation, and electronically monitoring the amplitude of the RF beat signal to indicate the strength of the narrow spectral feature. The device includes a polarization phase modulator and a polarization analyzer positioned on opposite sides of the sample. In a preferred embodiment the polarization phase modulator produces a frequency modulated optical spectrum with the sidebands polarized precisely orthogonal to the carrier.
METHOD AND DEVICE FOR DETECTING
SPECTRAL FEATURES
Abstract of the Disclosure A method and device for detecting dichroic and/or birefringent narrow spectral features in a sample is described. The method includes the steps of providing a beam of light having an optical frequency bandwidth which is narrow compared to the width of the narrow spectral feature and having a center frequency .omega.c which lies near the narrow spectral feature, polarization phase modulating a beam of light with a single RF frequency to provide a pure FM spectrum having upper and lower sidebands in which either the carrier and sidebands have been polarized differently with respect to one another, exposing the sample containing the narrow spectral feature to the polarized modulated light so that the FM sidebands probe the narrow spectral feature, polarization analyzing and then photodetecting the light emerging from the sample to detect a RF beat at the specific RF frequency used for the polarization phase modulation, and electronically monitoring the amplitude of the RF beat signal to indicate the strength of the narrow spectral feature. The device includes a polarization phase modulator and a polarization analyzer positioned on opposite sides of the sample. In a preferred embodiment the polarization phase modulator produces a frequency modulated optical spectrum with the sidebands polarized precisely orthogonal to the carrier.
Description
3~9 FREQUENCE MODULATION-POLARIZATION SPECTROSCOPY
METHOD AND DEVICE FOR DETECTING
SPECTRAL FEATURES
DESCRIPTION
Technical Field This invention relates to spectroscopy and more particularly to a method and apparatus utilizing frequency modulation-polarization spectroscopy for detecting one or more polarization anistropic spectral features in a sample.
Background Art Polarization spectroscopy has shown considerable promise as a sensitive tool for high resolution laser spectroscop~. Polarization spectroscopy has the advantage of allowing adjustable optical intens~ty and of minimal perturbation of the sample since the intense local oscillator beam is polarized orthogonal to the probe beam. However, polarization spectroscopy suffers from the disadvantage of sensitivity to low frequency ~0 laser power fluctuations from flicker or 1/~ noise and from the disadvantage of being sensitive to background birefringence.
l2~
Frequency modulation (FM) spectroscopy as described in the patent to Bjorklund U.S. 4,297,035 and assigned to the assignee of the present invention, has also shown considerable promise as sensikive tool for high resolution laser spectroscopy. This patent is included in this application by reference thereto. The apparatus for FM spectroscopy is shown in Fig. 1. The phase modulator provides two balanced sidebands 6 as shown in Fig. lA. The polarization of the sidebands 6 is always parallel to the carrier frequency component 8. When this modulated beam passes through the sample one of the sidebands 6A probes the narrow spectra]
feature and becomes unbalanced sideband 6A'. This difference in the sidebands is detected in the photodetector to produce an RF beat signal which is monitored by the analyzing electronics.
The primary advantage of FM spectroscopy of a zero background, that is, a signal only appears when the sidekands are unbalanced. In addition, only an FM
spectrum is useful for this purpose since the othex spectrums give non-zero background, that is, noise~ FM
spectroscopy, however, has three disadvantages. One disadvantage is that there is a r~sidual amplitude nlodulation (AM) produced because the phase modulator and other optical elements are imperfect and this results in some background noise~ A second disadvantage is that the carrier ~c is strong compared with the sidebands and as a result is destructive to the sample. This is a particularly serious problem when FM spectroscopy is used in holeburning memories.
~6J ~3~
The third disadvantaye is that FM spectroscopy is insensitive to polarization anisotropy, that is, birefringence, dichroism, and optical activity.
Summary of the Invention It is a primary object of this invention to provide an improved method and apparatus for detecting a single narrow anisotropic spectral feature.
It is another object of this invention to provide an improved frequency modulation spectroscopy method and apparatus having a low level of background noise.
It is still another object of this invention to provide an improved frequency modulation speetroscopy method and apparatus in which ~he carrier ~ c is not destructive.
It is a further object of this invention to provide an improved frequency modulation spectroscopy method and apparatus that is sensitive to birefringence, dichroism, and optical activity~
These and other objects are accomplishQd through a FM-polarization spectroscopy method and device for detecting dichroic and/or birefringent narrow spectral features in a sample. The method includes the steps of providing a beam of light having an optical frequency bandwidth which is narrow compared to the width of the narrow spectral feature and having a center frequency ~2~
~ c which lies near the narrow spectral feature, polarization phase modulating a beam of light with a single RF frequency to provide a pure F~ spectrum having upper and lower sidebands in which either the carrier and/or sidebands have been produced in different states of polariæation, exposing the sample containing the narrow spectral feature to the FM
polarization-modulated light so that the FM sidebands probe the narrow spectral feature, polarizing and then photodetecting the light emerging from the sample to detect a RF beat at the specific RF frequency used for the polarization phase modulation, and electronically monitoring the amplitude of the RF beat signal to indicate the strength of the narrow spectral feature.
The device includes a polarization phase modulator and a polarization analyzer positioned on opposite sides of the sample. The sidebands may be linearly polarized at an angle 0 with respect to the carrier where ~ is any angle between O and 9O. In a preferred embodiment the polarization phase modulator produces a frequency modulated optical spectrum with the sidebands polarized precisely orthogonal to the carrier (0 = 9O). The sidebands may also be polarized circularly in a counterclockwise or clockwise manner. Similarly the carrier may be polarized circularly in the same manner.
Other objects of this invention will be apparent rom the following detailed description, reference being made to the following drawinys in which specific embodiments of the invention are shown.
Brief Description of the Drawings FI&URE 1 is a schema~ic diagram of a prior art FM
spectroscopy device;
FIGURE lA is a diagram showing the carrier and the FM sidebands of the device shown in Figure l;
FIGURE 2 shows an anisotropic pho~ochemical hole burned in an inhomogeneous absorption band;
FIGURE 3 is a schematic diagram of the device in accordance with this invention;
FIGURE 3A through 3D illustrates examples of the polarization states of the F~ sidebands and carrier obtained with the device of Figure 2.
FIGURE 4 is one particular example of a polarization phase modulator capable of producing the polarization condition o Figure 3A with ~ = 9OO
Description of the Preferred Embodiment An embodiment will now be described in terms of the device and method for the readout of information encoded by the presence or absence of anisotropic photochemical holes burned in an inhomogeneous absorption band of a sample as shown in Figure 2. The inhomogeneous absorption band 9 is the same for both x and y polarized light. The anisotropic photochemical hole 10 only effects y polarized light. The FM
SA9~83-018 5 ~2~ 3~3 polarization modulated laser beam consists of a carrier component 11 at optical frequency ~ c polarized in the x direction and of two sidebands 12 and 12A at respective optical frequencies ~ c - ~ m and ~ c ~ ~ m polarized in the y direction. The laser beam is assumed to propagate in the z direction (not shown).
As shown in Figure 3, the first step is to provide a linearly polarized narrow band laser 24 having an optical frequency ~ c~ Examples of a narrow band laser are a single frequency dye laser and a fixed single mode ~requency solid state laser. The carrier frequency 15, ~cl as shown in Figure 3A, is chosen to lie within the inhomogeneous absorption band of the sample 16. An inhomogeneous absorption band with anisotropic holes is one example of a spectral feature whose dichroism and/or birefringence vary with optical frequency. The laser has a band which is narrow compared with the desired resolution of the information and the spectral feature, that is, for example, the width of a photoche~ical hole~
~ he laser passes light into the polarization phase modulator 17 which polarizes and modulates the light from the laser source to provide FM sidebands 13A and 13B.
Electronic means 14 drives the polariæation phase modulator 17 simultaneously with a single RF frequency to reproduce light with FM sidebands in a different state of polarization than the carrier~ An example of such an electronic means 14 is an RF oscillator.
This laser beam may ha~e the sidebands polarized an angle of 0 with respect to the carrier as shown in Figure 3~. As shown in Figure 3B~ the sidebands 13 are circular in a clockwise direction. In Figure 2C the sidebands 13 are circular in a counterclockwise direction. In Figure 3D the carrier 15 is circularly polarized whereas the sidebands are linearly polarized.
While four different combinations of polarized carriers and sidebands have been shown in Figures 3A through 3D, it is understood that other co~binations may be used.
Figures 3A - 3D show that this invention covers all types of linear and circular polarizations. It also includes elliptical polari~ations. In these cases, the polarization analyzer 18 can be oriented to transmit only a portion of the caxrier and none of the sidebands or only a portion o~ the sidebands and none of the carrier. Resonances with anisotropic spectral features alter these polarization transmission/rejection conditions to produce a beat frequency at the photodetector 20.
A bit of information in the sample 16 is encoded by the presence or absence of an anisotropic hole at a location which corresponds to a particular FM upper sideband. The presence of a hole will cause a differential polari~ation change, absorption or phase shift to be experienced between the upper FM sideband 13A and the lower FM sideband 13B which correspond to the hole location. Such a differential wil] produce a heterodyne amplified beat signal with the corresponding ~..2¢~ 3~9 RF frequency at the photodetector 20 after it has passed through the polarization analyzer 18. If there is no differential, no beat signal will be produced.
An example o a photodetector 20 is a solid state PIN diode. The design of electronics 22 is straightforward and well within the state of the art.
An example of the electronics 22 is a double balanced mixer, which is driven by a local oscillator at the RF
frequency used to modulate the laser.
The analyzing electronics 22 isolates the signals due to the differential absorption from those due to the differential phase shif~ by comparing the phase of the RF beat signal to the phase of the corresponding RF
driving frequency of the modulator. If there are a large number of hole locations, it is advantageous to use only that portion of tha beat signal which is due to the differential absorption, since the combined phase differentials caused by the presence of the holes at other frequency locations can cause a spurious differential signal. The length of the time necessary or the readout is of the order of V~ , where V~ is the typical frequency spacing between hole locations.
An example of a polarization phase modulator 17 is shown in Figure 4. ~he linearly polarized beam ~40) from the narrow bend laser (42) is incident on two electroptic phase modulators (44) and (46) with optical axes (c axes) (48) and (50) respectively~ orthogonal to one another. The applied RF electric field direction ~2~93~
is always parallel to the C axis of each crystal. The angle between the direction of polarization of the linearly polarized beam (40) and the optical axis of the first modulator is 45. Both phase modulators are driven by the same RF source (52), but a phase shifter ~54) allows the relative phases of the RF waveforms driving the crystals to be adjusted to arbitrary value.
This phase is adjusted until the sidebands produced by the two crystals cancel in the initial polarization direction while at ~he same time adding cons~ructively in the direction orthogonal to the initial polarization direction. The emerging beam (56) thus has a carrier ~ c polarized in the same direction as the original beam and sidebands polarized orthogonal to the carrier.
By using this method and/or device, all of the advantages found with a FM spectroscopy device are retained. In addition, a number of the disadvantages o~ the prior art system are eliminated, For example, a residual amplitude modulation (~M) produced because the phase modulator and other optical elements are imperfect resulting in background noise, is no longer a problem due to the fact that the polarization analyzer can be oriented to totally reject all sideband light which is not perturbed by the sample. Another advantage of this device is that the carrier ~c is no longer destructive to the information provided by the sidebands. Whereas in FM spectroscopy a carrier ~c is strong to such an extent that it can perturb the sample. This particular advantage is particularly useful for holeburning memorles. A third advantage is that this method is sensi1ive to polarization anisotrophy, that is, birefringence, dichroisrn, or optical activityO The FM polarization spectroscopy method and device described herein retains the advantages of FM spectroscopy while at the same time it does not have the disadvantages associated therewith.
This method and device is also useful for those applications where polariæation spectroscopy has been heretofore used. In particular, in high resolution spectroscopy (saturation spectroscopy) of gases, an orthogonal carrier polarization permits optical heterodyne detection of anisotropies without power broadening or light shifts. Optical heterodyne detection at an RF frequency also enhances sensitivity in ellipsometry and other classical spectroscopic techniques. The use of the FM polari2ation spectrum suppresses otherwise troublesome noise signals.
This inventlon provides a basis for spectrographic diagnostlc instruments, for example, for gas phase chemical reaction or molecular beam epitaxy. This invention describes a method for detecting dichroic and/or bixefringent spectral features. The first step in this method is providing a narrow beam of light having an optical frequen~y bandwidth which is narrower than the width of the absorption line and which has a center frequency ~. The next step is to polarization phase modulate the beam of light with a single RF
frequency to provide a pure FM spectrum havlng upper SA9-83-0l8 l0 ~%013g3~9 and lower sidebands in which the carrier and sidebands have been polarized differently from one another. The sample is exposed to the modulated light so that only one of the FM sidebands probes the absorption line.
~he light emerging from the sample is then passed through a polarization analyzer to a photodetector to detect an RF b~at at the specific RF frequency used for the phase modulation. The amplitude of ~he RF beat signal is electrically monitored as the sideband frequency is varied to indicate ~he strength of the absorption line.
This invention also provides a basis for multiplex detection of a plurality of polarization anisotropic spectral features, each located at different optical frequencles. An example of such a plurality of spectral features is a plural.ity of anisotropic photochemical holes burned into an absorption band~
The multiplex detection is accomplished by providing a narrow bearn of light having an optical frequency bandwidth which is naxrower than the width of the spectral features and which has a center frequency ~c~
The next step is to polari2ation phase modulate the beam of light with a plurality of RF frequencies to provide a pure FM optical spectrum having a plurality of upper and lower sidebands i.n which the carrier and side~ands have different state~ of polarization. The sample is exposed to the modulated liyht so that each one of the upper FM sidebands probes one of the ~\
~2~
spectral features. The light emerginy from the sample then passed through a polarization analyzer to a photodetector to detect a plurality of RF beats at the specific RF frequencies used for polarization phase modulation. The amplitudes of the RF beat signals are electronically monitored in a multiplex manner to indicate the strength of eac~ of the plurality of anisotropic spectral features.
Although preferred embodiments have been described, it is understood that numerous variations may be made in accordance with the principles of this invention.
METHOD AND DEVICE FOR DETECTING
SPECTRAL FEATURES
DESCRIPTION
Technical Field This invention relates to spectroscopy and more particularly to a method and apparatus utilizing frequency modulation-polarization spectroscopy for detecting one or more polarization anistropic spectral features in a sample.
Background Art Polarization spectroscopy has shown considerable promise as a sensitive tool for high resolution laser spectroscop~. Polarization spectroscopy has the advantage of allowing adjustable optical intens~ty and of minimal perturbation of the sample since the intense local oscillator beam is polarized orthogonal to the probe beam. However, polarization spectroscopy suffers from the disadvantage of sensitivity to low frequency ~0 laser power fluctuations from flicker or 1/~ noise and from the disadvantage of being sensitive to background birefringence.
l2~
Frequency modulation (FM) spectroscopy as described in the patent to Bjorklund U.S. 4,297,035 and assigned to the assignee of the present invention, has also shown considerable promise as sensikive tool for high resolution laser spectroscopy. This patent is included in this application by reference thereto. The apparatus for FM spectroscopy is shown in Fig. 1. The phase modulator provides two balanced sidebands 6 as shown in Fig. lA. The polarization of the sidebands 6 is always parallel to the carrier frequency component 8. When this modulated beam passes through the sample one of the sidebands 6A probes the narrow spectra]
feature and becomes unbalanced sideband 6A'. This difference in the sidebands is detected in the photodetector to produce an RF beat signal which is monitored by the analyzing electronics.
The primary advantage of FM spectroscopy of a zero background, that is, a signal only appears when the sidekands are unbalanced. In addition, only an FM
spectrum is useful for this purpose since the othex spectrums give non-zero background, that is, noise~ FM
spectroscopy, however, has three disadvantages. One disadvantage is that there is a r~sidual amplitude nlodulation (AM) produced because the phase modulator and other optical elements are imperfect and this results in some background noise~ A second disadvantage is that the carrier ~c is strong compared with the sidebands and as a result is destructive to the sample. This is a particularly serious problem when FM spectroscopy is used in holeburning memories.
~6J ~3~
The third disadvantaye is that FM spectroscopy is insensitive to polarization anisotropy, that is, birefringence, dichroism, and optical activity.
Summary of the Invention It is a primary object of this invention to provide an improved method and apparatus for detecting a single narrow anisotropic spectral feature.
It is another object of this invention to provide an improved frequency modulation spectroscopy method and apparatus having a low level of background noise.
It is still another object of this invention to provide an improved frequency modulation speetroscopy method and apparatus in which ~he carrier ~ c is not destructive.
It is a further object of this invention to provide an improved frequency modulation spectroscopy method and apparatus that is sensitive to birefringence, dichroism, and optical activity~
These and other objects are accomplishQd through a FM-polarization spectroscopy method and device for detecting dichroic and/or birefringent narrow spectral features in a sample. The method includes the steps of providing a beam of light having an optical frequency bandwidth which is narrow compared to the width of the narrow spectral feature and having a center frequency ~2~
~ c which lies near the narrow spectral feature, polarization phase modulating a beam of light with a single RF frequency to provide a pure F~ spectrum having upper and lower sidebands in which either the carrier and/or sidebands have been produced in different states of polariæation, exposing the sample containing the narrow spectral feature to the FM
polarization-modulated light so that the FM sidebands probe the narrow spectral feature, polarizing and then photodetecting the light emerging from the sample to detect a RF beat at the specific RF frequency used for the polarization phase modulation, and electronically monitoring the amplitude of the RF beat signal to indicate the strength of the narrow spectral feature.
The device includes a polarization phase modulator and a polarization analyzer positioned on opposite sides of the sample. The sidebands may be linearly polarized at an angle 0 with respect to the carrier where ~ is any angle between O and 9O. In a preferred embodiment the polarization phase modulator produces a frequency modulated optical spectrum with the sidebands polarized precisely orthogonal to the carrier (0 = 9O). The sidebands may also be polarized circularly in a counterclockwise or clockwise manner. Similarly the carrier may be polarized circularly in the same manner.
Other objects of this invention will be apparent rom the following detailed description, reference being made to the following drawinys in which specific embodiments of the invention are shown.
Brief Description of the Drawings FI&URE 1 is a schema~ic diagram of a prior art FM
spectroscopy device;
FIGURE lA is a diagram showing the carrier and the FM sidebands of the device shown in Figure l;
FIGURE 2 shows an anisotropic pho~ochemical hole burned in an inhomogeneous absorption band;
FIGURE 3 is a schematic diagram of the device in accordance with this invention;
FIGURE 3A through 3D illustrates examples of the polarization states of the F~ sidebands and carrier obtained with the device of Figure 2.
FIGURE 4 is one particular example of a polarization phase modulator capable of producing the polarization condition o Figure 3A with ~ = 9OO
Description of the Preferred Embodiment An embodiment will now be described in terms of the device and method for the readout of information encoded by the presence or absence of anisotropic photochemical holes burned in an inhomogeneous absorption band of a sample as shown in Figure 2. The inhomogeneous absorption band 9 is the same for both x and y polarized light. The anisotropic photochemical hole 10 only effects y polarized light. The FM
SA9~83-018 5 ~2~ 3~3 polarization modulated laser beam consists of a carrier component 11 at optical frequency ~ c polarized in the x direction and of two sidebands 12 and 12A at respective optical frequencies ~ c - ~ m and ~ c ~ ~ m polarized in the y direction. The laser beam is assumed to propagate in the z direction (not shown).
As shown in Figure 3, the first step is to provide a linearly polarized narrow band laser 24 having an optical frequency ~ c~ Examples of a narrow band laser are a single frequency dye laser and a fixed single mode ~requency solid state laser. The carrier frequency 15, ~cl as shown in Figure 3A, is chosen to lie within the inhomogeneous absorption band of the sample 16. An inhomogeneous absorption band with anisotropic holes is one example of a spectral feature whose dichroism and/or birefringence vary with optical frequency. The laser has a band which is narrow compared with the desired resolution of the information and the spectral feature, that is, for example, the width of a photoche~ical hole~
~ he laser passes light into the polarization phase modulator 17 which polarizes and modulates the light from the laser source to provide FM sidebands 13A and 13B.
Electronic means 14 drives the polariæation phase modulator 17 simultaneously with a single RF frequency to reproduce light with FM sidebands in a different state of polarization than the carrier~ An example of such an electronic means 14 is an RF oscillator.
This laser beam may ha~e the sidebands polarized an angle of 0 with respect to the carrier as shown in Figure 3~. As shown in Figure 3B~ the sidebands 13 are circular in a clockwise direction. In Figure 2C the sidebands 13 are circular in a counterclockwise direction. In Figure 3D the carrier 15 is circularly polarized whereas the sidebands are linearly polarized.
While four different combinations of polarized carriers and sidebands have been shown in Figures 3A through 3D, it is understood that other co~binations may be used.
Figures 3A - 3D show that this invention covers all types of linear and circular polarizations. It also includes elliptical polari~ations. In these cases, the polarization analyzer 18 can be oriented to transmit only a portion of the caxrier and none of the sidebands or only a portion o~ the sidebands and none of the carrier. Resonances with anisotropic spectral features alter these polarization transmission/rejection conditions to produce a beat frequency at the photodetector 20.
A bit of information in the sample 16 is encoded by the presence or absence of an anisotropic hole at a location which corresponds to a particular FM upper sideband. The presence of a hole will cause a differential polari~ation change, absorption or phase shift to be experienced between the upper FM sideband 13A and the lower FM sideband 13B which correspond to the hole location. Such a differential wil] produce a heterodyne amplified beat signal with the corresponding ~..2¢~ 3~9 RF frequency at the photodetector 20 after it has passed through the polarization analyzer 18. If there is no differential, no beat signal will be produced.
An example o a photodetector 20 is a solid state PIN diode. The design of electronics 22 is straightforward and well within the state of the art.
An example of the electronics 22 is a double balanced mixer, which is driven by a local oscillator at the RF
frequency used to modulate the laser.
The analyzing electronics 22 isolates the signals due to the differential absorption from those due to the differential phase shif~ by comparing the phase of the RF beat signal to the phase of the corresponding RF
driving frequency of the modulator. If there are a large number of hole locations, it is advantageous to use only that portion of tha beat signal which is due to the differential absorption, since the combined phase differentials caused by the presence of the holes at other frequency locations can cause a spurious differential signal. The length of the time necessary or the readout is of the order of V~ , where V~ is the typical frequency spacing between hole locations.
An example of a polarization phase modulator 17 is shown in Figure 4. ~he linearly polarized beam ~40) from the narrow bend laser (42) is incident on two electroptic phase modulators (44) and (46) with optical axes (c axes) (48) and (50) respectively~ orthogonal to one another. The applied RF electric field direction ~2~93~
is always parallel to the C axis of each crystal. The angle between the direction of polarization of the linearly polarized beam (40) and the optical axis of the first modulator is 45. Both phase modulators are driven by the same RF source (52), but a phase shifter ~54) allows the relative phases of the RF waveforms driving the crystals to be adjusted to arbitrary value.
This phase is adjusted until the sidebands produced by the two crystals cancel in the initial polarization direction while at ~he same time adding cons~ructively in the direction orthogonal to the initial polarization direction. The emerging beam (56) thus has a carrier ~ c polarized in the same direction as the original beam and sidebands polarized orthogonal to the carrier.
By using this method and/or device, all of the advantages found with a FM spectroscopy device are retained. In addition, a number of the disadvantages o~ the prior art system are eliminated, For example, a residual amplitude modulation (~M) produced because the phase modulator and other optical elements are imperfect resulting in background noise, is no longer a problem due to the fact that the polarization analyzer can be oriented to totally reject all sideband light which is not perturbed by the sample. Another advantage of this device is that the carrier ~c is no longer destructive to the information provided by the sidebands. Whereas in FM spectroscopy a carrier ~c is strong to such an extent that it can perturb the sample. This particular advantage is particularly useful for holeburning memorles. A third advantage is that this method is sensi1ive to polarization anisotrophy, that is, birefringence, dichroisrn, or optical activityO The FM polarization spectroscopy method and device described herein retains the advantages of FM spectroscopy while at the same time it does not have the disadvantages associated therewith.
This method and device is also useful for those applications where polariæation spectroscopy has been heretofore used. In particular, in high resolution spectroscopy (saturation spectroscopy) of gases, an orthogonal carrier polarization permits optical heterodyne detection of anisotropies without power broadening or light shifts. Optical heterodyne detection at an RF frequency also enhances sensitivity in ellipsometry and other classical spectroscopic techniques. The use of the FM polari2ation spectrum suppresses otherwise troublesome noise signals.
This inventlon provides a basis for spectrographic diagnostlc instruments, for example, for gas phase chemical reaction or molecular beam epitaxy. This invention describes a method for detecting dichroic and/or bixefringent spectral features. The first step in this method is providing a narrow beam of light having an optical frequen~y bandwidth which is narrower than the width of the absorption line and which has a center frequency ~. The next step is to polarization phase modulate the beam of light with a single RF
frequency to provide a pure FM spectrum havlng upper SA9-83-0l8 l0 ~%013g3~9 and lower sidebands in which the carrier and sidebands have been polarized differently from one another. The sample is exposed to the modulated light so that only one of the FM sidebands probes the absorption line.
~he light emerging from the sample is then passed through a polarization analyzer to a photodetector to detect an RF b~at at the specific RF frequency used for the phase modulation. The amplitude of ~he RF beat signal is electrically monitored as the sideband frequency is varied to indicate ~he strength of the absorption line.
This invention also provides a basis for multiplex detection of a plurality of polarization anisotropic spectral features, each located at different optical frequencles. An example of such a plurality of spectral features is a plural.ity of anisotropic photochemical holes burned into an absorption band~
The multiplex detection is accomplished by providing a narrow bearn of light having an optical frequency bandwidth which is naxrower than the width of the spectral features and which has a center frequency ~c~
The next step is to polari2ation phase modulate the beam of light with a plurality of RF frequencies to provide a pure FM optical spectrum having a plurality of upper and lower sidebands i.n which the carrier and side~ands have different state~ of polarization. The sample is exposed to the modulated liyht so that each one of the upper FM sidebands probes one of the ~\
~2~
spectral features. The light emerginy from the sample then passed through a polarization analyzer to a photodetector to detect a plurality of RF beats at the specific RF frequencies used for polarization phase modulation. The amplitudes of the RF beat signals are electronically monitored in a multiplex manner to indicate the strength of eac~ of the plurality of anisotropic spectral features.
Although preferred embodiments have been described, it is understood that numerous variations may be made in accordance with the principles of this invention.
Claims (11)
1. A method of detecting a single, narrow polarization anisotropic spectral feature in a sample comprising providing a beam of light having an optical frequency bandwidth which is narrow compared to the width of the narrow spectral feature and having a center frequence .omega.c which lies near the feature, polarization phase modulating the beam of light with a single RF frequency to provide a pure FM
spectrum having a carrier frequency and upper and lower sidebands where the sidebands and carrier are in different states of polarization, exposing the sample containing the narrow spectral feature to the polarized modulated light so that the FM sidebands probe the narrow spectral feature, polarization analyzing the light emerging from the sample, photodetecting the light from the polarizer to detect a beat at a specific RF frequency used for polarization phase modulation, and electronically monitoring the amplitude of the RF
beat signal to indicate the strength of the narrow spectral feature.
spectrum having a carrier frequency and upper and lower sidebands where the sidebands and carrier are in different states of polarization, exposing the sample containing the narrow spectral feature to the polarized modulated light so that the FM sidebands probe the narrow spectral feature, polarization analyzing the light emerging from the sample, photodetecting the light from the polarizer to detect a beat at a specific RF frequency used for polarization phase modulation, and electronically monitoring the amplitude of the RF
beat signal to indicate the strength of the narrow spectral feature.
2. A method as described in claim 1 whereby the carrier is linearly polarized in one direction and both sidebands are linearly polarized in another direction at an angle .theta. between 0° and 90° to the first direction.
3. A method as described in claim 2 whereby .theta. is exactly 90°.
4. A method as described in claim 1 whereby the sidebands are circularly polarized and the carrier is linearly polarized.
5. A method as described in claim 1 whereby the sidebands are elliptically polarized and the carrier is linearly polarized.
6. A method as described in claim 1 whereby the carrier is circularly polarized and both sidebands are linearly polarized in the same direction.
7. A method as described in claim 1 wherein the carrier is elliptically polarized and both sidebands are linearly polarized in the same direction.
8. A method as described in claim 1 wherein a plurality of narrow polarization anisotropic spectral features are detected using light which is polarization phase modulated with a plurality of RF frequencies and a plurality of RF beat signals at the specific RF frequencies used for phase modulation are electronically monitored in a multiplex manner.
9. A device for detecting a narrow polarization anisotropic spectral feature in a sample comprising a laser source having a bandwidth narrower than the width of the narrow spectral feature and having a center carrier frequency .omega.c which lies near a selected narrow spectral feature, polarization phase modulator means for modulating the light from said laser source to a pure FM
spectrum having upper and lower sidebands, means for driving said modulator means with a single RF frequency to produce FM sideband which probe the selected narrow spectral feature in the sample, polarizer means for analyzing the light after it has passed through said sample, photodetection means to receive the light after it has passed through the polarizer, and electronic means which is capable of monitoring the intensity of the RF electrical signals from said photodetection means to indicate the strength of the selected narrow spectral feature.
spectrum having upper and lower sidebands, means for driving said modulator means with a single RF frequency to produce FM sideband which probe the selected narrow spectral feature in the sample, polarizer means for analyzing the light after it has passed through said sample, photodetection means to receive the light after it has passed through the polarizer, and electronic means which is capable of monitoring the intensity of the RF electrical signals from said photodetection means to indicate the strength of the selected narrow spectral feature.
10. A device as described in claim 8 wherein the polarization phase modulator means includes at least one phase modulator each driven by an RF
frequency waveform with independently adjustable phase.
frequency waveform with independently adjustable phase.
11. A device as described in claim 8 wherein the polarization phase modulator beam consists of two phase modulator crystals with optical axes orthogonal to one another, each crystal driven by an RF frequency waveform with independently adjustable phase, and a polarization comparator.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/511,593 US4523847A (en) | 1983-07-07 | 1983-07-07 | Frequency modulation-polarization spectroscopy method and device for detecting spectral features |
| US511,593 | 1983-07-07 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1208930A true CA1208930A (en) | 1986-08-05 |
Family
ID=24035584
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000457031A Expired CA1208930A (en) | 1983-07-07 | 1984-06-20 | Frequency modulation-polarization spectroscopy method and device for detecting spectral features |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4523847A (en) |
| EP (1) | EP0131183B1 (en) |
| JP (1) | JPS6025423A (en) |
| CA (1) | CA1208930A (en) |
| DE (1) | DE3475225D1 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4614116A (en) * | 1985-02-04 | 1986-09-30 | International Business Machines Corporation | Phase sensitive ultrasonic modulation method for the detection of strain-sensitive spectral features |
| US4594511A (en) * | 1985-03-29 | 1986-06-10 | Sri International | Method and apparatus for double modulation spectroscopy |
| JPS61293972A (en) * | 1985-06-24 | 1986-12-24 | Nisshin Flour Milling Co Ltd | 1,4-dihydroxypyridinesulfone derivative and production thereof |
| US4856899A (en) * | 1985-12-20 | 1989-08-15 | Yokogawa Electric Corporation | Optical frequency analyzer using a local oscillator heterodyne detection of incident light |
| GB8612221D0 (en) * | 1986-05-20 | 1986-06-25 | Amersham Int Plc | Assaying species in sample |
| US5036204A (en) * | 1989-07-24 | 1991-07-30 | Philip Morris, Inc. | Continuous concentration monitoring by circular dichroism |
| US5124635A (en) * | 1990-02-15 | 1992-06-23 | Photon Dynamics, Inc. | Voltage imaging system using electro-optics |
| US4983911A (en) * | 1990-02-15 | 1991-01-08 | Photon Dynamics, Inc. | Voltage imaging system using electro-optics |
| US5097201A (en) * | 1990-02-15 | 1992-03-17 | Photon Dynamics, Inc. | Voltage imaging system using electro-optics |
| US5170127A (en) * | 1991-02-19 | 1992-12-08 | Photon Dynamics, Inc. | Capacitance imaging system using electro-optics |
| US5432461A (en) * | 1991-06-28 | 1995-07-11 | Photon Dynamics, Inc. | Method of testing active matrix liquid crystal display substrates |
| JPH0772700B2 (en) * | 1991-07-05 | 1995-08-02 | 日本分光株式会社 | Phase difference control device and method |
| US5406213A (en) * | 1991-09-10 | 1995-04-11 | Photon Dynamics, Inc. | Instrument for testing liquid crystal display base plates |
| US5444385A (en) * | 1991-09-10 | 1995-08-22 | Photon Dynamics, Inc. | Testing apparatus for liquid crystal display substrates |
| US5465052A (en) * | 1991-09-10 | 1995-11-07 | Photon Dynamics, Inc. | Method of testing liquid crystal display substrates |
| US5459409A (en) * | 1991-09-10 | 1995-10-17 | Photon Dynamics, Inc. | Testing device for liquid crystal display base plate |
| US5543729A (en) * | 1991-09-10 | 1996-08-06 | Photon Dynamics, Inc. | Testing apparatus and connector for liquid crystal display substrates |
| US5504438A (en) * | 1991-09-10 | 1996-04-02 | Photon Dynamics, Inc. | Testing method for imaging defects in a liquid crystal display substrate |
| TW342447B (en) * | 1996-11-11 | 1998-10-11 | Cherng Jou | Noninvasive polarized common path optical heterodyne glucose monitoring system |
| US5949480A (en) * | 1997-03-03 | 1999-09-07 | The United States Of America As Represented By The Secretary Of The Army | Broad band imaging spectroradiometer |
| US6064488A (en) * | 1997-06-06 | 2000-05-16 | Monitor Labs, Inc. | Method and apparatus for in situ gas concentration measurement |
| US6040914A (en) * | 1997-06-10 | 2000-03-21 | New Focus, Inc. | Simple, low cost, laser absorption sensor system |
| DE19827638A1 (en) * | 1998-06-20 | 1999-12-23 | Alcatel Sa | Method for measuring interference effects on fiber optic transmission links and transmission system |
| EP0967469B1 (en) * | 1998-06-22 | 2003-11-05 | Saint-Gobain Glass France | Measuring internal strains of a transparent material like window glazings |
| US6323950B1 (en) * | 1999-09-28 | 2001-11-27 | Lucent Technologies, Inc. | Chromatic dispersion measurement for optical components |
| US6856398B2 (en) * | 2001-10-24 | 2005-02-15 | Exfo Electro-Optical Engineering Inc. | Method of and apparatus for making wavelength-resolved polarimetric measurements |
| US7230712B2 (en) * | 2003-11-03 | 2007-06-12 | Battelle Memorial Institute | Reduction of residual amplitude modulation in frequency-modulated signals |
| FR2922307B1 (en) * | 2007-10-10 | 2017-03-24 | Centre Nat Rech Scient | METHOD AND DEVICE FOR CHARACTERIZING MICROSCOPIC ELEMENTS |
| TWI456184B (en) * | 2011-12-19 | 2014-10-11 | Ind Tech Res Inst | Sepctrum detecting device and method for operation |
| KR102506803B1 (en) * | 2018-11-23 | 2023-03-07 | 삼성전자주식회사 | Method of testing an interconnection substrate and apparatus for performing the same |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3239671A (en) * | 1962-05-21 | 1966-03-08 | Gen Telephone & Elect | Single-sideband light modulator |
| US4297035A (en) * | 1979-09-14 | 1981-10-27 | International Business Machines Corporation | Method and device for detecting a specific spectral feature |
-
1983
- 1983-07-07 US US06/511,593 patent/US4523847A/en not_active Expired - Lifetime
-
1984
- 1984-03-16 JP JP59049458A patent/JPS6025423A/en active Granted
- 1984-06-20 DE DE8484107044T patent/DE3475225D1/en not_active Expired
- 1984-06-20 CA CA000457031A patent/CA1208930A/en not_active Expired
- 1984-06-20 EP EP84107044A patent/EP0131183B1/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| EP0131183B1 (en) | 1988-11-17 |
| EP0131183A2 (en) | 1985-01-16 |
| EP0131183A3 (en) | 1986-04-30 |
| JPH041861B2 (en) | 1992-01-14 |
| JPS6025423A (en) | 1985-02-08 |
| US4523847A (en) | 1985-06-18 |
| DE3475225D1 (en) | 1988-12-22 |
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