WO2012142549A1

WO2012142549A1 - Laser multi-pass system with cell inside for spectroscopic measurements

Info

Publication number: WO2012142549A1
Application number: PCT/US2012/033709
Authority: WO
Inventors: Jacek Borysow; Manfred Fink
Original assignee: IsoSpec Technologies, LP
Priority date: 2011-04-15
Filing date: 2012-04-15
Publication date: 2012-10-18

Abstract

A laser multi-pass system comprising two spherical mirrors and a sample cell with flat windows in the middle is presented. For a window of thickness d and refractive index n, a precision adjustment of the mirror separation by ≈ 2d(l-[l/n]) provides laser beam alignment and tracing. This arrangement may be useful in Raman spectroscopy experiments and other applications.

Description

LASER MULTI-PASS SYSTEM WITH CELL INSIDE FOR SPECTROSCOPIC

MEASUREMENTS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application Serial No. 61/476, 166 filed 4/15/2011 and U.S. Patent Application Serial No. 13/016,966 filed 1/29/201 1, which are hereby incorporated by reference in their entirety for all purposes.

FIELD

[0002] Provided is a Raman spectral analyzer to measure the scattered light from a multi-pass Raman cell containing a vessel for containing the substance to be analyzed.

BACKGROUND

[0003] Raman scattering is a type of inelastic scattering of electromagnetic radiation, such as visible light, discovered in 1928 by Chandrasekhara Raman. When a beam of monochromatic light is passed through a substance some of the radiation will be scattered. Although most of the scattered radiation will have the same frequency as the incident radiation ("Rayleigh" scattering), some will have frequencies above ("anti-Stokes" radiation) and below ("Stokes" radiation) that of the incident beam. This effect is known as Raman scattering and is due to inelastic collisions between photons and molecules that lead to changes in the vibrational and/or rotational energy levels of the molecules. This effect is used in Raman spectroscopy for identifying and investigating the vibrational and rotational energy levels of molecules. Raman spectroscopy is the spectrophotometric detection of the inelastically scattered light.

[0004] "Stokes" emissions have lower energies (lower frequencies or a decrease in wave number (cm ¹)) than the incident laser photons and occur when a molecule absorbs incident laser energy and relaxes into an excited rotational and/or vibrational state. Each molecular species will generate a set of characteristic Stokes lines that are displaced from the excitation frequency (Raman shifted) whose intensities are linearly proportional to the density of the species in the sample.

[0005] "Anti-Stokes" emissions have higher frequencies than the incident laser photons and occur only when the photon encounters a molecule that, for instance, is initially in a vibrational excited state due to elevated sample temperature. When the final molecular state has lower energy than the initial state, the scattered photon has the energy of the incident photon plus the difference in energy between the molecule's original and final states. Like Stokes emissions, anti-Stokes emissions provide a quantitative fingerprint for the molecule involved in the scattering process. This part of the spectrum is seldom used for analytical purposes since the spectral features are weaker. However, the ratio of the Stokes to the anti- Stokes scattering can be used to determine the sample temperature when it is in thermal equilibrium.

[0006] The Stokes and anti-Stokes emissions are collectively referred to as spontaneous Raman emissions. Since the excitation frequency and the frequency of the Stokes (and anti- Stokes) scattered light are typically far off the excitation of any other component in the sample, fluorescence in near infrared (NIR) wavelengths is minimal. The sample is optically thin and will not alter the intensities of the Stokes emissions (no primary or secondary extinctions), in stark contrast to infrared spectroscopy.

[0007] Raman spectroscopy is a well-established technology to determine the presence of trace compounds down to very low (e.g. n mol/liter) levels. With Raman analysis, absolute densities can be determined, the sparse spectra minimize interferences, and overtones and combination lines are strongly suppressed.

[0008] Multi-pass cells are used in various scientific spectroscopy experiments as well as in industrial environments and in medical applications. Multi-pass cells are particularly important in absorption measurements of weakly absorbing species with low concentrations and Raman spectroscopy due to the extremely small cross sections. Frequently White cells are used, which typically use spherical mirrors; and Herriott cells, also known as an off axis resonator. Numerous variants of the latter one have been reported in the literature with either spherical mirrors or astigmatic mirrors. Many multi-pass absorption cells are available commercially. New designs of multi-pass cells for absorption spectroscopy are still being reported in the literature. Some of the latest are built with cylindrical mirrors, which may occupy very small volumes and have a path length of more than 50 meters. The signal-to- noise ratio in the absorption experiments typically increases with the path length. Path lengths as long as several hundred meters have been achieved in the past.

[0009] Raman Spectroscopy presents further challenges. In addition to the very small cross sections, a spectrometer analyzes the spectral components of the scattered light in the acceptance cone. Therefore, an advantageous property for a multi-pass configuration is that the excitation light source (e.g. laser) passes through the same very small scattering region. This is a main reason why the relatively large volume multi-pass cells developed for absorption measurements tend to be unsuitable for Raman spectroscopy without major modification.

[0010] One of the first multi-pass Raman tubes was reported by Waber et al. A design of multi-pass cell capable of producing a very high flux of laser light at small focal region imaged later on to the spectrometer slit was demonstrated by Hartley and Hill. Their light trapping system used an ellipsoidal mirror and a flat mirror positioned such that the laser light bouncing between two mirrors eventually collapsed on the major axis of the ellipsoidal mirror. However, this system requires custom manufacturing of ellipsoidal mirrors and tends to be difficult to align. The geometry of the present disclosure is relatively simply to align, can be built from off-the-shelf components, and offers very large signal gains (up to ~50) in comparison to single pass cell configuration.

SUMMARY

[0011] A multi-pass cell configuration was used during studies with Raman

spectroscopy on gases and liquids at low concentrations (at number densities below 10¹⁴cm^" ³). The major components of an embodiment of the presently disclosed Raman multi-pass cell are:

[0012] (1) A pair of 50.2 mm diameter concave mirrors with 100.0 mm radius of curvature, separated by a distance of about 200 mm. The nominal reflectivity of the mirrors at normal incidence is better than 99.99%.

[0013] (2) A small cylindrical vessel (5 cm long and 3.5 cm in diameter) containing gas or liquid samples placed between the mirrors. The vessel has 3mm thick windows made out of BK7 glass mounted on each side. The windows have anti-reflection coating to minimize losses. The alignment of the multi-pass cell without the sample vessel inside is

straightforward and not very complicated in the laboratory environment, as the entire system is in the open air and an experimentalist can put pieces of paper in to the laser beams and monitor the laser path.

[0014] Additional advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the aspects of the disclosure as described herein. The advantages can be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the aspects of the disclosure, as claimed. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The features, nature, and advantages of the disclosed subject matter will become more apparent from the detailed description set forth below when taken in conjunction with the accompanying drawings, wherein:

[0016] FIG. 1A shows a multi-pass system built from a pair of 50.2 mm diameter concave mirrors with 100.0 mm radius of curvature, separated by a distance of about 200 mm with a laser beam inside, without a sample cell inside;

[0017] FIG. IB shows the multi-pass system of FIG. 1A with a sample cell placed between the spherical mirrors, with the windows in the sample cell made out of BK7 glass of index of refraction equal to 1.5 and 3 mm thickness;

[0018] FIG. 1C shows shows the multi-pass system of FIG. IB after alignment correction according to the present disclosure;

[0019] FIG. 2 shows displacement of the laser beam passing through a glass window of thickness d;

[0020] FIG. 3 shows correction to alignment of a multi-pass cell with the glass window inside; and

[0021] FIG. 4 shows a graph of correction (horizontal shift of the spherical mirror) to the alignment of the multi-pass system as a function of the thickness of the windows of the sample cell.

DETAILED DESCRIPTION

[0022] The present disclosure may be understood more readily by reference to the following detailed description, examples, drawings, and claims, and their previous and following description. However, before the present devices, systems, and/or methods are disclosed and described, it is to be understood that this disclosure is not limited to the specific devices, systems, and/or methods disclosed unless otherwise specified, as such can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting.

[0023] As used in the specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an "analyzer" can include two or more such analyzers unless the context indicates otherwise. [0024] Ranges may be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0025] As used herein, the terms "optional" or "optionally" mean that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

[0026] Reference will now be made in detail to certain embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like parts.

[0027] A Raman apparatus in accordance with the present disclosure is designed to address how to modify the aligned multi-pass laser system after a cell containing a sample is placed in the middle between the two reflecting mirrors. Computations were done using the optical design program Zemax to find the changes in the optical paths. The addition of two windows alters significantly the alignment of the multi-pass configuration; this is seen by a comparison between FIG. 1A and FIG. IB.

[0028] FIG. 1A shows the laser beam travelling between two spherical mirrors crossing each time at one of the two very tight spots in the middle. FIG. IB shows trajectories of the laser beam travelling between the mirrors after the sample cell with 3 mm thick windows has been inserted. The end windows cause sufficient deviation from the original trajectories that the reflected laser beams don't cross the same spot inside the sample cell anymore, and thus the major principle of a multi-pass cell is lost. The scattering volume created by laser beams in the multi-pass system was greatly increased and the photon density in the scattering volume for the spectral analysis has become unacceptably small due to the insertion of the glass cell, which is apparent in FIG. IB.

[0029] The question arises: is there an elegant way to compensate for the distortion caused by the windows? This situation occurs in many optical systems and is quite general. At first sight this appears difficult to do as optical paths differ from ray to ray. Furthermore, the de-focusing has enlarged the size of the laser beam cross-over. However, while multi-pass cell operation involves several layers of approximation such as geometrical optics, paraxial optics, and beam diffraction, the alignment, per se, is governed by geometry only. [0030] The geometrical effect of the incident ray passage through the window is a phase delay, yielding a vertical shift (normal to the propagation direction of the laser), purely geometrically, and akin to the Goos-Hanchen shift.

[0031] FIG. 2 shows the laser beam displacement caused by the sample cell window of thickness d. In some embodiments, this window may be made of BK7 glass. We assume, for simplicity, that the initial laser beam travels along the radius 202 of left spherical mirror 200 in the multi-pass cell. The displaced beam arrives at the mirror below the normal and is reflected at the angle equal to the angle of incidence. The original beam travelling along the mirror radius would have been reflected onto itself without any deviation. The beam is displaced by:

Ay = (d / cos(P)) sin(a - β) (1) where d is the thickness of the window, a is the angle of incidence of the laser beam, β is the angle of the refracted beam computed from Snell's law, and n is the index of refraction of BK7 window at 780 nm (or whatever laser wavelength is appropriate).

[0032] The simulations show that moving the mirror horizontally will restore the alignment. This is shown in FIG. 1C. The ray tracing in that figure is the result of the Zemax calculations and was obtained by increasing the separation between the spherical mirrors until the laser beams travelling inside the cell crossed the same spots again.

[0033] After closer examination of the laser ray in FIG. 2, one can show that a move of left mirror 200 of the multi-pass cell along the optical axis will move the laser beam displaced by the sample cell window back to the radius of the left mirror. The left mirror of the multi-pass cell must be moved to the left by the distance CC as shown in FIG. 3, where the mirror is initial at position 210 and then is moved to position 212:

CC = 2(Ay)/sin(a) (2)

[0034] After using the result of Eq. 1, Eq. 2 becomes:

= (2d/(sin(a)cos( )))sini

2d(l - (tan( )/tan(a)) (3) [0035] The factor of 2 is due to the fact that there are two windows in the path of the laser beam.

[0036] After the correction described above, the laser beam arrives at the mirror at normal incidence and is reflected on itself and then displaced by the windows in the cell in such a way that it will arrive undisturbed at the right mirror of the multi-pass cell. For small incident angles Eq. 3 can be written as:

CC = 2d(l - (tan(P)/tan(a))

~ 2d(l - sin( )/sin(a))

~ 2d(l - 1/n) (4)

[0037] Where n is index of refraction of the windows assuming that Snell's law for small angles was used. The proposed alignment procedure for sample cell windows inside the multi-pass cell as thick as 6 mm was experimentally verified.

[0038] In the case where the sample cell is filled with liquid (i.e. when the sample has an index of refraction that is not approximately equal to 1), Eq. 4 is modified as follows:

CC ~ 2d(l - 1/n) + L(l - l/n_L) (5)

[0039] Where L is the length of the sample and ¾ is the index of refraction of the sample.

[0040] Additionally, the correction to the alignment of the multi-pass configuration was verified by placing two 3 mm thick windows inside it. An excellent agreement between experimental, simulations and model prediction computed from Eq. 4 is shown in FIG. 4. The solid line in FIG. 4 is the result of the model calculations. The circles are the results of simulation done using the Zemax software, and the triangles are the outcome of the laboratory experiment.

[0041] The present disclosure provides a simple solution to the seemingly very complex problem of making corrections to the alignment of a multi-pass laser system after placing a glass vessel with flat windows in the multi-pass path.

[0042] It will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. Other embodiments of the disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the disclosure being indicated by the following claims.

Claims

CLAIMS What is claimed is:

1. A multi-pass Raman cell comprising:

a laser source emitting a laser beam;

a pair of concave mirrors comprising a multi-pass cavity having an optical axis, said concave mirrors having approximately the same radius of curvature (r);

a cell for containing a substance to be measured via Raman spectroscopy, said cell positioned between said pair of concave mirrors and having two transparent faces approximately normal to said optical axis, said transparent faces having approximately the same thickness (d) and index of refraction (n);

at least one lens operable to collect Raman light and provide an image of said Raman light to a spectrograph operable to separate said Raman light according to wavelength; and a filter operable to remove at least a portion of Rayleigh scattered light from said Raman light,

wherein said concave mirrors are separated by a distance of approximately 2r + 2d(l -

1/n).

2. The multi-pass Raman cell of Claim 1, wherein said laser source comprises a laser diode.

3. The multi-pass Raman cell of Claim 2, wherein said laser diode comprises a grating-locked laser diode.

4. The multi-pass Raman cell of Claim 3, wherein said filter comprises a holographic notch filter.

5. The multi-pass Raman cell of Claim 4, wherein said holographic notch filter has an optical density of at least approximately 5 for said Rayleigh scattered light.

6. The multi-pass Raman cell of Claim 3, wherein said filter comprises an atomic vapor absorption filter.

7. The multi-pass Raman cell of Claim 6, wherein said atomic vapor absorption filter comprises a rubidium absorption filter.

8. The multi-pass Raman cell of Claim 3, further comprising a Dove prism operable to rotate said image of said Raman light.

9. The multi-pass Raman cell of Claim 3, wherein said spectrograph further comprises a chilled CCD camera, said chilled CCD camera comprising an avalanche photo diode.

10. The multi-pass Raman cell of Claim 9, wherein said chilled CCD camera is coupled to a programmed computer operable to analyze said Raman light.

11. A multi-pass Raman cell comprising:

a laser source emitting a laser beam;

a cell for containing a substance to be measured via Raman spectroscopy, said substance having an sample index of refraction (n_L), said cell positioned between said pair of concave mirrors and having two transparent faces approximately normal to said optical axis, said transparent faces having approximately the same thickness (d) and a window index of refraction (n), said cell having an interior length (L);

wherein said concave mirrors are separated by a distance of approximately 2r + 2d(l - l/n) + L(l - l/n_L).

12. The multi-pass Raman cell of Claim 1 1, wherein said laser source comprises a laser diode.

13. The multi-pass Raman cell of Claim 12, wherein said laser diode comprises a grating-locked laser diode.

14. The multi-pass Raman cell of Claim 13, wherein said filter comprises a holographic notch filter.

15. The multi-pass Raman cell of Claim 14, wherein said holographic notch filter has an optical density of at least approximately 5 for said Rayleigh scattered light.

16. The multi-pass Raman cell of Claim 13, wherein said filter comprises an atomic vapor absorption filter.

17. The multi-pass Raman cell of Claim 16, wherein said atomic vapor absorption filter comprises a rubidium absorption filter.

18. The multi-pass Raman cell of Claim 13, further comprising a Dove prism operable to rotate said image of said Raman light.

19. The multi-pass Raman cell of Claim 13, wherein said spectrograph further comprises a chilled CCD camera, said chilled CCD camera comprising an avalanche photo diode.

20. A method of aligning a multi-pass Raman cell, said method comprising:

providing a pair of concave mirrors comprising a multi-pass cavity having an optical axis, said concave mirrors having approximately the same radius of curvature (r);

providing a cell for containing a substance to be measured via Raman spectroscopy, said cell positioned between said pair of concave mirrors and having two transparent faces approximately normal to said optical axis, said transparent faces having approximately the same thickness (d) and index of refraction (n);

separating said concave mirrors by a distance of approximately 2r + 2d(l - 1/n).