Ring resonant cavities for spectroscopy

• US 5,912,740 A
• Filed: 06/20/1997
• Issued: 06/15/1999
• Est. Priority Date: 06/20/1997
• Status: Expired due to Term

• Current Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Inventors: Martin, Juergen, Xie, Jinchun, Zare, Richard N., Paldus, Barbara A.
• Primary Examiner(s): Rosenberger, Richard A.
• Application Number:
  US08/879,975
• Time in Patent Office: 725 Days
• Field of Search: 356/440, 356/432, 356/439, 356/437, 250/343
• US Class Current: 356/437
• CPC Class Codes: G01J 3/10 Arrangements of light sources specially adapted for spectrometry or colorimetry
• CPC Class Codes: G01J 3/42 Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry beam switching arrangements G01J3/08
• CPC Class Codes: G01N 21/39 using tunable lasers

Abstract

Ring-shaped resonant cavities for spectroscopy allow a reduction in optical feedback to the light source, and provide information on the interaction of both s- and p-polarized light with samples. A laser light source is locked to a single cavity mode. An intracavity acousto-optic modulator may be used to couple light into the cavity. The cavity geometry is particularly useful for Cavity Ring-Down Spectroscopy (CRDS).

Claims

1. An optical system for performing an absorption measurement, said system comprising:

a) a light source for generating light comprising a wavelength corresponding to an absorption region of interest of a sample;

b) a ring resonant cavity in optical communication with said light source, wherein said cavity defines an intracavity light path, light emitted by said light source is incident on an input of said cavity, follows said light path, and extends from an output of said cavity, and said sample is situated in said light path; and

c) a detection means in optical communication with said output, for detecting an intensity of light of said wavelength extending from said output;

wherein said light emitted by said light source is not normal to said input, whereby light extending from said input is directed away from said light source.

2. The system of claim 1 wherein said light source comprises a laser.

3. The system of claim 2 wherein said laser is a semiconductor laser.

4. The system of claim 3 wherein said light source further comprises a locking means in optical communication with said cavity and in electrical communication with said semiconductor laser, for locking said semiconductor laser to said cavity, thereby stabilizing said semiconductor laser.

5. The system of claim 4 wherein said locking means is in optical communication with said light extending from said input.

6. The system of claim 5 further comprising a data analysis means in electrical communication with said detection means, for receiving a signal characterizing said intensity and determining a ring-down rate using said signal.

7. The system of claim 2 wherein said light source further comprises an optical isolation means in optical communication with an output of said laser, for reducing an optical feedback to said laser.

8. The system of claim 1 further comprising a wavelength tuning means for varying said wavelength within a range, for generating an absorption spectrum of said sample for said range.

9. The system of claim 2 wherein said light source is monochromatic.

10. The system of claim 9 wherein said light source is tunable.

11. The system of claim 1 wherein said wavelength is in an infrared range.

12. The system of claim 1 wherein said cavity comprises:

a) a moderately-reflective input mirror, wherein light generated by said light source is incident on said input mirror;

b) a plurality of highly-reflective intermediate mirrors situated such that light passing through said input mirror is reflected sequentially by said set of intermediate mirrors, following said light path; and

c) a moderately-reflective output mirror situated such that light extending from an intermediate mirror is incident on said output mirror, light reflected by said output mirror is incident on said input mirror, and light passing through said output mirror is incident on said detection means.

13. The system of claim 1 wherein said light path has a substantially triangular shape.

14. The system of claim 1 wherein said light path has a bowtie shape.

15. The system of claim 1 further comprising a data analysis means in electrical communication with said detection means, for receiving a signal from said detection means characterizing said intensity, and determining a value of a parameter of interest using said signal.

16. The system of claim 15 wherein said parameter of interest is a ring-down rate.

17. The system of claim 1 further comprising a reactor enclosing said cavity, wherein said light source and said detection means are situated outside said reactor.

18. The system of claim 1 wherein said light along said light path consists of a traveling wave.

19. A system for determining an optical property of a sample, said system comprising:

a) a light source for generating light comprising a wavelength corresponding to an absorption region of interest of the sample;

b) a ring resonant cavity in optical communication with said light source, wherein said cavity defines an intracavity light path, light emitted by said light source is incident on an input of said cavity, follows said light path, and extends from an output of said cavity, and said sample is situated in said light path;

c) a detection means in optical communication with said output, for detecting an intensity of light of said wavelength extending from said output; and

d) a polarization control means for controlling a polarization of light incident on said detection means.

20. The system of claim 19 wherein said polarization control means is adapted to control a polarization of light incident on said input.

21. The system of claim 19 wherein said polarization control means is adapted to receive light extending from said output.

22. The system of claim 21 wherein said polarization control means comprises a polarization separation means in optical communication with said output, for selecting light of a predetermined polarization state for transmission to said detection means.

23. The system of claim 19 wherein said detection means comprises:

a) a first detector in optical communication with said output, for detecting s-polarized light extending from said output; and

b) a second detector in optical communication with said output, for detecting p-polarized light extending from said output.

24. An optical system for performing an absorption measurement, said system comprising:

a) a light source;

b) a ring resonant cavity defining an intracavity light path;

c) an input deflection means having an on state and an off state, wherein said input deflection means is situated within said intracavity light path such that;

(i) light generated by said light source is deflected onto said intracavity light path when said input deflection means is in said on state, such that intracavity light follows said intracavity light path unidirectionally, and

(ii) said light generated by said light source is not deflected onto said intracavity light path when said input deflection means is in said off state; and

d) a detection means for detecting light of a given wavelength extending from said cavity.

25. The system of claim 24 wherein said light source is adapted to generate a light pulse incident on said input.

26. The system of claim 24 wherein said input deflection means comprises an acousto-optic modulator.

27. The system of claim 24 wherein said detection means comprises a photovoltaic detector.

28. The system of claim 24 wherein said wavelength is in an infrared range.

29. The system of claim 24 wherein said light source comprises a laser.

30. An optical system for performing an absorption measurement, said system comprising:

a) a ring-down resonant cavity defining a non-linear unidirectional light path;

b) a tunable light source in optical communication with an input of said cavity such that light extending from said input is not incident on said light source, for generating light comprising a wavelength corresponding to an absorption region of interest of said sample, wherein said light of said wavelength is incident on said cavity and rings down within said cavity according to a ring-down rate determined by an absorption of said sample at said wavelength;

c) a detection means in optical communication with said cavity, for detecting a time-dependence of an intensity of light of said wavelength within said cavity; and

d) a data analysis means in electrical communication with said detection means, for determining said ring-down rate from said time-dependence.

31. The system of claim 30 wherein said tunable light source comprises a diode laser, and wherein said system further comprises:

a locking means in electrical communication with said diode laser and in optical communication with said light extending from said input, for locking said diode laser to said cavity, thereby stabilizing said diode laser.

32. A method of performing an absorption measurement, comprising the steps of:

a) generating light comprising a wavelength corresponding to an absorption region of interest of a sample;

b) using said light to illuminate said sample, wherein said sample is situated within a unidirectional intracavity light path defined by a ring resonant cavity;

c) detecting an intracavity intensity of s-polarized light of said wavelength; and

d) detecting an intracavity intensity of p-polarized light of said wavelength.