Laser based cavity enhanced optical absorption gas analyzer with laser feedback optimization

• US 8,659,758 B2
• Filed: 10/04/2011
• Issued: 02/25/2014
• Est. Priority Date: 10/04/2011
• Status: Active Grant

• Current Assignee: Li & S Company Limited (Yesco Holdings Co., Ltd.)
• Original Assignee: Li & S Company Limited (Yesco Holdings Co., Ltd.)
• Inventors: Koulikov, Serguei, Kachanov, Alexander
• Primary Examiner(s): STAFIRA, MICHAEL PATRICK
• Application Number:
  US13/252,915
• Publication Number:
  US 20130083328A1
• Time in Patent Office: 875 Days
• Field of Search: 356432-444
• US Class Current: 356/436
• CPC Class Codes: G01N 21/1702 with opto-acoustic detection, e.g. for gases or analysing solids
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01N 21/39 using tunable lasers
• CPC Class Codes: G01N 21/8507 Probe photometers, i.e. with optical measuring part dipped into fluid sample

Abstract

Optical feedback assisted cavity enhanced absorption spectroscopy systems and methods for measuring trace gases with improved long-term stability and reproducibility include a laser coupled with a resonant optical cavity containing a gaseous medium and having at least two cavity mirrors and a plurality of optical resonance cavity modes. The laser emits continuous wave laser light with a mean optical frequency of the laser being adjustable over a range of frequencies, and the laser is responsive to optical feedback light emerging from the cavity. The transmissivity of at least one of the cavity mirrors is selected such that the intensity of the optical feedback light impinging on the laser is below a threshold intensity value so as to ensure that a frequency hold interval range of the optical frequency of the laser is smaller than a free spectral range of the cavity.

Claims

1. A system for detecting one or more analyte species present in a gaseous or liquid medium, the system comprising:

a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, the cavity having a plurality of optical resonance cavity modes;

a laser that emits continuous wave laser light, wherein the laser is capable of being scanned whereby a mean optical frequency of the laser is adjustable over a range of frequencies, and wherein the laser is responsive to optical feedback light emerging from the cavity;

mode matching optics configured to couple the laser light to the cavity via the cavity coupling mirror; and

a detector configured to measure an intensity of the intracavity optical power of light circulating in the cavity and to generate a signal representing the intracavity optical power of light circulating in the cavity

wherein the transmissivity of at least one of the cavity mirrors is selected such that the intensity of the optical feedback light impinging on the laser is below a threshold intensity value so as to ensure that a frequency hold interval range of the optical frequency of the laser is smaller than a free spectral range of the cavity.

2. The system of claim 1, wherein the cavity has a configuration selected from the group consisting of a ring cavity having three or more cavity mirrors, a linear cavity having two or more cavity mirrors, and a V-shaped cavity having three cavity mirrors.

3. The system of claim 1, further including an acoustic sensor incorporated inside the optical cavity to measure a photo-acoustic signal generated by the gaseous or liquid medium within the cavity.

4. The system of claim 1, further including a means for adjusting the mean optical frequency of the laser so as to scan the mean optical frequency of the laser over a cavity resonance peak.

5. The system of claim 1, wherein the cavity is capable of being scanned whereby an optical frequency of a cavity resonance mode peak is adjustable over a range of frequencies.

6. The system of claim 5, further including a means for controlling a position of one of the cavity mirrors so as to scan the optical frequency of the cavity resonance mode peak.

7. The system of claim 1, wherein the laser has a linearly polarized output and wherein the cavity has two sets of linearly polarized modes orthogonal to each other.

8. The system of claim 7, wherein an orientation of the laser relative to the cavity is adjustable so that the output polarization of the laser makes a non-zero angle with respect to the polarization of the cavity modes so that the intensity of the optical feedback light impinging on the laser is below the threshold intensity value.

9. The system of claim 8, wherein the non-zero angle is selected so as to provide optical feedback intensity for each set of modes below the threshold value.

10. The system of claim 1, wherein as the mean optical frequency of the laser is sequentially adjusted (scanned) over the range of frequencies, the laser locks to sequential cavity modes.

27. The system of claim 1, wherein as the mean optical frequency of the laser is scanned over said range of frequencies of the laser, sequential locking to the plurality of optical resonance cavity modes that have a frequency within said range of frequencies of the laser occurs.

30. The system of claim 1, further including a means for adjusting a phase of the optical feedback light.

11. A system for detecting one or more analyte species present in a gaseous or liquid medium, the system comprising:

a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, the cavity having a plurality of optical resonance cavity modes, wherein the cavity has two sets of linearly polarized cavity modes orthogonal to each other;

a laser that emits continuous wave laser light, wherein the laser has a linearly polarized output, wherein the laser is capable of being scanned whereby a mean optical frequency of the laser is adjustable over a range of frequencies, and wherein the laser is responsive to optical feedback light emerging from the cavity;

mode matching optics configured to couple the laser light to the cavity via the cavity coupling mirror; and

a detector configured to measure an intensity of the intracavity optical power of light circulating in the cavity and to generate a signal representing the intracavity optical power of light circulating in the cavity

wherein an orientation of the laser relative to the cavity is selected so that the output polarization of the laser makes a non-zero angle with respect to the polarization of the cavity modes so that the intensity of the optical feedback light impinging on the laser is below a threshold intensity value so as to ensure that a frequency hold interval range of the optical frequency of the laser is smaller than a free spectral range of the cavity.

12. The system of claim 11, wherein the cavity has a configuration selected from the group consisting of a ring cavity having three or more cavity mirrors, a linear cavity having two or more cavity mirrors, and a V-shaped cavity having three cavity mirrors.

13. The system of claim 11, further including a first detector configured to measure an intensity of the intracavity optical power of light circulating in the cavity and to generate a signal representing the intracavity optical power of light circulating in the cavity.

14. The system of claim 11, further including a means for adjusting the mean optical frequency of the laser so as to scan the mean optical frequency of the laser over a cavity resonance peak.

15. The system of claim 11, wherein the cavity is capable of being scanned whereby an optical frequency of a cavity resonance peak is adjustable over a range of frequencies.

16. The system of claim 11, wherein as the mean optical frequency of the laser is sequentially adjusted (scanned) over the range of frequencies, the laser locks to sequential cavity modes.

17. The system of claim 11, wherein the orientation of the laser relative to the cavity is adjustable so that the output polarization of the laser makes a non-zero angle with respect to the polarization of the cavity modes so that the intensity of the optical feedback light impinging on the laser is below the threshold intensity value.

18. The system of claim 17, wherein the non-zero angle is selected so as to provide optical feedback intensity for each set of modes below the threshold value.

19. The system of claim 11, further including an acoustic sensor incorporated inside the optical cavity to measure a photo-acoustic signal generated by the gaseous or liquid medium within the cavity.

28. The system of claim 11, further including a means for adjusting the intensity of the optical feedback light.

29. The system of claim 28, wherein the means for adjusting includes an optical attenuator element positioned between the laser and the cavity along a path of the optical feedback light.

20. A system for detecting one or more analyte species present in a gaseous or liquid medium, the system comprising:

a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, the cavity having a plurality of optical resonance cavity modes;

a laser that emits continuous wave laser light, wherein the laser is capable of being scanned whereby a mean optical frequency of the laser is adjustable over a range of frequencies, and wherein the laser is responsive to optical feedback light emerging from the cavity;

mode matching optics configured to couple the laser light to the cavity via the cavity coupling mirror; and

a detector configured to measure an intensity of the intracavity optical power of light circulating in the cavity and to generate a signal representing the intracavity optical power of light circulating in the cavity

wherein the transmissivity of a laser output coupler is selected such that intensity of the optical feedback light impinging on the laser is below a threshold intensity value so as to ensure that a frequency hold interval range of the optical frequency of the laser is smaller than a free spectral range of the cavity.

21. A method for detecting one or more analyte species present in a gaseous or liquid medium using a laser that that emits continuous wave laser light and a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, wherein the laser is responsive to optical feedback light emerging from the cavity, and wherein a mean optical frequency of the laser is adjustable over a range of frequencies, the method comprising:

coupling the laser light to the cavity via the cavity coupling mirror using mode matching optics, the cavity having a plurality of optical resonance cavity modes;

adjusting a mean optical frequency of the laser so as to scan the mean optical frequency of the laser over one or more of the optical resonance cavity modes that have a frequency within said range of frequencies of the laser;

selecting the transmissivity of at least one of the cavity mirrors such that intensity of optical feedback light impinging on the laser is below a threshold intensity value so as to ensure that a frequency hold interval range of the optical frequency of the laser is smaller than a free spectral range of the cavity; and

measuring an intensity of the intracavity optical power of light circulating in the cavity and generating a signal representing the intracavity optical power of light circulating in the cavity.

22. The method of claim 21, wherein the cavity has a configuration selected from the group consisting of a ring cavity having three or more cavity mirrors, a linear cavity having two or more cavity mirrors, and a V-shaped cavity having three cavity mirrors.

23. The method of claim 21, wherein the laser has a linear polarized output and wherein the cavity has two sets of linearly polarized modes orthogonal to each other, the method further including adjusting an orientation of the laser relative to the cavity so that the output polarization of the laser makes a non-zero angle with respect to the polarization of the cavity modes.

24. A method for detecting one or more analyte species present in a gaseous or liquid medium using a laser that that emits continuous wave laser light and a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, wherein the laser is responsive to optical feedback light emerging from the cavity, wherein the laser has a linear polarized output, and wherein a mean optical frequency of the laser is adjustable over a range of frequencies, the method comprising:

coupling the laser light to the cavity via the cavity coupling mirror using mode matching optics, wherein the cavity has a plurality of optical resonance cavity modes, and wherein the cavity has two sets of linearly polarized cavity modes orthogonal to each other;

adjusting a mean optical frequency of the laser so as to scan the mean optical frequency of the laser over one or more of the optical resonance cavity modes that have a frequency within said range of frequencies of the laser;

adjusting an orientation of the laser relative to the cavity so that the output polarization of the laser makes a non-zero angle with respect to the polarization of the cavity modes such that the intensity of optical feedback light impinging on the laser is below a threshold intensity value so as to ensure that a frequency hold interval range of the optical frequency of the laser is smaller than a free spectral range of the cavity; and

measuring an intensity of the intracavity optical power of light circulating in the cavity and generating a signal representing the intracavity optical power of light circulating in the cavity.

25. The method of claim 24, wherein the cavity has a configuration selected from the group consisting of a ring cavity having three or more cavity mirrors, a linear cavity having two or more cavity mirrors, and a V-shaped cavity having three cavity mirrors.

26. The method of claim 24, wherein the non-zero angle is selected so as to provide optical feedback intensity for each set of cavity modes below the threshold value.