Low noise cavity enhanced absorption spectroscopy apparatus and method

Patent metadata

Field	Value
Publication number	US-11499868-B2
Application number	US-202117177146-A
Country	US
Kind code	B2
Filing date	Feb 16, 2021
Priority date	Feb 16, 2021
Publication date	Nov 15, 2022
Grant date	Nov 15, 2022

Abstract

Noise in cavity enhanced spectroscopy due to higher order mode excitation in a resonant cavity is reduced. There are two main points. The first point is that the source and detector are both fiber coupled, to provide the spatial filtering and other general advantages of fiber coupling. The second point is that the cavity is designed to ensure sufficient separation in frequency between the desired TEM00 mode and the first few higher order spatial modes.

First claim

The invention claimed is:

1. Apparatus for performing low noise cavity enhanced absorption spectroscopy, the apparatus comprising:
a resonant optical cavity configured to include a gas sample, wherein the resonant optical cavity supports TEM mnq modes having longitudinal index q and transverse indices m and n, wherein f mnq is a frequency of the TEM mnq mode;
an optical source configured to provide light to the resonant optical cavity, wherein the light from the optical source has a full-width half-maximum line width Δf L , and wherein the light from the optical source has a tunable output frequency;
an optical detector configured to receive light from the resonant optical cavity;
wherein light is coupled from the optical source to the optical cavity via first coupling optics including a first single mode optical fiber coupled to the resonant optical cavity;
wherein the first coupling optics provides first mode selective coupling of the optical source to a selected TEM mnq mode (TEM 00q0 ) of the resonant optical cavity having m=n=0 and q=q 0 , wherein the TEM 00q0 mode has a frequency f 00q0 and has a full-width half-maximum line width Δf 00q0 ;
wherein light is coupled from the resonant optical cavity to the optical detector via second coupling optics including a second single mode optical fiber coupled to the resonant optical cavity;
wherein the second coupling optics provides second mode selective coupling of the TEM 00q0 mode to the optical detector;
wherein a round trip path length of the resonant optical cavity is selected such that the modes TEM mnq for 0<m+n<13 and for all q satisfy a design condition given by |f mnq −f 00q0 |>max (Δf L , Δf 00q0 ); and
a processor configured to determine concentration of one or more gas analytes in the gas sample from measurements of loss in the resonant optical cavity.

2. The apparatus of claim 1 , wherein the first mode selective coupling provides a coupling efficiency η1 from the optical source to the TEM 00q0 mode, and wherein the first mode selective coupling provides a coupling efficiency of 0.1η1 or less from the optical source to any TEM mnq mode of the resonant optical cavity having m+n>0.

3. The apparatus of claim 1 , wherein the second mode selective coupling provides a coupling efficiency η2 from the TEM 00q0 mode to the optical detector, and wherein the second mode selective coupling provides a coupling efficiency of 0.1η2 or less from any TEM mnq mode of the resonant optical cavity having m+n>0 to the optical detector.

4. The apparatus of claim 1 , wherein the first coupling optics includes a polarizer configured to provide polarization selective excitation of modes of the resonant optical cavity.

5. The apparatus of claim 1 , wherein the first single mode optical fiber is a polarization-maintaining fiber.

6. The apparatus of claim 1 , wherein the second single mode optical fiber is a polarization-maintaining fiber.

7. The apparatus of claim 1 , wherein the resonant optical cavity is selected from the group consisting of: two mirror cavities, three-mirror ring cavities and three-mirror folded linear cavities.

8. The apparatus of claim 1 , wherein the processor is configured to determine the loss in the resonant optical cavity from a ring-down time of the resonant optical cavity.

9. The apparatus of claim 1 , wherein the processor is configured to determine the loss in the resonant optical cavity from absorption in the resonant optical cavity.

10. The apparatus of claim 1 , wherein the first coupling optics includes a Faraday isolator configured to provide optical isolation of the first single mode optical fiber from light emitted from the resonant optical cavity toward the first single mode optical fiber.

11. The apparatus of claim 1 , wherein the second coupling optics includes a Faraday isolator configured to provide optical isolation of the resonant optical cavity from light reflected by the second single mode optical fiber.

12. The apparatus of claim 1 , wherein the first single mode optical fiber and the second single mode optical fiber are selected from the group consisting of: polyimide coated optical fibers and infrared optical fibers.

13. The apparatus of claim 1 , wherein the processor is configured to determine a temperature and a pressure of the gas sample with a spectroscopic method.

14. The apparatus of claim 1 , wherein the resonant optical cavity is disposed within a pressure regulated box configured to regulate a pressure of a gas surrounding the cavity.

15. The apparatus of claim 1 , wherein the resonant optical cavity is disposed within a temperature regulated box configured to regulate a temperature of the cavity.

16. The apparatus of claim 1 , wherein the first single mode optical fiber and the second single mode optical fiber are CTE (coefficient of thermal expansion) matched to the resonant optical cavity to 10% or better.

17. The apparatus of claim 1 , wherein a volume between the first single mode optical fiber and the resonant optical cavity is sealed.

18. The apparatus of claim 1 , wherein a volume between the second single mode optical fiber and the resonant optical cavity is sealed.

19. The apparatus of claim 1 , further comprising a temperature sensor configured to measure a temperature of the resonant optical cavity.

20. The apparatus of claim 1 , further comprising a pressure sensor configured to measure a pressure of the gas sample in the resonant optical cavity.

21. The apparatus of claim 1 , further comprising an optical aperture placed in the resonant optical cavity, wherein FWHM is a full width half maximum beam diameter of the TEM 00q0 mode at a location of the aperture, wherein a beam radius parameter w=FWHM/sqrt(2 ln 2), and wherein a distance between any point on an edge of the aperture and an axis of the TEM 00q0 mode is in a range from 3 w to 12 w.

Assignees

• Picarro Inc

Inventors

• Koulikov Serguei
• Farinas Alejandro Dario

Classifications

• G01J2003/1291

  polarised, birefringent · CPC title

• G01J3/021 Primary

  using plane or convex mirrors, parallel phase plates, or particular reflectors · CPC title

• G01J3/42 Primary

  Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry (beam switching arrangements G01J3/08) · CPC title

• G01J3/0224

  using polarising or depolarising elements · CPC title

• G01J3/0218

  using optical fibers · CPC title