Cavity enhanced laser based isotopic gas analyzer

• US 8,665,442 B2
• Filed: 06/29/2012
• Issued: 03/04/2014
• Est. Priority Date: 08/18/2011
• Status: Active Grant

• Current Assignee: Li & S Company Limited (Yesco Holdings Co., Ltd.)
• Original Assignee: Li & S Company Limited (Yesco Holdings Co., Ltd.)
• Inventors: Kachanov, Alexander, Koulikov, Sergeui
• Primary Examiner(s): Geisel, Kara E
• Application Number:
  US13/538,620
• Publication Number:
  US 20130044314A1
• Time in Patent Office: 613 Days
• Field of Search: 356 72- 73, 356/437, 356/454, 356/519
• US Class Current: 356/437
• CPC Class Codes: G01J 3/4338 Frequency modulated spectrometry
• CPC Class Codes: G01N 2021/1704 in gases
• CPC Class Codes: G01N 21/1702 with opto-acoustic detection, e.g. for gases or analysing solids
• CPC Class Codes: G01N 21/39 using tunable lasers
• CPC Class Codes: G01N 21/59 Transmissivity G01N21/25 takes precedence
• CPC Class Codes: G01N 21/61 Non-dispersive gas analysers G01N21/3504 takes precedence
• CPC Class Codes: G01N 2201/023 Controlling conditions in casing
• CPC Class Codes: G01N 2201/06113 Coherent sources; lasers
• CPC Class Codes: G01N 2291/021 Gases
• CPC Class Codes: G01N 29/2425 optoacoustic fluid cells therefor

Abstract

Systems and methods for measuring the isotope ratio of one or more trace gases and/or components of gas mixtures such as different gas species present in a gas mixture. The system includes a resonant optical cavity having two or more mirrors and containing a gas, the cavity having a free spectral range that equals the difference between frequencies of two measured absorption lines of different gas species in the gas, or of two different isotopes, divided onto an integer number. The system includes a continuous-wave tunable laser optically coupled with the resonant optical cavity and a detector system for measuring an absorption of laser light by the gas in the cavity. The detector system includes a photo-detector to measure an intensity of the intra-cavity light, or both a photo-acoustic sensor to measure photo-acoustic waves generated in the cavity and a photo-detector to measure an intensity of the intra-cavity light.

Claims

1. A method of performing an absorption measurement in a cavity having two or more cavity mirrors, the method comprising the steps of:

a) tuning a cavity mode of the cavity to a first wavelength corresponding to an absorption line of a first one of at least two different isotopes that have equal or close quantum numbers;

b) coupling light of the first wavelength with the cavity;

c) measuring a signal corresponding to an absorption coefficient at the first wavelength;

d) tuning the cavity mode to a second wavelength corresponding to an absorption line of a second isotope;

e) coupling light of the second wavelength with the cavity;

f) measuring a signal corresponding to an absorption coefficient at the second wavelength; and

g) calculating the isotope ratio based on two measured signals.

2. The method of claim 1, wherein measuring a signal corresponding to an absorption coefficient includes 1) measuring the intracavity optical power at the corresponding wavelength, or 2) measuring photo-acoustic waves generated in the cavity and measuring the intracavity optical power at the corresponding wavelength.

3. The method of claim 2, wherein the difference between the quantum numbers is one of −

2, −

1, 0, +1, or +2.

12. The method of claim 1, further including determining a baseline by measuring an absorption coefficient at a wavelength that does not correspond with an absorption line of any of the isotopes being measured or analyzed.

4. A method of performing an absorption measurement in an optical cavity having two or more mirrors, the method comprising the steps of:

a) selecting or adjusting an optical cavity length such that the difference between frequencies of measured absorption lines of different isotopes is a product of an integer number and free spectral range of the cavity;

b) coupling light of a first wavelength with the cavity, the first wavelength corresponding to an absorption line of one of the isotopes;

c) measuring a first signal corresponding to an absorption coefficient at the first wavelength;

d) coupling light of a second wavelength with the cavity, the second wavelength corresponding to an absorption line of a second isotope;

e) measuring a second signal corresponding to an absorption coefficient at the second wavelength; and

f) calculating an isotope ratio based on the first and second measured signals.

5. The method of claim 4, wherein measuring a signal corresponding to an absorption coefficient includes 1) measuring the intracavity optical power at the corresponding wavelength, or 2) measuring photo-acoustic waves generated in the cavity and measuring the intracavity optical power at the corresponding wavelength.

6. The method of claim 4, further including determining a baseline.

7. The method of claim 6, wherein determining a baseline includes measuring an absorption coefficient at a wavelength that does not correspond with an absorption line of any of the isotopes being measured or analyzed.

8. A method of performing an absorption measurement of two different gas species in a cavity having two or more mirrors, the method comprising of the steps of:

a) selecting or adjusting a cavity length such that a difference between frequencies of the measured absorption lines of the two different gas species is a product of an integer number and the free spectral range of the cavity;

b) coupling light of a first wavelength with the cavity, the first wavelength corresponding to an absorption line of a first one of the gas species;

c) measuring a first signal corresponding to an absorption coefficient at the first wavelength;

d) coupling light of a second wavelength with the cavity, the second wavelength corresponding to an absorption line of a second one of the gas species;

e) measuring a second signal corresponding to an absorption coefficient at the second wavelength; and

f) determining concentrations or absorption coefficients of the first and second gas species based on the first and second measured signals.

9. The method of claim 8, wherein measuring signals corresponding to absorption coefficients includes 1) measuring an intracavity optical power at the corresponding wavelengths, or 2) measuring photo-acoustic waves generated in the cavity and measuring an intracavity optical power at the corresponding wavelengths.

10. The method of claim 8, further including determining a baseline.

11. The method of claim 10, wherein determining a baseline includes measuring an absorption coefficient at a wavelength that does not correspond with an absorption line of any of the gas species being measured or analyzed.