Cavity enhanced laser based isotopic gas analyzer
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 also 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 one of a photo-detector configured to measure an intensity of the intra-cavity light or both a photo-acoustic sensor configured to measure photo-acoustic waves generated in the cavity and a photo-detector configured to measure an intensity of the intra-cavity light.
1. A gas analyzer system for measuring a concentration of a component in a gas mixture, the system comprising:
a resonant optical cavity having two or more mirrors and containing a gas having a chemical species to be measured, wherein the resonant optical cavity defines a plurality of cavity modes having a free spectral range (FSR);
control circuitry configured to adjust or tune an optical path length of the resonant optical cavity so that a frequency of one of the plurality of cavity modes corresponds to an absorption frequency of the chemical species;
a continuous-wave tunable laser optically coupled with the resonant optical cavity, wherein an output of the laser is scanned across a range of frequencies including the frequency of said one of the plurality of cavity modes; and
a detector system for measuring an absorption of laser light by the gas in the resonant optical cavity.
2. The system of claim 1 , further including;
a temperature sensor for measuring a temperature of the gas in the resonant optical cavity; and
a pressure sensor for measuring a pressure of the gas in the resonant optical cavity.
3. The system of claim 1 , wherein the detector system includes a photo-detector configured to measure an intensity of intra-cavity light.
4. The system of claim 1 , wherein the absorption frequency of the chemical species corresponds to an absorption frequency of an isotope of the chemical species.
5. The system of claim 1 , wherein the resonant optical cavity is disposed in a housing that provides an airtight seal for the resonant optical cavity, and wherein the temperature and the pressure of the gas in the housing are actively controlled.
6. The system of claim 1 , further comprising a temperature control element configured to control a temperature of the gas in the resonant optical cavity and a pressure control element configured to control a pressure of the gas in the resonant optical cavity.
7. A method of performing an absorption measurement in a cavity having two or more cavity mirrors, wherein the cavity has a plurality of resonant cavity modes having a free spectral range (FSR), the method comprising the steps of:
a) tuning the cavity so that a frequency of one of the plurality of cavity modes corresponds to an absorption frequency of a first chemical species in a gas to be measured;
b) coupling output light of a continuous-wave tunable laser with the cavity;
c) scanning the output light of the laser across a range of frequencies including the frequency of said one of the plurality of cavity modes;
d) measuring a signal corresponding to an absorption at the frequency of said one of the plurality of cavity modes; and
e) calculating an absorption coefficient of the chemical species in the gas based on the measured signal.
8. The method of claim 7 , wherein tuning includes adjusting an optical path length of the cavity.
9. The method of claim 8 , wherein adjusting an optical path length includes adjusting a relative position of one of the cavity mirrors and/or adjusting a pressure of the gas in the cavity and/or adjusting a temperature of the gas in the cavity.
10. The method of claim 7 , wherein scanning includes adjusting a current applied to the laser and/or adjusting a temperature of a laser medium of the laser.
11. The method of claim 7 , further including calculating a concentration of the chemical species based on the absorption coefficient.