Trace gas detection with 2-photon, 2-color, cavity ring-down spectroscopy

1. A method of detecting a trace gas, the method comprising:

containing the trace gas in an optical cavity;

injecting a first laser light from a first laser into the optical cavity causing the trace gas to transition from an energy state lower than a first excited energy state to the first excited energy state;

injecting a second laser light from a second laser into the optical cavity causing the trace gas to transition from the first excited energy state to a second excited energy state; and

measuring, by a detector, a first cavity ringdown intensity as a function of time after turning off the second laser.

2. The method of claim 1 , further comprising:

turning off the first laser or stopping the injecting the first laser light into the optical cavity;

injecting the second laser light from the second laser into the optical cavity to transition from the first excited energy state to the second excited energy state, in the absence of the first laser light;

turning off the second laser; and

measuring, by the detector, a second cavity ringdown intensity as a function of time, wherein the second cavity ringdown indicates a cavity ringdown rate for the cavity with no absorption from the trace gas, and with absorption from molecular species other than the trace gas and with cavity absorption.

3. The method of claim 2 , wherein the trace gas is identified based on the first cavity ringdown intensity and the second cavity ringdown intensity over a measurement time.

4. The method of claim 1 , wherein the first laser is a pump laser and the second laser is a probe laser, and wherein the pump and probe lasers are each frequency-locked to a cavity resonance with frequency close to a molecular transition frequency.

5. The method of claim 1 , wherein the second energy state is a higher energy state than the first energy state.

6. The method of claim 1 , wherein the optical cavity is high finesse.

7. The method of claim 4 , wherein the pump laser is horizontally polarized and the probe laser is vertically polarized, or the pump laser is vertically polarized and the probe laser is horizontally polarized.

8. The method of claim 1 , wherein the optical cavity comprises two mirrors.

9. The method of claim 1 , wherein the optical cavity is configured as a ring comprising three or more mirrors.

10. The method of claim 1 , wherein the detector is a mercury cadmium telluride (MCT) detector or an indium antimonide (InSb) detector.

11. The method of claim 1 , wherein the first laser or the second laser is a quantum cascade (QCL) laser.

12. The method of claim 1 , wherein the transitions are coherent two-photon transitions or stepwise transitions with a populated intermediate state.

13. The method of claim 1 , wherein after the injecting the first laser light and the injecting the second laser light and before measuring the first cavity ringdown intensity, collisions involving the trace gas cause energy transitions from the first excited energy state to one or more third excited energy states different from the second excited energy state.

14. A trace gas detection apparatus comprising:

an optical cavity containing a trace gas;

a first laser configured to inject a first laser light into the optical cavity to cause the trace gas in the analyte to pump to a first excited energy state;

a second laser configured to inject a second laser light into the optical cavity to cause the trace gas to pump from the first excited energy state to a second excited energy state; and

a detector configured to measure a first cavity ringdown intensity as a function of time after the second laser is turned off.

15. The apparatus of claim 14 , wherein the detector is further configured to measure a second cavity ringdown after the first laser is turned off or injection of the first laser light into the optical cavity is stopped, wherein the second cavity ringdown indicates a cavity ringdown rate for the cavity with no absorption from the trace gas, and with absorption from molecular species other than the trace gas and with cavity absorption.

16. The apparatus of claim 15 , wherein the trace gas is identified based on the first cavity ringdown intensity and the second cavity ringdown intensity over a measurement time.

17. The apparatus of claim 14 , wherein the first laser is a pump laser and the second laser is a probe laser, and wherein the pump and probe lasers are each configured to operate frequency-locked to a cavity resonance with a frequency close to a molecular transition frequency.

18. The apparatus of claim 14 , wherein the optical cavity is high finesse.

19. The apparatus of claim 17 , wherein the pump laser and the probe laser are configured to produce one of:

output light from the pump laser that is horizontally polarized and output light from the probe laser that is vertically polarized,

output light from the pump laser that is vertically polarized and output light from the probe laser that is horizontally polarized,

output light from the pump laser that is right-hand circularly polarized and output light from the probe laser that is left-hand circularly polarized, or

output light from the pump laser that is left-hand circularly polarized and output light from the probe laser is right-hand circularly polarized.

20. The apparatus of claim 14 , wherein the optical cavity comprises two mirrors.

21. The apparatus of claim 14 , wherein the optical cavity is configured as a ring comprising three or more mirrors.

22. The apparatus of claim 14 , wherein the detector is a mercury cadmium telluride (MCT) detector or an indium antimonide (InSb) detector.

23. The apparatus of claim 14 , wherein the first laser or the second laser is a quantum cascade (QCL) laser.