A novel, compact, ultrasensitive, and inexpensive He-Ne laser intracavity photothermal deflection spectrometer has been developed. In this apparatus a gas sample is placed inside a cavity of the He-Ne laser, which serves as a probe laser. A CO2 excitation laser coaxially overlaps with the He-Ne laser beam inside the sample cell. The detection limit for ethanol vapor in air at 1 atm excited with 1.5 W of CO2 laser chopped at 13.3 Hz is estimated to be 2.5 ppb.
Chieu D. Tran, "Intracavity Helium-Neon Laser Photothermal Deflection as a Sensitive Technique for Trace Gas Analysis," Appl. Spectrosc. 40, 1108-1110 (1986)