An optical parametric oscillator based Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) system suitable for addressing fundamental vibrational transitions in the 3.2–3.9 μm mid-infrared (MIR) region has been realized. An Allan–Werle analysis provides a detection sensitivity of methane of 1.5×10−9 cm−1 with a 20 s integration time, which corresponds to 90 ppt of CH4 if detected at the strongest transition addressed at 40 Torr. This supersedes that of previous Db MIR NICE-OHMS demonstrations and suggests that the technique can be suitable for detection of both the environmentally important CH413 and CH3D isotopologues. It also opens up for detection of many other molecular species at ppt and sub-ppt concentration levels.
Silander, I., Hausmaninger, T., Ma, W., Harren, F. J. M., & Axner, O. (2015). Doppler-broadened mid-infrared noise-immune cavity-enhanced optical heterodyne molecular spectrometry based on an optical parametric oscillator for trace gas detection. Optics Letters, 40(4), 439-442. https://doi.org/10.1364/OL.40.000439