Difference-frequency-based laser spectrometer for trace CH4 detection

A laser spectrometer based on difference-frequency generation in periodically poled LiNbO₃ (PPLN) has been developed. The mid-infrared laser source was pumped by complementally two commercial narrow linewidth diode laser, to produce about 1μw of output power in the tuning region from 3.2μm to 3.7μm. The spectrometer was used for trace CH₄ measurement, a CH₄ absorption line at 3028.751 cm^-1 of v₃ band was chosen as this line is relatively free of interference by other gas species. The selected absorption line was verified using a single-pass cell detection. The detection sensitivity is estimated to be 0.8ppm for 10cm optical path length at atmospheric pressure, and is limited mainly by the etalon noise of optical surfaces.