Combining a difference-frequency source with an off-axis high-finesse cavity for trace-gas monitoring around 3 micron

High-sensitivity spectroscopy of methane around 3 mm was carried out by means of a 5.5-mW cw difference-frequency generator in conjunction with a high finesse cavity in off-axis alignment. By cavity-output integration a minimum detectable absorption coefficient of 5.7×10-9 cm-1Hz-1/2 was achieved, which well compares with results already reported in literature. Detection of methane in natural abundance was also performed in ambient air, for different values of total pressure, allowing direct concentration measurements via evaluation of the integrated absorbance of the spectra. In particular, at atmospheric pressure, a minimum detectable concentration of 850 parts per trillion by volume (pptv)×Hz-1/2 was demonstrated.