A cavity ringdown spectrometer for methane isotope analysis using a 1.65 µm distributed feedback diode laser with fiber optical feedback loop Available to Purchase

The development of a 1.65 µm cavity ringdown methane spectrometer for methane isotope analysis is reported. In order to reduce the laser linewidth, simple optical feedback with an 11 m external fiber cavity using a retroreflector was implemented and it improved the sensitivity. The detection limit at the ppt level for both ¹²CH₄ and ¹³CH₄ concentrations at 100 Torr gas pressure was evaluated from the Allan–Werle plot calculated from the dataset obtained at the fixed laser frequency. In contrast, the detection limit estimated from the baseline noise on the absorption spectrum was a few ppb for both methane isotopologues due to the periodic background oscillations that remained even after baseline correction. The system demonstrated the direct measurement of ambient methane in atmospheric room air, and the estimated ¹³CH₄ ratio as well as the methane concentration were in good agreement with the reference values of ambient air.