Continuous-Wave Frequency-Shifted Interferometry Cavity Ring-Down Gas Sensing With Differential Optical Absorption

This paper proposes a novel frequency-shifted interferometry (FSI) fiber cavity ring-down (CRD) gas sensing with dual-wavelength differential optical absorption that requires no modulation to a continuous-wave source or without fast detection and switching electronics. The fiber cavity is constructed from standard fiber optical components that include a micro-optical gas cell. FSI-CRD experiments are carried out with two wavelengths at 1531.770 and 1532.000 nm. The technique is successfully carried out by measuring acetylene-nitrogen mixtures with acetylene concentrations varying from 0% to 1.0%. A resolution of 7.8125%/dB is obtained. A minimum detectable acetylene concentration of 105.25 ppm was achieved with a 48-mm gas cell. The results show a good linear relationship between acetylene concentration and absorption loss and are in good agreement with existing theories. Dual-wavelength differential optical absorption can enhance measurement precision efficiently and eliminate the influence of various external factors. The relative deviation of measured concentration is less than ±0.29%, measured at 1.0% acetylene concentration over 70 min.