Detection of Methane and Ethane by Continuous-Wave Cavity Ring-Down Spectroscopy Near 1.67 μm

We report the simple detection method of the small hydrocarbons, methane and ethane, by continuous-wave cavity ring-down spectroscopy near 1.67 μm using an external cavity diode laser. The absorption lines of methane between 6002.48 cm-1 and 6003.37 cm-1 and ethane between 5955.65 cm-1 and 5956.4 cm-1 have been resolved and employed for the gas detection. The largest absorption cross sections were found to be 6.5×10-20 cm2 and 7.4×10-21 cm2 for methane and ethane, respectively, in each spectral range. The minimum detectable absorption limit of our spectrometer was 4.8×10-9 cm-1/ , which corresponds to the detection limits of 3 ppb/ and 27 ppb/ for methane and ethane, respectively. The near-IR continuous-wave cavity ring-down spectroscopic detection method of the small hydrocarbons can be applied for medical diagnosis and environmental monitoring as a fast and convenient method.