Wide-range and calibration-free H2S volume fraction measurement based on combination of wavelength modulation and direct absorption spectroscopy with cavity ringdown spectroscopy

Combining cavity ring down spectroscopy (CRDS) and wavelength modulated direct absorption spectroscopy (WM-DAS), a wide range and calibration-free gas concentration detection method is established, which has the advantages of high signal-to-noise ratio of CRDS and fast speed and measurable absolute concentration of WM-DAS. The baseline ring down time (tau(0)) can be calculated by measuring the absorptivity (WM-DAS) and ring down time (CRDS) at the central frequency of the spectral line, without real-time calibration, which greatly improves the speed of CRDS measurement. The measurement results of different H2S concentrations at 6336.617 cm(-1) at room temperature and atmospheric pressure show that the dynamic measurement range of this method can be extended to more than 4 orders of magnitude, the measurement accuracy is improved in comparison with WM-DAS, and the detection limit can reach 1 x 10(-9) in 40 s.