A new technique for photo-acoustic Raman spectroscopy (PARS) and coherent anti-Stokes Raman spectroscopy (CARS) is proposed and demonstrated for the detection of H2 and CH4 at atmospheric pressure. Conventionally, these types of nonlinear Raman spectroscopy require two lasers whose frequency difference is tuned to the Raman frequency. In the proposed scheme, only a pulsed Nd: YAG laser is used as a pumping source, and a Raman shifter filled with the same gas to be detected is combined. This allows automatic generation of the Raman-shifted radiation. In the case of CH4, the measurement with the optimized scheme shows that detection limits up to 1 ppm for PARS and 15 ppm for CARS are achieved. The proposed PARS technique allows the measurement of the CH4 concentration in the natural air. Although the sensitivity of CARS is lower than that of PARS, the signal to noise ratio (S/N) for higher concentrations is better.
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