Mid-Infrared Doppler-Free Saturation Absorption Spectroscopy of the Q Branch of CH4 ν3=1 Band using a Rapid-Scanning Continuous-Wave Optical Parametric Oscillator

We have developed a mid-infrared Doppler-free saturation absorption spectroscopy apparatus using a continuous-wave optical parametric oscillator (CW-OPO). Here we report a comprehensive spectral scan of the Q branch transitions of the ν3=1 band of methane (CH4) with an average line width (FWHM) of 4.5 MHz. The absolute frequency calibration was achieved using previously reported transition frequencies determined using optical frequency combs, while a Fabry-Pérot etalon was used for relative frequency calibration. We report 12 transitions with improved accuracies of ~3 MHz.