By using free-running independent femtosecond OPOs with a repetition-rate difference of 500 Hz we demonstrate methane absorption spectroscopy with spectral coverage simultaneously spanning the methane P, Q and R branches and with a resolution of 0.5 cm-1. Absolute optical frequency calibration with an accuracy of 0.25 cm-1 (0.27 nm) is achieved from simultaneous repetition-rate and carrier-envelope-offset frequency measurements, without the need for any optical reference. The calibration technique allows registration and averaging of consecutively acquired dual-comb spectra, leading to a high quality and low-noise absorbance measurement in good agreement with the HITRAN database.
Kara, O., Zhang, Z., Gardiner, T., & Reid, D. T. (2017). Dual-comb mid-infrared spectroscopy with free-running oscillators and absolute optical calibration from a radio-frequency reference. Optics Express, 25(14), 16072-16082. https://doi.org/10.1364/OE.25.016072