Sensitive, calibration-free and multi-channel wavelength scanning spectroscopy based on free-running laser frequency comb with Gaussian process regression algorithm for gas sensing

A new kind of wavelength scanning spectroscopy was proposed and demonstrated, based on a free-running fiber laser frequency comb centered at 1560 nm with bandwidth of 25 nm working as a highly coherent broadband light source. A phase-shifted fiber Bragg grating (PS-FBG) was used with a tunable filter to form an ultra-narrow-bandwidth transmission module and to generate narrow-linewidth probe light, scanning across the gas absorption line. The absolute gas absorption spectrum was measured by using the radio-frequency (RF) pulse intensity demodulation in the forms of frequency beating and lock-in amplification. Gaussian process regression (GPR) algorithm was developed for spectrum recovery and gas concentration retrieval, especially for conditions with low signal-to-noise ratio (SNR). The absorbance detection limits were tested as 2 × 10−4 for integration time of 0.4 s and 8.1 × 10−6 at 100 s. Multi-channel gas monitoring for CO, CO2 and NH3 was conducted via cascading PS-FBGs with different π-phase-shift wavelengths. The proposed wavelength scanning spectroscopy with capacity of absolute gas absorption spectrum measurement has advantages of calibration free, broadband, high precision and high stability, which is suitable for multi-channel gas sensing applications.