Non-dispersive optical gas sensing based on free-running fiber laser frequency comb with a narrowband filter

An optical gas sensing method without absorption spectrum scanning is proposed based on optical frequency comb (OFC). The light emission of OFC goes through a narrow-band-pass optical filter to target a molecular absorption line, and intensity of the transmitted laser pulse through the gas medium is measured for absorbance detection. For measurement of this high-speed pulse, a two-stage frequency down-conversion scheme including electronical frequency beating and lockin amplification is employed. A dual-channel optical path with light-gas interaction cell and reference light channel is configured for normalization of the light signal to suppress the intensity noise. Meanwhile, the beat frequency of the reference light channel is tracked and fed to the lock-in amplifier as a real-time frequency compensation, to further stabilize the sensing system. Carbon dioxide (CO2) is chosen as the gas under test by filtering the output spectrum of a fiber laser frequency comb at ~ 1572.33 nm with a bandwidth of 0.1 nm. The high repetition frequency of ~ 41 MHz is downconverted to ~ 50kHz and the generated beat note is then lock-in amplified to calculate the CO₂ concentration. After calibration within the concentration range of 0% - 30%, stability of the system is evaluated. According to an Allan deviation analysis based on a long-term zero-gas measurement, the minimum detectable absorbance is ~ 0.0031 for an integration time of 0.1 s. Then a repeatability test indicates a high detection linearity of 0.99953.