Near infrared ammonia sensing system based on on-axis cavity-enhanced absorption spectroscopy using board-level mode-locked circuit

We proposed a near-infrared gas sensing system based on mode-locked cavity-enhanced absorption spectroscopy. A distributed feedback fiber laser (DFB-FL) with a central wavelength of 1550 nm was employed as the light source, coupling with a Fabry–Perot (F-P) cavity with an effective optical path length of 42.7 m served as the gas cell. By utilizing a homemade mode-locked electric circuit and Pound–Drever–Hall (PDH) technique, the mode-locking between the DFB-FL and F-P cavity was achieved with long-term fluctuation of 3.2%. System performance was verified using ammonia as the analyte gas with a target line at 1550.17 nm. Allan variance analysis revealed that the minimum detection limit was 15.8 ppm with an averaging time of 216 s, corresponding to an absorption coefficient of 2.3 × 10⁻⁷ cm⁻¹.