Photoacoustic Spectroscopy Dual-Gas Sensor for CH ₄ and NH ₃ Based on Dual-Path Dense Spot Multipass Cell and Dual-Frequency Photoacoustic Cell

In this paper, a highly sensitive methane (CH₄) and ammonia (NH₃) dual-gas photoacoustic spectroscopy (PAS) gas sensor based on a dual-path high-spot-density multipass cell (MPC) and dual-frequency photoacoustic cell (PAC) was demonstrated for the first time. A dual-path MPC mathematical model was established to achieve multiple independent rings and 223 excitations of dual gases. In order to integrate with the MPC more conveniently, the designed dual-frequency PAC was used to excite the Helmholtz resonance mode. A Raman fiber amplifier (RFA) and an erbium-doped fiber amplifier (EDFA) were employed to amplify the output power of the two diode lasers to enhance the excitation intensity. The experimental results indicated that when the output power was set as 50 and 300 mW, respectively, the 2<i>f</i> signals for CH₄ and NH₃ detection with the high-spot-density MPC showed 41.91-fold and 41.30-fold enhancements compared to the system without MPC, demonstrating significant performance improvement. The minimum detection limits (MDLs) of CH₄ and NH₃ were found to be 65 and 374 ppb, respectively, when the average time of the CH₄/NH₃-PAS dual-gas sensor was 1000 s.