A ppb-level H2S photoacoustic sensor with highly accurate and robust resonance frequency tracking using phase characteristics of acoustic excitation.

Photoacoustic spectroscopy (PAS) systems that employ a resonant photoacoustic cell (PAC) require accurate tracking of the resonance frequency to maintain optimal photoacoustic detection sensitivity. However, conventional frequency scanning relies on amplitude-peak searching, which is slow and susceptible to amplitude fluctuations and peak-position uncertainty. To address this, a photoacoustic gas sensor with highly accurate and robust resonance frequency tracking using the phase characteristics of acoustic excitation is proposed. A loudspeaker integrated into the PAC provides acoustic excitation independent of laser modulation and gas concentration. The resonance frequency is identified from the zero crossing of the corrected phase response, providing a highly accurate and robust resonance criterion. The method was experimentally demonstrated for H2S detection in an SF6 background. When the gas flow rate was changed from 75 to 50 sccm, phase-based resonance frequency correction increased the H2S sensitivity from 0.0318 to 0.0547 V/ppm, an improvement of approximately 72%, and the minimum detection limit was reduced from 349 ppb to 203 ppb at an optimal integration time of 277 s. The proposed method advances PAS resonance tracking from amplitude-peak searching to phase-criterion-based identification and is well suited for real-time closed-loop frequency control. It is readily extensible to other resonant photoacoustic systems, paving the way for continuous and unattended PAS operation in field environments.