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

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.