Resonant Cell-Based 1<i>f</i> Photoacoustic Gas Analyzer Immune to Light Power Fluctuation and Frequency Mismatch

To overcome the light power fluctuation and frequency mismatch in photoacoustic spectroscopy (PAS), we proposed a self-corrected 1<i>f</i>-only resonant cell-based PAS (1<i>f</i>-RCPAS) gas analyzer. Based on the theoretical analysis of the 1<i>f</i> signal, a signal processing algorithm considering laser power-current nonlinearity is proposed. The 1<i>f</i>-only algorithm is well-tailored for the resonant systems, requiring no time-division multiplexing. The algorithm is further improved to extend the dynamic range. The T-type resonant cell incorporating a graphene sticker is utilized for effectively amplifying the acoustic signals from both the gas and solid to achieve normalization. No optical path alignment is needed. For the low resonance frequency, a digital orthogonal-vector lock-in amplifier is used, further simplifying the system setup. The gas analyzer is used to measure methane (CH₄) with the near-infrared absorption peak at 1651 nm. The experiments demonstrated immunity to fiber coupling loss, laser power drift over time, and frequency mismatch caused by property differences between air and standard gases. The <i>R</i>² value in the concentration calibration reaches 0.99995, and the minimum detection limit given by the Allan variance reaches 3.5 ppb at an average time of 105 s.