Stepped-Type Photoacoustic Resonator for Simultaneous Dual-Gas Detection

A stepped-type resonator-based photoacoustic (PA) sensor is proposed for simultaneous dual-gas detection. The sensor employs a dual-resonance architecture, with a stepped resonant tube capable of acoustic amplification at two different frequencies. An optimized design of the PA resonator response is conducted using finite element analysis. When the amplitudes of the two resonance peaks are high and the relative signal strength is also high, the simulated values of the two resonance frequencies are 2370 and 4220 Hz, respectively. A dual-channel synchronous digital lock-in amplifier is employed to achieve simultaneous detection in the two channels. Two lasers for methane and acetylene detection are modulated simultaneously, with the modulation frequencies set to half of these two resonant frequencies. The crosstalk evaluation results show that the signal crosstalk between the dual-gas detection channels is -125.26 dB, and the frequency crosstalk is -129.70 dB. Under a 1-s integration time, the minimum detection limits are 25.2 ppb for C₂H₂ and 51.5 ppb for CH₄, corresponding to normalized noise-equivalent absorption coefficients (NNEAs) of 1.62 × 10^-9 Wcm^{- 1} Hz^{- 1/2} and 9.98 × 10^-10 Wcm^{- 1} Hz^{- 1/2}, respectively.