High-Sensitivity Photoacoustic Spectroscopy Gas Sensor Enhanced by a Microphone-Array-Coupled Differential Resonator

A high-sensitivity photoacoustic spectroscopy (PAS) gas sensor based on a differential photoacoustic cell (DPAC) and microphone array is reported in this article. The DPAC, with an inner volume of only 4.3 mL, was especially designed to couple with 40 microphones to enhance photoacoustic detection. Finite element analysis (FEAs) was used to optimize the acoustic characteristics inside the DPAC and then determine the distribution of the microphone array. For assessing the performance, an acetylene (C₂H₂) PAS sensor was developed using a 1531 nm distributed feedback (DFB) laser with an optical power of 30 mW, operating in the mode of wavelength modulation spectroscopy (WMS). Compared to the sensor with a single microphone, the microphone array realized 39.2 times photoacoustic enhancement and 13.7 times improvement in signal to noise ratio (SNR). The minimum detection limit (MDL) of the developed C₂H₂ PAS sensor can be improved to 8.6 ppb when the average time is extended to 754 s, which is equal to a normalized noise equivalent absorption (NNEA) of 3 × 10−10 cm−10 W/Hz1/2.