High-sensitivity photoacoustic spectroscopy gas sensor enhanced by a microphone-array-coupled differential resonator

A high-sensitivity photoacoustic spectroscopy (PAS) gas sensor based on differential photoacoustic cell (DPAC) and microphone array is reported in this paper. The DPAC with inner volume of only 4.3 mL was specially designed to couple with 40 microphones to enhance the photoacoustic detection. Finite element analysis was utilized to optimize the acoustic characteristics inside the DPAC, and then determine the distribution of microphone array. To assess the performance, an acetylene (C
₂
H
₂
) PAS sensor was developed using a 1531 nm DFB laser with an optical power of 30 mW, operating in the mode of wavelength modulation spectroscopy. Compared to the sensor with single microphone, the microphone array realized 39.2 times photoacoustic enhancement and 13.7 times improvement on signal to noise ratio (SNR). The minimum detection limit (MDL) of the developed C
₂
H
₂
PAS sensor can be improved to 8.6 ppb when the average time is extended to 754 s, which equals to a normalized noise equivalent absorption (NNEA) of 3×10
^-10
cm
^-1
W/Hz
^1/2
.