Ultrasensitive Fiber-Optic Photoacoustic Gas Sensor Based on Miniaturized Dual-Enhancement Spectrophone

A fiber-optic photoacoustic sensor (FOPAS) based on a miniaturized dual-enhancement spectrophone is proposed for high-performance sensing and analysis of trace gases in confined spaces or scenarios with fewer samples. A compact non-resonant photoacoustic cell (PAC) with a volume of 1 mL is employed in conjunction with a distributed feedback (DFB) laser to excitation of photoacoustic signals. To improve the performance of the FOPAS, a Herriott multi-pass reflection structure is constructed in the compact PAC based on two concave mirrors which effectively enhanced the excitation power. A fiber-optic acoustic sensor (FOAS) with Fabry-Perot (F-P) interference structure is applied to sense the weak photoacoustic signals and the sensitivity of trace gas detection is enhanced by the mechanical resonance enhancement of the silicon cantilever acoustic sensitive element. This dual-enhancement strategy effectively combines multi-pass excitation and resonant cantilever amplification, providing a solution to the persistent challenge of maintaining high sensitivity while achieving sensor miniaturization. Furthermore, the all-optical detection scheme employed in this gas sensing system enhances its immunity to electromagnetic interference and broadens its applicability. The experimental results indicate that the minimum detection limit (MDL) of acetylene (C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>) is 4.5 ppb when the average time is 100 s and the normalized noise equivalent absorption (NNEA) coefficient is 6×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> W/Hz<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup>.