All Fiber-Optic Photoacoustic Spectroscopy Sensor Combining Resonance and Nonresonance Photoacoustic Cells for High-Sensitivity Simultaneous Dual-Gas Sensing

In this article, an all fiber-optic photoacoustic spectroscopy (PAS) sensor combining resonance and nonresonance (RNR) photoacoustic cells (PACs) is reported for high-sensitivity simultaneous dual-gas detection, where the single-fiber-type nonresonant photoacoustic sensor and the T-type resonant photoacoustic sensor are employed to measure CH4 and C2H2, respectively. The sensor mainly consists of a silicon cantilever fiber-optic microphone (FOM), a joint RNR PAC. The silicon cantilever FOM acts not only as an acoustic sensing unit for T-type resonant photoacoustic sensor but also as a nonresonant photoacoustic sensing unit. The performance of the silicon cantilever FOM and the RNR PAC has been optimized by theoretical and finite element analysis (FEA) methods. The detection limits of the all fiber-optic RNR-PAS sensor for CH4 and C2H2 are 1.06 and 0.37 ppm, with a 1-s integration time, corresponding to normalized noise equivalent absorption (NNEA) values of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.18\times 10^{-{9}}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.38\times 10^{-{9}}$ </tex-math></inline-formula> W<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> cm<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}} \cdot $ </tex-math></inline-formula> Hz−1/2, respectively. In addition, the detection limits for CH4 and C2H2 can be extrapolated to 109.1 and 35.18 ppb, respectively, when the averaging time is prolonged to 100 s. The developed all fiber-optic RNR-PAS sensor features an all-optical configuration with high sensitivity and all-fiber optic signal excitation and transmission, paving the way for remote trace dual-gas simultaneous sensing.