In this paper, a novel single fiber-type double cavity enhanced photoacoustic spectroscopy (SFDCE-PAS) sensor has been proposed and demonstrated for trace methane (CH ₄ ) sensing. The sensor mainly consists of a Fabry-Perot (F-P) chamber, a fiber-optic ceramic pin, a polyethylene terephthalate (PET) film, and a photoacoustic (PA) cavity. The PET film acts as an acoustic sensing unit and the excitation light can transmit into the PA cavity through this PET film to increase the gas absorption range. The performance of the SFDCE-PAS sensor has been carefully evaluated in comparison with a single fiber-type photoacoustic spectroscopy (SF-PAS) sensor with one cavity in a PAS system for CH ₄ sensing. It has been found out that the PA signal detected by the proposed SFDCE-PAS sensor is about 6.2 times higher than the one of the SF-PAS sensor, demonstrating superior sensitivity with a compact architecture. The proposed sensor is suitable for long-distance in-situ real-time measurement of trace CH ₄ gas in harsh environments with narrow space such as coal mines, due to its unique architecture with a single fiber and dual cavities.

We report, what we believe to be, a novel miniaturized 3D-printed Y-type resonant photoacoustic cell (YRPAC) consisting of a frustum of cone-type buffer chamber and a cylindrical resonant chamber. The volume of the designed YRPAC is about 7.0 cm³, which is only about a half of the T-resonant photoacoustic cell (TRPAC). The finite element simulation of the sound field distribution of the TRPAC and YRPAC based on COMSOL shows that the photoacoustic signal is enhanced with the shape of the buffer chamber changing from the traditional cylinder to a frustum of cone. The photoacoustic spectroscopy (PAS) system, utilizing the YRPAC and TRPAC as the photoacoustic reaction units, a 1653.7 nm distributed feedback (DFB) laser as the excitation light source, a cantilever beam acoustic sensor as the acoustic sensing unit, and a high-speed spectrometer as the demodulation unit, has been successfully developed for high-sensitivity trace CH₄ sensing. When the CH₄ concentration is 1000 ppm, the 2f signal of YRPAC in the first-order resonance mode is 2.3 nm, which is 1.7 times higher than the 2f signal amplitude of TRPAC. The detection sensitivity and minimum detection limit for the PAS system are 2.29 pm/ppm and 52.8 parts per billion (ppb) at 100 s of averaging time. The reported YRPAC has higher sensitivity, smaller size, and faster response time compared to the conventional TRPAC, which can provide a new solution for PAS development.

In this paper, an all fiber-optic photoacoustic spectroscopy (PAS) sensor combining resonance and non-resonance photoacoustic cells is reported for high-sensitivity simultaneous dual-gas detection, where the single fiber type non-resonant photoacoustic sensor and the T-type resonant photoacoustic sensor are employed to measure CH ₄ and C ₂ H ₂ , respectively. The sensor mainly consists of a silicon cantilever fiber-optic microphone (FOM), a joint resonance and non-resonance (RNR) photoacoustic cell. The silicon cantilever FOM acts not only as an acoustic sensing unit for T-type resonant photoacoustic sensor but also as a non-resonant photoacoustic sensing unit. The performance of the silicon cantilever FOM and the RNR photoacoustic cell has been optimized by theoretical and finite element analysis (FEA) methods. The detection limits of the all fiber-optic RNR-PAS sensor for CH ₄ , C ₂ H ₂ are 1.06 ppm and 0.37 ppm, with a 1 s integration time, corresponding to normalized noise equivalent absorption (NNEA) values of 6.18 × 10 ^-9 W·cm ^-1 ·Hz ^-1/2 and 4.38 × 10 ^-9 W·cm ^-1 ·Hz ^-1/2 , respectively. In addition, the detection limits for CH ₄ and C ₂ H ₂ can be extrapolated to 109.1 ppb 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.