We demonstrate a fiber-optic Fabry-Perot acoustic sensor based on a Parylene-C diaphragm of 500 nm in thickness. The excellent diaphragm forming ability and good adhesion of Parylene-C make the sensor fabricate up to 9 mm in diameter. Acoustic testing demonstrates a high dynamic pressure sensitivity of 2239 nm/Pa and a noise limited equivalent acoustic signal level of 22.1 μPa/Hz1/2 at the frequency of 20 Hz. The frequency response exhibits an obvious peak at the frequency of 13 Hz. The sensor can be used for low-frequency acoustic pressure detections, which is necessary in surveillance and security systems.

This paper reports a mini-resonant photoacoustic sensor for high-sensitivity trace gas sensing. The sensor primarily contains a sphere-cylinder coupled acoustic resonator, a cylindrical buffer chamber, and a fiber-optic acoustic sensor. The acoustic field distributions of this mini-resonant photoacoustic sensor and the conventional T-type resonant photoacoustic sensor have been carefully evaluated, showing that the first-order resonance frequency of the present mini-resonant photoacoustic sensor is reduced by nearly a half compared to that of the T-type resonant photoacoustic sensor. The volume of the developed photoacoustic cavity is only about 0.8 cm³. Trace methane is selected as the target analytical gas and a detection limit of 101 parts-per-billion at 100-s integration time has been achieved, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 1.04 × 10^-8 W·cm^-1·Hz^-1/2. The developed mini-resonant photoacoustic sensor provides potential for high-sensitivity trace gas sensing in narrow spaces.

We present an all-optical photoacoustic (PA) multi-gas analyzer by using a digital fiber-optic acoustic detector, which consists of a spectrum demodulation based fiber Fabry-Perot (F-P) pressure sensor and a digital virtual lock-in amplifier. The concentration of multi-gas is quantified by using an intensity modulated infrared thermal source and two wavelength modulated laser diodes. Light beams from the three sources are coupled to a single nonresonant PA cell. The generated PA pressure is detected by the fiber F-P pressure sensor, which is demodulated through a near-infrared spectrometer. A lock-in white-light interferometer is developed for detecting first-harmonic (1f) and second-harmonic (2f) signals. The noise equivalent detection limits with a 100-s averaging time are 37 ppb, 9 ppb, 6 ppb, 17 ppb, 4 ppb and 60 ppb for CH4, C2H2, C2H4, C2H6, CO and CO2, respectively.

Detection of weak acoustic signals is of great significance. To achieve ultrahigh sensitivity acoustic detection, a silicon cantilever-based fiber-optic acoustic sensor (FOAS) formed by a Fabry–Perot interferometric structure is proposed in this work. Theoretical analysis and finite element analysis are used to assist the sensor design. The cantilever is fabricated by the microelectro-mechanical system (MEMS) processing technology on a silicon-on-insulator (SOI) wafer. A white light interference (WLI) demodulation system based on an amplified spontaneous emission (ASE) source is used to demodulate the cavity length of the sensor. The acoustic pressure sensitivity of the sensor was measured to be $1.753~\mu \text{m}$ /Pa at a frequency of 1 kHz and $28.75~\mu \text{m}$ /Pa at the resonance frequency of the cantilever. Experimental results indicated that the minimum detectable pressure (MDP) level of the fabricated sensor was $0.21~\mu $ Pa/Hz ^1/2 at 1 kHz, which is the lowest reported value. The silicon-based FOAS proposed in this article demonstrates its ability to detect ultraweak acoustic signals due to its extremely high sensitivity.

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.

A highly sensitive optical fiber photoacoustic (PA) sensor was proposed and demonstrated for in situ detection of dissolved gas in oil. Two optical fibers were used for the transmission of pump light and probe light, respectively. The miniature detection module integrated by optical fiber sensor and oil–gas separation membrane made it passive and anti-electromagnetic interference. The dissolved gas in oil passed through the organic membrane into the micro-chamber of the PA sensor. The PA signal generated by the gas absorption of laser energy was detected by a Fabry–Perot interferometric cantilever. To reduce the equilibrium time of oil–gas separation and improve the PA response, structural parameters of the sensor were optimized. The experimental results showed that the temperature had an influence on the response time and PA response. The response time was ~1.8 h at 50 °C. To realize temperature compensation, the temperature was measured simultaneously by averaging the measured Fabry–Perot cavity length. In addition, the detection limit of dissolved acetylene (C ₂ H ₂ ) gas reached $0.5~\mu \text{L}$ /L. Since oil pumping was not required and the oil did not contact with the ambient air, the oil was not polluted and consumed to achieve in situ measurement.