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 presents an optimized T-type resonant photoacoustic (PA) cell for methane (CH4) gas detection. The noise transmission coefficients and PA field distributions of the T-type resonant PA cell have been evaluated using the finite element method and thermoviscous acoustic theory. The optimized T-type resonant PA cell, together with a near-infrared (NIR) distributed feedback (DFB) laser source, a high-speed spectrometer and a fiber-optic acoustic sensor constitutes a PAS system for CH4 detection. The sensitivity is measured to be 1.8 pm/ppm and a minimum detectable limit (MDL) of 9 parts per billion (ppb) can be achieved with an averaging time of 500 s. The optimized T-type longitudinal resonant PA cell features of high PA cell constant, fast response time and simple manufacturing process.

A fiber-optic Fabry-Perot (F-P) accelerometer with high resolution, high dynamic range, high speed and absolute measurement capability was developed and demonstrated in elevator health monitoring. The sensor element is based on a mesh diaphragm mass-loaded compact structure, and the F-P cavity interrogation was achieved by utilizing a high-speed white light interferometry (WLI) demodulation algorithm. The displacement of inertial mass representing the change of F-P cavity was measured and translated to acceleration in real time. The results indicate a resonance frequency of 270 Hz and axial sensitivity of 3.86 μm/g within the frequency bandwidth of 10-120 Hz for the sensor. In experimental tests, the acceleration resolution achieved 8.5 μg within a $\pm 30 \rm{g}$ maximum measurement range. The performance of the sensing system was compared with a commercialized piezoelectric accelerometer in monitoring the acceleration of an elevator.

An all-optical photoacoustic spectroscopy based on lock-in white-light interferometry is proposed for trace gas detection. The cavity length of the fiber-optic Fabry–Perot cantilever microphone is demodulated by a high-speed white-light interferometer, whose spectral sampling is synchronously triggered by a phased locked signal. To improve the signal-to-noise ratio, the demodulated digital photoacoustic signal is further processed by a specially designed virtual lock-in amplifier. The designed photoacoustic spectrometer has been tested for trace acetylene (${\mathrm{C}}_2{\mathrm{H}}_2$) detection in the near-infrared region. The normalized noise equivalent absorption coefficient for ${\mathrm{C}}_2{\mathrm{H}}_2$ is achieved to be $1.1\times {10}^{-9}{\mathrm{cm}}^{-1}\mathrm{W}{\mathrm{Hz}}^{-1/2}$.

We present a fiber-optic photoacoustic (PA) sensor for remote monitoring of gas micro-leakage. The gas sensing head is a miniature ferrule-top PA cavity with a cantilever beam. Gas diffuses into the cavity from the gap around the cantilever beam, and a small hole opens on the side wall. The volume of the optimized PA cavity is only 70 μL. An erbium-doped fiber amplified laser is used as a light source of acoustic excitation. The PA pressure signal is obtained by measuring the deflection of the cantilever beam with a fiber-optic white-light interferometric readout. The experimental result of leaking acetylene (C₂H₂) gas measurement shows a real-time response of 11.2 s. A detection limit is achieved to be 20 ppb with a 1 s lock-in integration time and a 1 km conductive fiber. Since both the excitation light and probe light are transmitted by the optical fiber, the designed sensing system has the advantages of remote detection and intrinsic safety.

We present a novel T-type half-open resonant photoacoustic (PA) cell for trace gas detection. The T-type PA cell has just one buffer volume, and a fiber-optic acoustic sensor is placed at one end of the resonator. Compared with the conventional H-type PA cell, the first-order resonant frequency of the T-type PA cell is reduced by half and the PA signal is enhanced with the same resonator. The T-type resonant PA cell was used in acetylene (C₂H₂) gas detection system based on PA spectroscopy. Experimental results show that the minimum detectable limit of C₂H₂ is calculated to be 0.70 parts per billion (ppb), which is lower than the traditional H-type PA cell.