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.

The femtosecond laser has emerged as a powerful tool for micro- and nanoscale device fabrication. Through nonlinear ionization processes, nanometer-sized material modifications can be inscribed in transparent materials for device fabrication. This paper describes femtosecond precision inscription of nanograting in silica fiber cores to form both distributed and point fiber sensors for sensing applications in extreme environmental conditions. Through the use of scanning electron microscope imaging and laser processing optimization, high-temperature stable, Type II femtosecond laser modifications were continuously inscribed, point by point, with only an insertion loss at 1 dB m⁻¹ or 0.001 dB per point sensor device. High-temperature performance of fiber sensors was tested at 1000 °C, which showed a temperature fluctuation of ±5.5 °C over 5 days. The low laser-induced insertion loss in optical fibers enabled the fabrication of a 1.4 m, radiation-resilient distributed fiber sensor. The in-pile testing of the distributed fiber sensor further showed that fiber sensors can execute stable and distributed temperature measurements in extreme radiation environments. Overall, this paper demonstrates that femtosecond-laser-fabricated fiber sensors are suitable measurement devices for applications in extreme environments.

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, we realize a wideband fiber-optic Fabry-Perot (F-P) acoustic sensing (FPAS) scheme by utilizing a high-speed absolute cavity length demodulation with a 70-kHz maximum line rate spectrometer. The wideband FPAS is made of a pre-stress stainless-steel diaphragm based on F-P interferometric structure. The real-time absolute F-P cavity lengths are calculated by a phase demodulation method, which is realized by processing the interference image at a 70-kHz frame rate. Acoustic signal is obtained by extracting the AC component of the demodulated cavity length. The experimental results show that the spectrometer can be running at a 50-kHz line rate, and an acoustic detection wideband of 20 Hz to 20 kHz is obtained. The noise-limited minimum detectable sound pressure level is 18.8 dB, which is sensitive enough for the communication of human voice. The proposed wideband acoustic sensing scheme achieves good robustness, which is promising as a speech-sound microphone for communication during the magnetic resonance imaging procedure.

This Letter presents an approach to produce multiplexable optical fiber chemical sensor using an intrinsic Fabry–Perot interferometer (IFPI) array via the femtosecond laser direct writing technique. Using the hydrogen-sensitive palladium (Pd) alloy as a functional sensory material, Pd alloy coated IFPI devices can reproducibly and reversibly measure hydrogen concentrations with a detection limit of 0.25% at room temperature. Seven IFPI sensors were fabricated in one fiber and performed simultaneous temperature and hydrogen measurements at seven different locations. This Letter demonstrates a simple yet effective approach to fabricate multiplexable fiber optical chemical sensors for use in harsh environments.