As one of the earliest natural phenomena to be studied, sound waves carry abundant information and energy. Acoustic sensors play a key role in fields such as fire safety, structural health monitoring, extreme environment communication, smart grid security detection, and medical imaging. With the rapid development of information technology, the demand for high-sensitivity and wide-bandwidth acoustic signal detection has increased. Recently, the research focus in acoustic sensing has shifted from electroacoustic to photoacoustic technology. Compared to electroacoustic sensors, fiber-optic acoustic sensors offer advantages such as high sensitivity, wide dynamic range, fast response, and excellent anti-electromagnetic interference performance. As these sensors are made from non-metallic, insulating materials, they are suitable for environments where electroacoustic sensors may fail, including those with strong electromagnetic interference, flammable or explosive conditions, and high temperatures or pressures.

The fiber-optic Fabry?Perot (FP) interferometer integrates optical elements into the fiber, retaining the high sensitivity and precision of traditional FP interferometers, while overcoming the challenges of size and environmental sensitivity. The fiber-optic acoustic sensor uses a double-reflector setup to create the FP cavity, leveraging the diaphragm’s simple structure, ease of fabrication, and high sensitivity. When an external sound pressure signal affects the diaphragm, it induces slight deformation, altering the length of the FP cavity, which in turn causes fluctuations in the interference spectrum and changes in the intensity and phase of the reflected light. Detailed information about the external acoustic signal can be derived by detecting these changes. The material and structure of the diaphragm are crucial to the performance of the FP cavity. Silicon nitride membranes, known for their high mechanical stiffness and superior elasticity, offer highly sensitive responses to vibrations caused by small sound waves and can effectively detect acoustic signals over a wide frequency range. MEMS processing, with its high precision, miniaturization, high yield, and ease of mass production, has gained considerable attention. It uses silicon as base material, with most diaphragm-sensitive structures being made from silicon, silicon nitride, or silicon oxide. In MEMS processing, silicon nitride films are commonly deposited using chemical vapor deposition (CVD) and are widely employed as sensitive membrane materials.

The results show that both sensor structures respond well to sound waves, with the 50 nm thick, 2 mm×2 mm exhibiting a higher response. The output voltage signals from both sensors increase linearly with sound pressure, indicating good linearity in response. The minimum detectable pressure (MDP) of the 50 nm (2 mm×2 mm) sensor is 2.57 μPa/Hz^1/2 @ 16.5 kHz, while that of the 30 nm (1 mm×1 mm) sensor is 3.71 μPa/Hz^1/2 @ 14.5 kHz, demonstrating their potential for acoustic wave sensing applications.

Using MEMS technology, two types of silicon nitride membranes are fabricated, leading to the development of fiber-optic FP acoustic wave sensors with reflection spectral extinction ratios of 26.5 and 25.0 dB, respectively. Both sensors show excellent frequency response within the human voice frequency range in audio experiments. The resonant frequency of the 50 nm thick, 2 mm×2 mm sensor is about 16.5 kHz, with a sound pressure sensitivity of 60.175 mV/Pa and MDP of 2.57 μPa/Hz^1/2. These MEMS-based fiber-optic acoustic sensors offer high sensitivity, reliability, and strong linearity, presenting great potential for future applications in acoustic sensing.