Optical Fiber Fabry-Perot Underwater Acoustic Sensor Based on Dual Hetero-Structured Silicon Micro-Diaphragm

This paper presents a MEMS-based underwater acoustic sensor featuring a dual hetero-structure silicon diaphragm, operating on the principle of extrinsic Fabry-Perot interferometer(EFPI). The sensor exhibits miniaturized dimensions, high sensitivity, and excellent consistency. The hetero-structure consists of SiO₂ and Si₃N₄ layers fabricated through thermal oxidation and chemical vapor deposition(CVD) on silicon wafers. Photolithography and reactive ion etching(RIE) techniques were employed to create a composite microstructure combining a central planar region with concentric annular corrugations. This dual hetero-structure design effectively mitigates internal stresses while maintaining structural integrity and enabling linear high-sensitivity response. A step structure was integrated to precisely control the Fabry-Perot(F-P) cavity length and mirror alignment, ensuring stable optical performance. The sensor incorporates bend-insensitive small mode-field fiber to construct the EFPI, significantly enhancing anti-interference capability during signal transmission. Three prototype sensors with diaphragm diameters of ϕ1.3mm, ϕ1.1mm, and ϕ0.9mm (uniform thickness: 1.3μm) were fabricated. The quartz fiber ferrules were precisely aligned and bonded using epoxy resin to form complete acoustic sensors. Broadband interference spectra characterization revealed an average F-P cavity length of 46μm with less than 1μm variation and optical visibility exceeding 0.94, demonstrating excellent optical stability. Acoustic sensitivity tests showed a maximum sensitivity of -150dB (ref rad/μPa) with frequency response fluctuations within ±2dB across the 20Hz-2kHz band. The inter device sensitivity variation was also maintained below ±2dB, with phase inconsistency limited to ±5° throughout the operational bandwidth. Additional measurements indicated a noise floor below -90dB (ref rad) and a linear dynamic range exceeding 60dB. The proposed MEMS sensor architecture and fabrication methodology demonstrate significant potential for cost-effective, high-performance mass production of underwater acoustic sensors.