High-Sensitivity MEMS Fiber-Optic Acoustic Sensor With Submicrometer Multilayer Diaphragm

The conventional acoustic emission (AE) sensors, such as piezoelectric and capacitive sensors, are susceptible to electromagnetic interference (EMI), while previously reported fiber-optic AE sensors encounter issues with reproducibility and high cost. To overcome these limitations, we have developed a micro-electromechanical system (MEMS) fiber-optic Fabry–Perot (FP) acoustic sensor using bulk-silicon-micromachined 710-nm diaphragms. The thin diaphragm was fabricated in a multilayer (Au/Ti/Si₃N₄) configuration to achieve high sensitivity and mechanical strength for acoustic sensing applications. More than 250 diaphragms with high reflectivity and easy-to-align features, such as sloped walls and alignment grooves, could be produced from a single batch process. Using a tunable laser, we confirmed the consistent spectral properties of the integrated FP sensors, including a uniform free spectral range (FSR) and an interference contrast greater than 21 dB. From acoustic sensing experiments, the sensitivity and minimum detectable pressure (MDP) were measured to be 179.3 mV/Pa and 247.6 μPa/√Hz at 140 kHz, respectively. The frequency response was almost flat in the 20–150-kHz range.We constructed a four-point sensor array to test its performance in detecting and localizing acoustic events. Using the generalized cross correlation with phase transform (GCC-PHAT) algorithm, the average errors of 0.95 cm (1.34%) and 1.4 cm (1.82%) were obtained in 2-D and 3-D location tests, respectively.