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

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 systems (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 4-point sensor array to test its performance in detecting and localizing acoustic events. Using the generalized cross-correlation with phase transform (GCC-PHAT) algorithm, average errors of 0.95 cm (1.34 %) and 1.4 cm (1.82 %) were obtained in two-dimensional (2D) and three-dimensional (3D) location tests, respectively.