Extrinsic Fabry–Pérot Underwater Acoustic Sensor Based on Micromachined Center-Embossed Diaphragm

Most research works emphasized on the acoustic sensor miniaturization are not optimized for applications under high surrounding pressure. For underwater acoustic sensing, measuring the small acoustic pressure from the huge hydrostatic pressure makes the design of the sensor challenging. In this paper, we attempt to solve the problem by adopting a center-embossed diaphragm as the sensitive structure. The deformation angular error is defined to evaluate the optical performance degradation and the optical sensitivity reduction under hydrostatic pressure. Simulations indicate that the central embossment is beneficial to maintain optics-related properties, although the structural sensitivity is reduced. Then, the diaphragm design guideline for underwater acoustic sensing is regulated. An optical fiber extrinsic Fabry-Perot interferometer probe based on the diaphragm, which was micromachined by double-side etching of the silicon on insulator, was designed and assembled. Experimental results show that the interferometric fringe preserves similar shapes at any tested depth from 0 (in air) to 50 cm. The recovered signal detected by the sensor coincides well with the corresponding transmitted signal. The pressure sensitivity response is flat in frequency range from 10 to 2 kHz, of which value is about -154.6 dB re. 1/μPa. It agrees well with the theoretical predication. These results demonstrated that the designed sensor according to the guideline can be used as an underwater acoustic sensor. Moreover, the sensor has potential applications in smart unmanned platforms and swiftly deployable arrays.