Numerical simulations on the performance of optical-acoustic sensors of minimal dimensions

Abstract

This paper presents analytical and numerical methods for developing optical-acoustic transducers of minimal dimensions. One can find acoustic sensors used as microphones in various electronic devices such as smartphones or smartwatches. Therefore, it is highly desirable to minimize their size while ensuring high-quality sound reception. The optical-acoustic sensor relies on laser detection of membrane vibrations and consists of a membrane that vibrates in the presence of an acoustic field and reflects the radiation emitted by the laser back to the laser. We focus on methods to optimize the membrane's design and the cavity (back volume) that separates the laser from the membrane. The back volume compliance significantly affects the sensitivity of the membrane. In addition, it is a noise source due to acoustic and viscous damping. Using calculations and simulations, we show the possibilities of reducing the membrane size and the air-filled back volume size while achieving the desired acoustic properties. We employ analytical calculations for the mechanical vibration of the diaphragm, back-volume compliance and resistance, and precise FEM simulations of the interaction between membrane vibration and the acoustic field. We build on similar techniques used for micromachined capacitive microphones, but we apply these methods newly to a specific setup of backplate-less optical-acoustic sensors. Based on the theoretical results, we can conclude that optical-acoustic devices achieve the same maximum noise level with smaller dimensions than the current industry standard.