Design of a MEMS Optical Microphone Transducer Based on Light Phase Modulation

Acoustic sensing through optical transduction represents a promising alternative to the conventional capacitive sensing used in MEMS microphones, especially when aiming at ultralow-noise applications. In fact, the traditional acoustic to electrical transduction stages are decoupled by the intermediate conversion of the signal into the optical domain. As a result, the mechanical design of the sensor has no direct influence on the electrical readout performance, and this allows for a significant reduction of the MEMS transducer noise through aggressive acoustically semi-transparent stator designs that represent one of the limits of the standard capacitive technologies. This article reports the design and the modeling of the sensing elements of a MEMS optical microphone. The basic transduction mechanism is presented, and the main design parameters and challenges are explained and analyzed with advanced modeling techniques; the measurement results are finally compared to the expected performance.

I. Introduction

MEMS-based silicon microphones are currently the most popular solution for consumer electronics applications due to their extraordinary miniaturization, relatively low production costs, and reliability. However, the constantly growing market of consumer electronics is continuously demanding for higher performance devices; one of the main trends is the increase of microphones’ signal-to-noise ratio (SNR). This trend seems intrinsically impossible to be satisfied by the present MEMS technology. A potential option for satisfying such demand is the development of disruptive technologies that allow to overcome the performance plateau that capacitive MEMS solutions have been facing in the last years. An increasingly popular audio sensing alternative is represented by optical-based solutions. Optical microphones solutions and design techniques were proposed in [1], [2], [3], and [4]. The initial approaches for the design of a MEMS-based optical microphone, including the optimum placement of the photodetectors with respect to the reflector and the method for calculating the optimum operating point of the device, were illustrated in [5] and [6]. The above aspects are here addressed, further developed, and optimized with the final target of defining a solution compatible with mass production.