Integration and characterization of micromachined optical microphones
Jeelani, Mohammad Kamran
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The focus of this study is the optoelectronic integration of a micro-optical displacement detection architecture with a biomimetic MEMS microphone membrane based on the directional hearing mechanism of the parasitic fly Ormia Ochracea. The micromachined microphones feature optical interferometric displacement detection achieved using a commercially available Vertical Cavity Surface Emitting Laser (VCSEL) coupled with a custom designed silicon photodiode array. This design is shown to have significant advantages over conventional hearing aid microphones, which employ capacitive detection. A Multi-Chip Module (MCM) optoelectronic package is designed to integrate the biomimetic membrane with the optical displacement detection electronics in order to produce a fully integrated acoustic sensor. The modular package components, which are fabricated using high resolution stereolithography apparatus (SLA) equipment, provide accurate optical alignment of the optoelectronic components and allow complete device integration in a package with a total volume under 0.5cc. Characterization of the integrated microphones is described in detail, including measurements of sensitivity, noise floor and directivity. A displacement resolution of 3.5x10⁻¹³ m/√Hz was measured between 4kHz and 16kHz in an anechoic test chamber, corresponding to a dynamic range of 115dB for the optical detection architecture. The total noise SPL of the device is 35.9dBA. Unlike capacitive microphones with similar noise levels, the device developed in this work exhibits first order dipole directivity patterns between 250Hz-1kHz, with an ideal Directivity Index of 4.8dB @ 1kHz and directional attenuation exceeding 25dB. With these results the optoelectronic package presented in this work demonstrates the viability of the integrated optical biomimetic microphones in compact, low power applications, specifically directional hearing aids.