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.

Measuring acoustic pressure gradients is critical in many applications such as directional microphones for hearing aids and sound intensity probes. This measurement is especially challenging with decreasing microphone size, which reduces the sensitivity due to small spacing between the pressure ports. Novel, micromachined biomimetic microphone diaphragms are shown to provide high sensitivity to pressure gradients on one side of the diaphragm with low thermal mechanical noise. These structures have a dominant mode shape with see-saw like motion in the audio band, responding to pressure gradients as well as spurious higher order modes sensitive to pressure. In this dissertation, integration of a diffraction based optical detection method with these novel diaphragm structures to implement a low noise optical pressure gradient microphone is described and experimental characterization results are presented, showing 36 dBA noise level with 1mm port spacing, nearly an order of magnitude better than the current gradient microphones. The optical detection scheme also provides electrostatic actuation capability from both sides of the diaphragm separately which can be used for active force feedback. A 4-port electromechanical equivalent circuit model of this microphone with optical readout is developed to predict the overall response of the device to different acoustic and electrostatic excitations. The model includes the damping due to complex motion of air around the microphone diaphragm, and it calculates the detected optical signal on each side of the diaphragm as a combination of two separate dominant vibration modes. This equivalent circuit model is verified by experiments and used to predict the microphone response with different force feedback schemes. Single sided force feedback is used for active damping to improve the linearity and the frequency response of the microphone. Furthermore, it is shown that using two sided force feedback one can significantly suppress or enhance the desired vibration modes of the diaphragm. This approach provides an electronic means to tailor the directional response of the microphones, with significant implications in device performance for various applications. As an example, the use of this device as a particle velocity sensor for sound intensity and sound power measurements is investigated. Without force feedback, the gradient microphone provides accurate particle velocity measurement for frequencies below 2 kHz, after which the pressure response of the second order mode becomes significant. With two-sided force feedback, the calculations show that this upper frequency limit may be increased to 10 kHz. This improves the pressure residual intensity index by more than 15 dB in the 50 Hz-10 kHz range, matching the Class I requirements of IEC 1043 standards for intensity probes without any need for multiple spacers.