An integrated-optical silicon microphone with a Fabry–Pérot cavity as modulator

Abstract

A new construction principle for an integrated-optical microphone is presented. It uses a Fabry–Pérot cavity as modulator. This combines the very sensitive interferometric multiple reflection scheme with a direct intensity modulation. The microphone consists of two chips, the carrier chip and the membrane chip. Both are fabricated in silicon using standard semiconductor technology. The membrane chip is mounted on top of the carrier chip, so that both chips together form the Fabry–Pérot cavity. Unmodulated primary light is generated by a laser diode and partially transmitted to the microphone via a 3-dB coupler. A single fiber, which is attached to the bottom side of the carrier chip, connects the coupler and the microphone. The incoming light is modulated in the Fabry–Pérot cavity by the membrane deflection and is reflected back into the fiber. It passes the coupler in reverse direction and is thereby partially transmitted to a receiver device, e.g., a photo diode. The main advantage of this kind of microphone is that a very low inherent noise level (<30 dB/A) can be achieved with a relatively simple design.