A practical optical force-feedback microphone

A novel technique of measuring diaphragm displacement in microphones using a laser-diode Fabry-Perot system is presented. The microphone diaphragm is held within an electrostatic force-feedback loop so as to improve the performance. A second loop operates around the laser, for noise minimisation purposes. Following a brief introduction to the subject of optical detection in microphones, the modified Fabry-Perot interferometer is described, and the complete system with the two feedback loops presented. The characteristics of the laser driver and laser diode are examined. Theoretical responses are shown, for different cases, and noise calculations yield the dynamic ranges without feedback. The effect of feedback is then examined, both around the laser (electronic) and around the microphone capsule (electrostatic), and the dynamic ranges are recalculated to show the advantages of feedback. Intended directions for further research are also discussed.

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