A prosthetic hearing device comprising a biocompatible housing having a surface that vibrates in response to sound waves traveling through tissue; and an interferometer mounted in the housing, the interferometer is constructed and arranged to generate a light beam that impinges on a reflective interior surface of the vibrating surface, and to receive light reflected from the reflective interior surface. The device detects ambient sound by impinging a light beam on a portion of the vibrating surface; receiving light reflected from the reflective portion; measuring the movement of the vibrating surface based on an interference pattern of the impinging and reflected light; and determining at least a frequency of the incident sound wave based on the interference pattern.