FIBEROPTIC AND INTEGRATED-OPTICAL MICROPHONES BASED ON INTENSITY MODULATION BY BEAM DEFLECTION AT A MOVING MEMBRANE

The realization of an acoustic sensor in which the incident sound waves directly modulate light guided in glass fibers without the need for any energy other than light is reported. The movement of a mirrored membrane influences the optical coupling between two waveguides, thus producing intensity modulation of the light in the output fiber. Two powerful approaches for the realization of such a device are presented: a microphone with single-mode fibers utilizing a gradient-index lens for beam focusing, and an integrated-optical microphone. For the latter, a new waveguide processing technology based on the polymer PMMA has been developed, resulting in low-loss waveguides with intensity distributions similar to those of common single-mode fibers. With the gradient-index-lens microphone, a noise-equivalent pressure level of 38 dB(A) and an almost flat frequency response within the audio range have been achieved.