A comparative study on miniature retroreflectors for totally implantable hearing aids using an optical detection of the incus vibration

Optical interferometric detection of the middle ear ossicle vibration is used to get an acoustical signal needed for a totally implantable hearing aid (TIHA). In order to improve the reflectivity, we have attached a lightweight piece of retroreflector to the incus. Three different kinds of retroreflectors were investigated here, using a modified fiber-optic Michelson interferometer, based on a single mode 3×3 coupler and a high-coherence VCSEL light source. The sinusoidal vibration of the target was produced by a PZT, in the acoustic frequency range with amplitudes in the range of several picometers to hundreds of nanometer. The quadrature signals were captured and demodulated while the position of the probing beam to the retroreflector were changing. The evaluation of retroreflectors and probe beam collimating techniques was made in respect of the maximum signal-to-noise ratio and the minimum detectable amplitude of vibrations.