Low-coherence fibre heterodyne interferometer for both dc and high-frequency vibration measurements in the inner ear

Abstract

A coherence encoded heterodyne interferometer with a fibre-optic sensor head of 1.8mm diameter designed to detect both high-frequency vibrations as well as low-frequency non-harmonic phase shifts in the inner ear is presented. The light source was a laser diode with a multimode spectrum of width 4.6nm. In order to achieve low sensitivity to spurious phase drifts, a coherence selective configuration was used, comprising an entrance Mach–Zehnder interferometer, so that object and reference light travel the same path in the fibre. Phase measurements yielded a square root of the Allan variance of [sgrave] z = 0.26 Å for an integration time of 0.1 s and [sgrave] z = 3.6 Å for an integration time of 3.2s. A phase measurement, at an object of 1.18 × 10−4 reflectivity, showed a rms resolution of 0.46° over a measurement duration of 10s, corresponding to a distance resolution of 5.2 Å.