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

A coherence encoded heterodyne interferometer with a fibre-optic sensor head of 1.8 mm 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 4.6 nm. 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 sigma(z) = 0.26 Angstrom for an integration time of 0.1 s and sigma(z) = 3.6 Angstrom for an integration time of 3.2 s. A phase measurement, at an object of 1.18 x 10(-4) reflectivity, showed a rms resolution of 0.46 degrees over a measurement duration of 10 s, corresponding to a distance resolution of 5.2 Angstrom.