Measurement of distance changes using a fibre-coupled common-path interferometer with mechanical path length modulation

We present a novel type of fibre-coupled interferometric sensor. A micro-optical probe acts as a common-path interferometer and focuses the measuring light emitted by a laser diode onto the surface under investigation. The probe is mounted on a bending beam and deflected periodically by a piezoelectric actuator. This generates a phase-modulated interference signal, which is transformed to an electrical signal by a photo diode. The measurement principle is based on the fact that a distance change between the probe and the measurement object leads to a characteristic phase-shift of the measured signal. The photo diode signal is analysed in different ways in order to enable both high accuracy in the nanometre range and high measurement speeds, where the data rate of the sensor is finally limited by the conversion rate of the used analogue-to-digital converter. In order to extend the range of unambiguity of the sensor and to improve the robustness of measurement, we apply a dual-wavelength technique using two laser diodes emitting at different wavelengths simultaneously. Repeatability measurements, responses to distance changes and, finally, the measurement of calibration specimens with well-defined surface profiles demonstrate the performance of the system. (paper)