Measurement of changes in optical path length and reflectivity with phase-shifting laser feedback interferometry

The operating characteristics of a novel phase-shifting interferometer are presented. Interference arises by reflecting the light from a sample back into the cavity of a cw He-Ne laser. Changes in phase and fringe visibility are calculated from an overdetermined set of phase-shifted intensity measurements with the phase shifts being introduced with an electro-optic modulator. The interferometer is sensitive enough to measure displacements below 1 Hz with a rms error of approximately 1 nm from a sample that reflects only 3% of the 28 microW that is incident on its surface. The interferometer is applied to the determination of cantilever bending of a piezoelectric bimorph.