Compact differential plane interferometer with in-axis mirror tilt detection

Abstract

Besides the environmental fluctuations, the typical sources of significant uncertainty in the laser interferometry systems are the geometrical errors. These are stemming, among others, from the misalignment of measurement axes, the thermo-mechanical influences of the system components and the mounting, guidance errors of the translation mechanism that carries the measurement mirror or vibrations. We report on a compact double-pass differential plane interferometer that features an original optical arrangement with four parallel and coplanar beams, where the beam pairs in the two arms are coaxial. The differential arrangement minimizes the dead path and shortens the metrological loop so that the sensitivity to thermal drifts and vibrations is reduced. The arm symmetry allows for the preservation of the Abbe principle, and the common path mitigates the influences of the environmental disturbances. The interferometer optics is designed as a self-contained single-piece assembly made using optical contacting from the low-expansion materials. The interferometer system integrates the homodyne receiver (even though the optical arrangement is well-suited for heterodyne detection too) and also a tilt-detection electronics that allows for detection of pitch and roll of the interferometer mirror so that the parasitic movement of the measuring mirror could be compensated for. The experimental characterization revealed a good optical performance of the interferometer with sub-nanometre cyclic error and the resolution of tilt detection in order of a few microradians.