Polarization-modulated grating interferometer by conical diffraction.

The grating interferometer in the Littrow configuration uses quarter wave plates (QWPs) to modulate the polarization in the measurement system to determine the autocollimation optical path. Fabrication errors and mounting errors of the QWPs lead to phase changes in the grating interferometer that generate measurement errors. As an alternative, we propose a grating interferometer that produces conical diffraction. Using the grating instead of QWPs to modulate the beam's polarization bypasses this source of error. A 45 mm range experiment was performed that yielded a repeated measurement error of 40 nm. Experiments show that the system has a simple structure and good repeatability and is capable of high-precision displacement measurements.