Micro displacement measured by the grating interferometer with rings pattern

Abstract

Micro displacements are measured with holographic gratings by using a grating interferometer of one order that detects the phase changes in the diffracted orders caused by movement of the grooves in the diffraction gratings. The period can be on the order of fractions of a micron, with high reproducibility and an error of a half period. The basic operating principle involves the superposition of order +1, with order 0. The interferometer system produces standing waves; it works by measuring the intensity variations at the center of a ring-shaped interference pattern, which indicate the phase shift introduced by displacement of the grating grooves. When these rings move to the center of the pattern or to the border, the direction of the grating displacement can be detected; the interferometer system has no moving parts, except for the diffraction grating, and is very stable and robust. This system has the ability to measure micro displacements even with damaged gratings, as long as the gratings diffract evenly.