Optical measurement system and method for measuring optical grating

Abstract

An optical measurement system for measuring an optical grating (102) comprises a light source (100) that outputs collimated light beam (200) that is monochromatic and coherent. An interferometer arrangement (5) splits collimated light beam (200) and guides first and second light beams (104, 106) to the optical grating (102) for back-diffraction to the interferometer (5). The system causes interference between each of grating light beams (122G, 122G’) diffracted from the optical grating (102) set in two positions (G, G’), and a reference light beam (122M) reflected from a reference (116). The system also causes at least four predetermined relative wavefront phase shifts between the first and second light beams. A camera (124) forms interferograms of the interference in the two different positions (G, G’) of the optical grating (102) with at least two predetermined wavefront shifts. And a data processing unit (140) processes the interferograms, and present data on the optical grating (102) based on the interferograms.