Heterodyne grating interferometric method and system for two-degree-of-freedom with high alignment tolerance

Abstract

Present disclosure relates to a heterodyne grating interferometric method and system for two-degree-of-freedom with high tolerance. The system comprises a separately modulated heterodyne laser (), an optical prism () and a photoelectric detection and signal processing unit (). The separately modulated heterodyne laser () simultaneously outputs two laser beams at different frequencies, which are incident in parallel to a first beamsplitting surface so as to be split, and then a part thereof is incident to a retro-reflector () to produce reference beams (), which are incident to a third beamsplitting surface, and the other part traverses a double-diffraction structure formed by a measured grating () and retro-reflectors () to obtain two measured beams (), which are incident to a second beamsplitting surface and then are divided into two parts. Wherein one part is converged to form a first interference beam (), and the other part is incident to the third beamsplitting surface and is converged with the corresponding reference beams () to form second and third interference beams (). Photoelectric detection and signal processing is performed on the interference signals of the three interference beams (), so as to calculate horizontal and vertical displacement of the grating (). The present measurement method and system improve the angular tolerance of tip and tilt of the optical grating () while increasing the fold factors.