Heterodyne planar grating encoder with high alignment tolerance, especially insensitivity to grating tilts

Planar grating encoder, composed of one two-dimensional grating and the optical system (grating interferometer), is able to measure two-dimensional displacements. Grating interferometer errors are the errors caused by grating tilts, which possibly make phase changes of optical paths, and interfering beams non-parallelism. A new design of planar grating encoder based on optical heterodyne interferometry with a symmetric setup and double diffraction is proposed. It can measure the planar displacement with high resolution and high stability. With help of retro-reflectors, the tilts of the grating do not change the interference pattern and bring no measurement errors theoretically. The planar grating encoder has been set up, and the measurement results are compared with a two-axis laser interferometer. The repeatability of the grating encoder is about 10nm, and the laser interferometer comparing results confirm its error insensitivity to grating tilts.