The grating interferometer with separated beams in joule balance

The multi-axis grating interferometry for multi-axial displacement measurement is investigated. Two spatially separated laser beams are used to ensure that the two frequencies of light do not share a common optical path before interference, which eliminates optical mixing and thus eliminates periodic nonlinearity error. Besides, decoupling of displacement along different axis is analyzed. The experimental setups are built for the test of the periodic nonlinearity. Periodic nonlinear error is smaller than 0.05 nm.