Straightness Measurement Method by Using an Optical Heterodyne Interferometer with a Gratiing.

A new optical system for measuring motion errors such as the straightness of a slide table has been developed. The optical system mainly consists of three components : (i) The He-Ne Zeeman laser that emits two partial beams of different frequencies polarized orthogonally to each other, (ii) A grating mounted on a slide table and (iii) A heterodyne interferometer where the beams of the ±1st order diffracted on a grating are superimposed and a beat signal, which has a phase shift δφ depending on an in plane displacement δx of the grating, is generated. The straightness of a slide table can be measured, because the phase shift δφ is proportional to the in plane displacement δx in principle. In this study, two different types of interferometers, which we call "Mach Zehnder type" and "Michelson type", are developed. Using each interferometer, the straightness of a slide table is measured experimentally. As a result, in "Michelson type", a straightness measured by the new optical system is good agreement with one measured by a laser interferometer commercially obtained. The effectiveness of the new optical system is confirmed by this experimental result.