Patent ID: 8780357
Filing Date: 2014-07-15
Classification: G01B,G01D

Abstract:
1. An optical displacement measurement device including a multi-mode semiconductor laser light source oscillating at a plurality of frequencies, and a diffraction grating moved relative to the multi-mode semiconductor laser light source; in which the laser light radiated from the multi-mode semiconductor laser light source via the diffraction grating and diffracted by the diffraction grating is made to interfere and measurement is made of the amount of movement of the diffraction grating; wherein the device further comprises a transparent protective layer of a refractive index n and a thickness L; the transparent protective layer being formed to cover a surface on which the diffraction grating is formed; wherein an angle θ between a line normal to the protective layer and an incident light beam and an angle θ′ between diffracted light generated by the diffraction grating and reflected by a boundary surface of the protective layer to become stray light to be re-incident on the diffraction grating and a line normal to the diffraction grating, the protective layer has a range of thickness L such that, the interference signal intensity for an optical path difference between the stray light and light interfering with the stray light represented by Δ=2L(n/cos θ′+tan θ′·sin θ) will amount to not higher than 2% of the interference signal intensity for the optical path length difference Δ=0.