Patent ID: 8687202

Claim:
A displacement detecting device comprising: a substantially plate-like diffraction grating adapted to diffract light; a grating interferometer adapted to irradiate light on the diffraction grating where the irradiated light is diffracted into two beams of light and to cause the two beams of diffracted light to interfere with each other, and generate an interference signal; and a relative position information output section adapted to detect relative position information of the diffraction grating based on the interference signal generated by the grating interferometer, wherein the grating interferometer includes: a light source adapted to irradiate light on the diffraction grating, perpendicular to the diffraction grating; two reflectors adapted to reflect two one-time diffracted lights diffracted once by the diffraction grating, and cause the reflected one-time diffracted lights to be incident again on the diffraction grating at a position substantially identical to the point at which the light from the light source is irradiated, each reflector including a first mirror to reflect the one-time diffracted light diffracted once by the diffraction grating in a direction different from an incident direction of the one-time diffracted light and a second mirror to reflect the one-time diffracted light reflected by the first mirror toward the diffraction grating so as to be incident again on the diffraction grating at the position substantially identical to the point at which the light from the light source is irradiated, and the first mirror and the second mirror arranged such that the one-time diffracted light reflected by the second mirror toward the diffraction grating is incident on the diffraction grating at an angle different from both an incident angle of the light incident from the light source onto the diffraction grating and an angle at which the one-time diffracted light diffracted once by the diffraction grating is transmitted through or reflected away from the diffraction grating toward the first mirror, and thereby an optical path through which the one-time diffracted light reflected by the second mirror toward the diffraction grating passes is different from an optical path through which the one-time diffracted light diffracted once by the diffraction grating and reflected away from the diffraction grating toward the first mirror passes and an optical path through which the light incident from the light source onto the diffraction grating passes; a beam splitter adapted to superimpose two two-time diffracted lights diffracted twice by the diffraction grating on each other; and a receiver adapted to receive the two-time diffracted lights superimposed on each other by the beam splitter to generate the interference signal.