Optical encoder with dual diffraction grating

摘要

An optical encoder including a light source and a first grating plate having a first diffraction grating for diffracting a light beam emitted from the light source. The optical encoder further includes a second grating plate having a second diffraction grating including a blazed diffraction grating for further diffracting the light beam diffracted by the first diffraction grating so as to allow the light beam to be incident on the first grating plate. The optical encoder also includes a light-receiving portion for receiving the light beam reentering the first grating plate and diffracted by the first grating plate. The second diffraction grating is designed so that the greater part of the diffracted light is concentrated in diffracted light beam of a predetermined order among the light beams from the first diffraction grating, and the diffracted light beam of the predetermined order travels from the second diffraction grating in a direction which is parallel with a direction in which the light beam is incident on the second diffraction grating from the first grating plate. The light-receiving portion generates an electric signal in accordance with an amount of plus and minus mth-order diffracted light beams of the further diffracted light beam.