Encoder.

摘要

An encoder of this invention has a first optical means including a first polarizing beam splitter for splitting a beam from a light source into a beam (R1) and a beam (R2), and a first lambda /4 plate arranged in optical path (L1) for the beam (R1) and an optical path (L2) for the beam (R2), the first optical means causing the beams (R1, R2) to be incident on the diffraction grating so that a positive first mode diffracted beam obtained by diffracting the beam (R1) on a diffraction grating travels along the optical path (L1) and a negative second mode diffracted beam generated by diffracting the beam (R2) on the diffraction grating travels along the optical path (L2), and for superposing the first and second mode diffracted beams on each other through the first polarizing beams splitter; a second optical means including a second polarizing beam splitter for splitting the first and second mode diffracted beams from the first optical means from each other, and a second lambda /4 plate arranged in an optical path (L3) for the first mode diffracted beam and an optical path (L4) for the second mode diffracted beam, the second optical means causing the first and second mode diffracted beams to be incident on the diffraction grating so that a positive first mode rediffracted beam generated by diffracting the first diffracted beam on the diffraction grating travels along the optical path (L3) and a negative second mode rediffracted beam obtained by diffracting the second mode diffracted beam on the diffraction grating travels along the optical path (L4), and for superposing the first and second mode rediffracted beams on each other through the second polarizing beam splitter; and a converting means for causing the first and second mode rediffracted beams to interfere with each other to form an interfered beam, and converting the interfered beam into an electrical signal, wherein a difference between optical path lengths of beams incident on the converting means through the optical paths (L1, L4) and of beams incident on the converting means through the optical paths (L2, L4), and a difference between optical path lengths of beams incident on the converting means through the optical paths (L1, L3) and of beams incident on the converting means through the optical paths (L2, L3) are larger than a coherent length of a beam from the light source.