Patent ID: 8081315

Claim:
A displacement measuring instrument, comprising: a light source that emits two laser beams of different wavelengths having a multiple relationship; a polarization splitter that splits the laser beams emitted from the light source depending on polarizing directions of the laser beams so that one of split laser beams is irradiated to a measurement optical path in a direction toward a target object and the other of the split laser beams is irradiated to a reference optical path in a direction different from the direction of the measurement optical path; a measurement light splitter provided in the measurement optical path to reflect a long-wavelength laser beam and transmit a short-wavelength laser beam out of the two laser beams having different wavelengths; a long-wavelength reflector provided in the measurement optical path to reflect the long-wavelength laser beam split by the measurement light splitter; a measurement optical path length changing unit provided in the measurement optical path to change an optical path length of the long-wavelength laser beam; a measurement reflector that is fixed to the target object and displaceable along the measurement optical path to reflect the short-wavelength laser beam; a reference reflector that is displaceable in the reference optical path to reflect the laser beam; a reference optical path length changing unit provided in the reference optical path to change the optical path length of the long-wavelength laser beam; a light superposing unit that superposes the laser beams reflected by the measurement reflector and the long-wavelength reflector in the measurement optical path on the laser beam reflected by the reference reflector in the reference optical path; a detector that receives interference waves generated from the laser beams superposed by the light superposing unit and outputs interference signals corresponding to the received interference waves; a sum signal computer that calculates a sum signal of an interference signal corresponding to the long-wavelength laser beam and an interference signal corresponding to the short-wavelength laser beam out of the interference signals outputted from the detector; a displacement controller that displaces the reference reflector so as not to change a phase of the sum signal; a reference displacement detector that detects a displacement of the reference reflector; and a measurement displacement computer that calculates a displacement of the measurement reflector based on the displacement of the reference reflector.