US Patent 7911889 Optical disc apparatus

An optical disc apparatus includes a laser diode, a first optical component which branches beams from the laser diode into a first beam and a second beam, an objective lens which focuses the first beam on an optical information recording medium, an actuator which drives the objective lens in an optical axis direction, a reference mirror provided in an optical path of the second beam, an optical path length varying unit which varies an optical path length of the second beam, a first photo detector, a second photo detector, a second optical component which branches beams obtained by combining the first beam which is reflected by the optical information recording medium and the second beam which is reflected by the reference mirror and which makes the branched beams incident on the first and second photo detectors, and a signal processing part.