Patent ID: 8274875

Claim:
An optical disc device comprising: a reference beam source to emit a reference optical beam so as to irradiate a reference layer provided in an optical disc; an information beam source to emit an information optical beam so as to reproduce information from a mark layer on which recording marks representing the information are arranged inside a recording layer provided in the optical disc at a predetermined distance from the reference layer; an objective lens to focus the reference optical beam and the information optical beam; a beam shaping section to shape the information optical beam entering the objective lens such that, in terms of a direction of an optical axis of the information optical beam, a distance between focal points of the reference optical beam and the information optical beam focused by the objective lens becomes equal to the predetermined distance; a motion control section to move the objective lens in the direction of the optical axis of the information optical beam; a reference beam receiving section to receive a reflected reference optical beam, which is the reference optical beam reflected by the reference layer of the optical disc, and generate a reference detection signal; an information beam receiving section to receive a reflected information optical beam, which is the information optical beam reflected by the mark layer of the optical disc, and generate an information detection signal; a signal generation section to generate a reflected reference beam intensity signal representing the intensity of the reflected reference optical beam based at least in part on the reference detection signal, and generate a focus error signal, which varies depending on a distance from the mark layer to the focal point of the detected optical beam in a proximity of the mark layer, based at least in part on a plurality of information detection signals; and a servo control section to start focus control based on the focus error signal when a change in the reflected reference beam intensity signal satisfies a condition.