Patent ID: 7978569
Filing Date: 2011-07-12
Classification: G11B

Abstract:
1. An optical disc device comprising: a reference beam source that emits a reference optical beam to an optical disc's reference layer on which concentric or spiral reference tracks are formed; an information beam source that emits an information optical beam in order to reproduce information from a mark layer on which an information track that contains recording marks representing the information is formed such that the information track corresponds to a reference track of the reference layer, wherein the mark layer is positioned inside a recording layer which is a different layer from the reference layer; an objective lens that focuses the reference optical beam and the information optical beam; a beam formation section that shapes the reference optical beam and the information optical beam which enter the objective lens, so 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 a distance between the mark layer containing the information track and the reference layer; a reference beam reception section that receives a reference reflection optical beam which is the reference optical beam reflected by the reference layer of the optical disc; an information beam reception section that receives an information reflection optical beam which is the information optical beam reflected by the mark layer of the optical disc; a focus control section that performs a focus control process in order to move the objective lens close to or away from the optical disc; and a tracking control section that calculates a tracking error signal representing a distance from the information track to the focal point of the information optical beam from a result of receiving the information reflection optical beam, and moves the objective lens toward the outermost or innermost point of the optical disc according to the tracking error signal, wherein the focus control section first performs a reference focus control process in which the reference optical beam is focused on the reference layer according to a result of receiving the reference reflection optical beam, and then switches from the reference focus control process to an information focus control process in which the information optical beam is focused on the mark layer according to the result of receiving the information reflection optical beam, and the focus control section switches from the reference focus control process to the information focus control process after the focus control section detects that an amplitude of the tracking error signal becomes less than a predetermined level and that a information optical focus error signal representing a distance between the focal point of the information optical beam and the mark layer becomes available.