Patent ID: 9007881
Filing Date: 2015-04-14
Classification: G11B

Abstract:
1. An optical disc device comprising: an optical head device; a disc drive unit for spinning an optical disc; a signal processing unit for generating a tracking servo control signal on a basis of a signal detected by the optical head device; and a servo control unit for performing control to shift an objective lens in a radial direction of the optical disc in response to the tracking servo control signal; wherein the optical head device includes a semiconductor laser, an optical element for transmissively diffracting a light beam output from the semiconductor laser to generate a zero-order diffracted light beam, a +1-order diffracted light beam, and a −1-order diffracted light beam, an objective lens for focusing the +1-order diffracted light beam and the zero-order diffracted light beam to form a focused spot on an information recording layer in the optical disc, and a photodetector for receiving the +1-order diffracted light beam and the zero-order diffracted light beam after reflection from the optical disc; the photodetector includes a primary light receiving section having a first primary light receiving surface and a second primary light receiving surface aligned in a first direction corresponding to the radial direction of the optical disc, a first secondary light receiving section disposed in a position outwardly separated from the primary light receiving section in a positive direction of the first direction, and a second secondary light receiving section disposed in a position outwardly separated from the primary light receiving section in a negative direction of the first direction; the first primary light receiving surface and the second primary light receiving surface perform photoelectric conversion of a received light spot of the zero-order diffracted light beam and output, respectively, a first main detected signal and a second main detected signal; the first secondary light receiving section has a plurality of secondary light receiving surfaces aligned in the first direction; the plurality of secondary light receiving surfaces include a first outer secondary light receiving surface and a first inner secondary light receiving surface, the first outer secondary light receiving surface being disposed in a position most widely separated from the primary light receiving section in the positive direction of the first direction, the first inner secondary light receiving surface being disposed in a position most closely separated from the primary light receiving section in the positive direction of the first direction; the second secondary light receiving section has a plurality of secondary light receiving surfaces aligned in the first direction; the plurality of secondary light receiving surfaces include a second outer secondary light receiving surface and a second inner secondary light receiving surface, the second outer secondary light receiving surface being disposed in a position most widely separated from the primary light receiving section in the negative direction of the first direction, the second inner secondary light receiving surface being disposed in a position most closely separated from the primary light receiving section in the negative direction of the first direction; the first outer secondary light receiving surface performs photoelectric conversion of an outer portion, in the positive direction of the first direction, of a received light spot of the +1-order diffracted light beam and outputs a first secondary detected signal; the second outer secondary light receiving surface performs photoelectric conversion of an outer portion, in the negative direction of the first direction, of a received light spot of the −1-order diffracted light beam and outputs a second secondary detected signal; among the plurality of secondary light receiving surfaces included in the first secondary light receiving section, the first inner secondary light receiving surface has a larger light receiving area than the secondary light receiving surfaces other than the first inner secondary light receiving surface, the first inner secondary light receiving surface having a larger length than the first primary light receiving surface and the second primary light receiving surface in a second direction orthogonal to the first direction; among the plurality of secondary light receiving surfaces included in the second secondary light receiving section, the second inner secondary light receiving surface has a larger light receiving area than the secondary light receiving surfaces other than the second inner secondary receiving surface, the second inner secondary light receiving surface having a larger length than the first primary light receiving surface and the second primary light receiving surface in the second direction; and the signal processing unit generates a main push-pull signal on a basis of the first primary detected signal and the second primary detected signal, detects an offset component due to relative displacement of the objective lens with respect to the light receiving element, on a basis of by the first secondary detected signal and the second secondary detected signal, and generates the tracking error signal by deducting the offset component from the push-pull signal.