Patent ID: 8189436

Claim:
An optical drive device comprising: a laser light source; a diffraction grating through which an optical beam passes from the laser light source; an optical detector having a main-beam receiving surface, a plus-first-order sub-beam receiving surface, and a minus-first-order sub-beam receiving surface receiving respectively zero-order diffracted light, plus-first-order diffracted light, and minus-first-order diffracted light of the optical beam obtained by diffraction of the optical beam through the diffraction grating; and a tracking-error signal generator configured to generate a tracking error signal based on a light receiving amount of the optical beam received in each of the receiving surfaces, wherein each of the light receiving surfaces is formed point-symmetrically to a spot center of a corresponding one of the diffracted light, and line-symmetrically to a straight line passing through the spot center and parallel with a signal light tangent direction, and is further divided into two signal-light receiving areas by the straight line, the tracking-error signal generator being configured to generate a main push-pull signal based on a light receiving amount at a portion of a non-zero width W 1 centered on the spot center of the zero-order diffracted light and extending perpendicularly to the signal light tangent direction in the main-beam receiving surface, to generate a sub push-pull signal based on a light receiving amount at a portion of a non-zero width W 2 that is centered on the spot center of the plus-first-order diffracted light and the minus-first-order diffracted light and extending perpendicularly to the signal light tangent direction in the ±first order sub-beam receiving surfaces, and to generate the tracking error signal based on the main push-pull signal and the sub push-pull signal, at least one of the width W 1 and the width W 2 being smaller than 100% of a diameter of a corresponding spot.