Patent ID: 7929386

Claim:
An optical disc device, comprising: a light source, an objective lens, an optical splitter, and a photodetector; a light beam emitted from the light source passes through the objective lens so as to be focused on a track on a signal plane of an optical disc, the light beam reflected and diffracted by the track on the signal plane passes through the objective lens so as to enter the optical splitter, and the light beam emitted from the optical splitter is focused on the photodetector; wherein the optical splitter is divided into a region A′ where a zeroth-order diffracted light from the optical disc enters alone and the remaining region A, the photodetector is divided into a detection region B′ for detecting a light beam emitted from the region A′ of the optical splitter and a detection region B for detecting a light beam emitted from the region A of the optical splitter; a focus error signal is generated by a detection signal from the detection region B; a tracking error signal is generated by a detection signal from the detection region B, or the detection regions B and B′; the tracking error signal is corrected by the detection signal from the detection region B′; and a focus control is performed based on the focus error signal so that when a substrate thickness of the optical disc has a positive error (thicker than a reference value), the objective lens is located closer to the signal plane of the optical disc; and when a substrate thickness of the optical disc has a negative error (thinner than a reference value), the objective lens is located away from the signal plane of the optical disc; wherein the optical splitter is divided into four quadrants k (k=1, 2, 3, 4 counterclockwise) by two straight lines (a y-axis parallel to the radial direction of the optical disc and an x-axis orthogonal to the y-axis) crossing an optical axis; each of the quadrants k is divided further into a region Ak′ where a zeroth-order diffracted light from the optical disc enters alone and the remaining region Ak; light beams emitted from the regions Ak and Ak′ of the optical splitter are detected respectively by detection cells Bk, Bk′ on the detection regions B, B′ and outputted as detection signals Ck, Ck′: and a tracking error signal TE is generated through expressions (Formula 1) to (Formula 3) using α as a constant: TE 1=( C 2 ′+C 3′)−( C 1 ′+C 4′) (Formula 1) TE 2=( C 2 +C 3)−( C 1 +C 4) or TE 2=( C 2 +C 2 ′+C 3 +C 3′)−( C 1 +C 1 ′+C 4 +C 4′) (Formula 2) TE=TE 2 −α×TE 1 (Formula 3).