A DVD-ROM (digital versatile disc read-only memory), a DVD-RAM (digital versatile disc random access memory), a DVD±R (digital versatile disc plus/minus recordable), and a DVD±RW (digital versatile disc plus/minus rewritable) are high-density large-capacity recording media that have been developed in recent years. Other optical discs, such as a blu-ray disc, have been developed as recording media with even larger capacity to record large volumes of data including high-definition videos.
An optical disc apparatus may have optical crosstalk, which is the phenomenon in which a track crossing signal (a tracking error signal (hereafter may be referred to as a “TE signal”)) leaks into a focus error signal (hereafter may be referred to as an “FE signal”) when a light spot focused on an optical disc crosses a track. When the optical crosstalk occurs, the light spot fluctuates. The light spot may defocus when the light spot fluctuates greatly.
One method often used in the optical disc apparatus to detect a focus error signal is an astigmatic method. With the astigmatic method, the light receiving surface of a light receiving element is divided in four light receiving surfaces, light intensity signals corresponding to each pair of light receiving surfaces that are arranged diagonally are added to obtain a sum signal corresponding to each pair of light receiving surfaces (obtain two sum signals corresponding to the two pairs of light receiving surfaces), and a focus error signal is detected based on a difference between the two sum signals.
When, for example, the optical disc apparatus using the astigmatic method includes the light receiving element that is mounted off the optical axis in the direction tangent to a track of the optical disc, track crossing components (optical crosstalk) leak into the two sum signals by different amounts. In this case, the track crossing components leaking into the two additional signals do not cancel out. As a result, the focus error signal will have a track crossing component. If the track crossing component of the focus error signal is large, the light spot of the optical apparatus may defocus. To prevent defocusing, the optical crosstalk needs to be corrected.
A conventional optical disc apparatus corrects optical crosstalk while tracking control is off, such as while seeks are being performed. Such a method of correcting optical crosstalk will now be described with reference to FIG. 29. FIG. 29 shows the structure of a conventional optical disc apparatus 600. The optical disc apparatus 600 mainly includes an optical head (mainly composed of a light source 104, a coupling lens 106, a polarization beam splitter 108, a polarization plate 110, a converging lens 112, a focusing lens 114, and a light receiving element 116), a focus error detection unit 2901, a focus control unit 2902, a vertical movement unit 140, a tracking error detection unit 2904, a tracking control unit 2905, a horizontal movement unit 142, and an optical crosstalk correction unit 2911. The optical head reads information from an optical disc 102 or records information onto the optical disc 102.
In FIG. 29, the focus error detection unit 2901 detects a focus error signal based on reflection light from an optical disc through the light receiving element 116, and outputs the focus error signal. The focus control unit 2902 generates a control signal that enables the light spot to focus on the optical disc in a substantially uniform state based on an output of the focus error detection unit 2901. The focus control unit 2902 drives the vertical movement unit 140 based on the control signal to execute focus control. The tracking error detection unit 2904 detects a tracking error signal based on reflection light from the optical disc through the light receiving element 116, and outputs the tracking error signal. The tracking control unit 2905 generates a control signal that enables the light spot to be within a substantially uniform range from the center of a track of the optical disc based on an output of the tracking error detection unit 2904. The tracking control unit 2905 drives the horizontal movement unit 142 based on the control signal to execute tracking control. When the light spot crosses a track, optical crosstalk occurs from a path linking the light receiving element 116 and the tracking error detection unit 2904 to a path linking the light receiving element 116 and the focus error detection unit 2901. In FIG. 29, a box 2910 indicates a circuit portion equivalent to the state of such optical crosstalk. Due to the optical crosstalk, a signal component of a tracking error signal corresponding to the optical crosstalk (optical leaking signal component) leaks from the tracking error signal into a focus error signal.
The optical crosstalk correction unit 2911 includes a coefficient multiplier 2908 and an adder 2912. To reduce the optical leaking signal component generated by the optical crosstalk, the optical crosstalk correction unit 2911 electrically corrects the focus error signal output from the focus error detection unit 2901. The coefficient multiplier 2908 receives the tracking error signal, multiplies the tracking error signal by a gain value that would reduce the optical leaking signal component generated by the optical crosstalk, and outputs the resulting signal to the adder 2912. The adder 2912 adds the output from the focus error detection unit 2901 and the output from the coefficient multiplier 2908 to obtain a focus error signal with a reduced optical leaking signal component generated by the optical crosstalk. The adder 2912 outputs the focus error signal with the reduced optical leaking signal to the focus control unit 2902.
In this manner, the optical disc apparatus 600 executes focus control while reducing undesired effects of optical crosstalk.
One technique of correcting a focus error signal known in the art is to calculate an amount of a tracking error signal that leaks into a focus error signal while the tracking control is off, and subtract a tracking error signal with a level adjusted according to the calculated leaking amount from the focus error signal while seeks are being performed (see, for example, Patent Citation 1).
Another technique of correcting a focus error signal known in the art is to use a different focus offset amount depending on whether the focus error signal corresponds to a land or a groove (see, for example, Patent Citation 2).
Still another technique of correcting a focus error signal known in the art is to use a differential astigmatic method instead of the astigmatic method. With the differential astigmatic method, the light receiving surface of a light receiving element is divided in four light receiving surfaces, a first focus error signal involving optical crosstalk (a focus error signal obtained with a first beam) and a second focus error signal involving optical crosstalk whose phase is opposite to the phase of the optical crosstalk of the first focus error signal (a sum signal of a focus error signal obtained with a second beam and a focus error signal obtained with a third beam) are obtained, and the first focus error signal and the second focus error signal are added to reduce optical crosstalk. In this manner, the optical leaking signal component generated by optical crosstalk is prevented from leaking into the focus error signal.    Patent Citation 1: Japanese Unexamined Patent Publication No. 2001-67682 (p2)    Patent Citation 2: Japanese Unexamined Patent Publication No. H8-180429
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