Methods for increasing the storage capacity of an optical disc include, for example, reducing the wavelength of laser light that is used for data write and read, reducing the laser spot size by increasing the numerical aperture of an objective lens, and increasing the number of data layers. Examples of multilayer optical discs include DVDs and Blu-ray discs (hereinafter abbreviated as BDs), in which dual layer structure is now in practical use. In recent years, BDs with three or more data layers have also been proposed.
The optical systems for BDs are configured such that the laser wavelength is 405 nm and the numerical aperture is 0.85, and BDs often suffer the problem of spherical aberration that occurs due to the difference in the distance from the disc cover surface to each data layer. Because the spherical aberration affects the data recording and reproducing capabilities of an optical disc, BD-compatible optical disc apparatuses include spherical aberration correction mechanisms to appropriately correct the spherical aberration, thereby ensuring adequate recording and reproducing capabilities.
In the case of multilayer optical discs, the optimal spherical aberration correction amount differs for each data layer because the distance from the disc cover surface to each data layer differs. This requires optical disc apparatuses to change their spherical aberration correction amount for each data layer so that the amount is optimal for each data layer.
As above, various challenges need to be met by optical disc apparatuses that are compatible with spherical-aberration-prone multilayer optical discs. Another challenge for such optical disc apparatuses is to increase the speed at which the apparatuses lock the focus of a laser beam onto a target data layer with focus servo control. To meet this challenge, Japanese Unexamined Patent Application Publication No. 2003-16660 (Patent Document 1) proposes an optical disc apparatus in which focus lock operation is performed after the spherical aberration correction amount is set in advance so as to be optimal for a desired data layer. Specifically, the spherical aberration correction mechanism is driven before the focus lock operation so as to be optimal for the desired data layer. Thereafter, a so-called focus sweep is performed to bring the objective lens closer to the optical disc, and the number of peaks of a sum signal is counted during this focus sweep. After the desired number is reached, a sum signal level and zero-crossing of a focus error signal are detected to close the focus servo loop.