Patent ID: 8179752

Claim:
A method for locking the focus of a laser beam of an optical disc apparatus onto a target data layer of an optical disc having a plurality of data layers, wherein a peak level ratio is defined as the ratio of a second peak level to a first peak level, the second peak level being a peak level of an S-shaped focus error signal waveform for a data layer adjacent to the target data layer through which the laser beam passes earlier than through the target data layer during focus lock operation, the first peak level being a peak level of an S-shaped focus error signal waveform for the target data layer, the method comprising steps of: setting a spherical aberration correction amount to a second spherical aberration correction amount so that the peak level ratio is smaller than that when the spherical aberration correction amount is set to a first spherical aberration correction amount that is optimal for the target data layer; driving an objective lens in an optical-axis direction of the laser beam; and closing a focus servo loop after a focus error signal obtained when the laser beam approaches the target data layer exceeds a predetermined level.