US Patent 7272084 Optical disc apparatus and tracking control method

In a configuration in which side beams scan in positions that are offset with respect to a main beam by approximately (¼+n)P or (¾+n)P in the radial direction of an optical disk, a differential signal is generated from push-pull signals of the return lights of the side beams to generate a track-cross signal. In a configuration in which side beams scan in positions that are offset with respect to a main beam by approximately (¼+n)P or (¾+n)P in the radial direction of an optical disk, far-side or near-side light detection results of the return lights of the side beams are processed and computed with respect to a push-pull signal of the return light of the main beam.