Substrate thickness error and radial tilt detection using a five-beam optical head.

In an optical disk system that uses a blue laser diode and a high numerical aperture objective lens, it is necessary to detect and correct a substrate thickness error and a radial tilt of the disk, because a spherical aberration and a coma aberration deteriorate the read/write characteristics. We present a newly developed method to detect the substrate thickness error and the radial tilt of the disk using a five-beam optical head. This detection method features a high signal-to-noise ratio for a readout signal and enables combination with differential focusing and tracking error detection methods. Experimental results demonstrate the validity of this detection method and a correction method using a liquid-crystal panel.