Patent ID: 7355932

Claim:
An optical disk device for recording or reproducing information on an optical disk with a recording layer formed on a transparent substrate, comprising: an objective lens for condensing light for recording or reproducing information on said recording layer via a transparent substrate of the optical disk; a signal detector for detecting a focus error signal from all rays of return light reflecting from said recording layer; a thickness error detector for detecting a thickness error of said transparent substrate with reference to a specified value or its sign, based on a difference between the absolute value of the positive peak of said focus error signal and the absolute value of the negative peak of said focus error signal; a spherical aberration compensator for compensating for spherical aberration caused by the thickness error of said transparent substrate; and a controller for calculating a compensating amount for said spherical aberration at each radial position of said optical disk based on a thickness error of said transparent substrate detected at said radial position of said optical disk prior to recording or reproducing information, and driving said spherical aberration compensator to compensate for said spherical aberration based on said compensation amount during recording or reproduction of said optical disk.