Patent ID: 7369465
Filing Date: 2008-05-06
Classification: G11B

Abstract:
1. An optical head unit comprising: a light source that supplies light of a predetermined wavelength; an objective lens that focuses the light from the light source to one of recording layers of a recording medium; an objective lens moving mechanism that moves the objective lens in the optical axis direction and in the direction intersecting a signal mark string formed in the recording medium; a defocus detecting system that detects a defocus in the objective lens; a thickness unevenness detecting system that detects a thickness unevenness in a transparent resin layer of the recording medium provided nearest the objective lens; and a thickness unevenness correcting mechanism that changes a focusing characteristic of light impinging upon the objective lens from the light source based on a change in the thickness of the transparent resin layer of the recording medium detected by the thickness unevenness detecting system, wherein, as a defocus correction control, the objective lens moving mechanism moves the objective lens from the recording layer to a farthest position in accordance with a specific voltage supplied to the objective lens moving mechanism, and thereafter gradually brings the objective lens near the recording layer in accordance with a gradual change in the supplied voltage; and upon confirmation of an output of a photo detector that the objective lens has been moved to a just-on-focus position against the recording layer through the defocus correction control of the objective lens moving mechanism, the thickness unevenness correcting mechanism is activated.