Patent ID: 7675825

Claim:
An optical head unit comprising: a light source configured to supply light of a predetermined wavelength; an objective lens that focuses the light from the light source to the recording layer of a recording medium; an objective lens moving mechanism configured to move 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 configured to detect a defocus in the objective lens; a thickness unevenness detecting system configured to detect a thickness unevenness in a transparent resin layer of the recording medium provided nearest to the objective lens, the thickness unevenness detecting system including a photodetector divided into at least two detecting areas; a thickness unevenness correcting mechanism configured to change 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; and wherein the thickness unevenness correcting mechanism operates with a smaller gain relative to a servo gain of a DC level of the frequency characteristic of a defocus correcting control circuit.