OPTICAL PICKUP DEVICE CAPABLE OF DETECTING CHANGE IN THICKNESS OF RECORDING MEDIUM AND/OR CORRECTING SPHERICAL ABERRATION CAUSED BY CHANGE IN THICKNESS

摘要

PROBLEM TO BE SOLVED: To provide an optical pickup capable of detecting change in thickness of a recording medium and/or correcting spherical aberration caused by a change in thickness. SOLUTION: The optical pickup is equipped with, as a light-splitting/detecting means, a photodetector or a light-splitting element installed for the purpose of splitting/ detecting a light beam, which is reflected from a recording medium and which is passed through an objective lens and an optical-path changing means, by taking account of change in light-quality distribution on the thickness change in the recording medium, and also equipped with a plurality of photodetectors for receiving each beam split by the light splitting element; the optical pickup is designed to detect the signals of change in thickness by calculating the detection signals outputted from the photodetectors. The optical pickup device is also capable of detecting the change in thickness of the recording medium even for an optical system structure having no astigmatic lens that causes astigmatism on the light-receiving side, is provided with an actuator that drives a spherical aberration correcting element or a collimator lens along the optical axis, and is able to correct the spherical aberration caused by the change in thickness of the recording medium.