| 摘要 |
PROBLEM TO BE SOLVED: To attain stable detection of spherical aberrations which is hardly affected by the change of environment, such as lapse of time or temperature, by arranging a light-shielding means using a liquid crystal element, in a nearly parallel light path without increasing the number of photodetectors, and to realize lowering of cost by reducing the number of components and the assembling process. SOLUTION: The optical system is operated, in such a manner that laser beams from a semiconductor laser 2 are reflected by a multilayer information recording medium 1 through a beam splitter 4 and an objective lens 6, and the reflected beams are deflected by the beam splitter 4, and the center area or the peripheral area of the reflected beam is polarized by a rotating means 7, and either one of the center area or the peripheral area is made incident on the photodetector 10 by a polarizing plate 8; then the reflected beam of the peripheral area is detected, after the reflected beam of the center area is detected, and on the basis of the difference between both beams, a spherical aberration signal is produced by a spherical aberration control circuit 13, and a spherical aberration correcting means 12 is driven on the basis of this signal. |
