OPTICAL DEVICE

摘要

PROBLEM TO BE SOLVED: To provide an optical device for reproducing disks of plural formats by one optical system in an optical device optically recording information in a disk or reproducing information recorded in the disk. SOLUTION: A twisted element 32 which can change the optical anisotropy is arranged in an objective lens holder 14 and the optical anisotropy is changed. Consequently, by changing the diameter of a laser beam 3 made incident on an objective lens, plural numbers of numerical apertures suitable for disks 9 of plural formats are distinguished for their use.