Optical information system focal point displacement detecting means having Fresnel zone plates with rectilinear grating

摘要

This invention pertains to a displacement detecting device for an optical head for detecting focal point displacement of irradiation light. The displacement detecting device has optical means for converting reflected rays from an irradiated face into an astigmatic bundle of rays and a photodetector for detecting a change in the the focused shape. The displacement detecting device thus detects focal point displacement of irradiation light. The optical means is obtained by integrally forming a plurality of Fresnel zone plates having rectilinear gratings, focusing reflected rays from the irradiated face, and having different focal lengths.