Patent ID: 7920456
Filing Date: 2011-04-05
Classification: G02B,G11B

Abstract:
1. An objective lens used for an optical pickup device, wherein the optical pickup device comprises: a light source; and a converging optical system including the objective lens for converging a light beam emitted from the light source to an information recording surface of an optical information recording medium, wherein the optical pickup device is capable of recording and/or reproducing information by converging the light beam emitted from the light source to the information recording surface of the optical information recording medium with the converging optical system, wherein the objective lens is a plastic single lens that comprises a ring-shaped phase structure on at least one optical surface, the ring-shaped phase structure comprising a plurality of ring surfaces and formed so that adjacent ring surfaces generate a predetermined optical path difference for incident light, wherein when a ring surface including an optical axis is called a central ring surface, a ring surface adjacent to an outside of the central ring surface is formed to be displaced in the optical axis direction so as to have a shorter optical path length than the central ring surface, a ring surface at a maximum effective diameter position is formed to be displaced in the optical axis direction so as to have a longer optical path length than a ring surface adjacent to an inside thereof, wherein the objective lens satisfies following formulas: where NA is an image-side numerical aperture of the objective lens, which is required for recording and/or reproducing information for the optical information recording medium, f (mm) is a focal length of the objective lens, λ wherein X1 is a plus in a case of measuring X1 in a direction of the optical information recording medium with reference to the tangent plane, and minus in a case of measuring X1 in a direction of the light source, and wherein X2 is plus in a case of measuring X2 in a direction of the optical information recording medium with reference to the tangent plane and minus in a case of measuring X2 in a direction of the light source.