Patent ID: 8270279
Filing Date: 2012-09-18
Classification: G11B

Abstract:
1. An objective lens for an optical information recording/reproducing apparatus for recording information to and/or reproducing information from two types of optical discs including first and second optical discs differing in recording density by selectively using two types of light beams including first and second light beams respectively having first and second wavelengths, the first light beam being a substantially collimated light beam and the second light beam being one of a substantially collimated beam and a diverging light beam, when λ when t when NA wherein: at least one of optical surfaces of the objective lens comprises a diffraction surface having a diffraction structure defined by an optical path difference function: where P the diffraction surface has a first region contributing to converging the first light beam onto a recording surface of the first optical disc and converging the second light beam onto a recording surface of the second optical disc; the first region has a diffraction structure configured such that each of a diffraction order at which a diffraction efficiency is maximized for the first light beam and a diffraction order at which a diffraction efficiency is maximized for the second light beam is 1 the diffraction surface has a second region located outside the first region; the second region is configured to contribute to converging the first light beam onto the recording surface of the first optical disc and not to contribute to convergence of the second light beam; when P