Patent ID: 8068401
Filing Date: 2011-11-29
Classification: G11B

Abstract:
1. An objective optical system used for an optical information recording/reproducing device for recording information to and/or reproducing information from three types of optical discs, by selectively using one of three types of substantially collimated light beams including a first light beam having a first wavelength λ 1 (nm), a second light beam having a second wavelength λ 2 (nm) and a third light beam having a third wavelength λ 3 (nm), the three types of optical discs including a first optical disc for which information recording or information reproducing is executed by using the first light beam, a second optical disc for which information recording or information reproducing is executed by using the second light beam, and a third optical disc for which information recording or information reproducing is executed by using the third light beam, the first, second and third wavelengths λ when protective layer thicknesses of the first, second and third optical discs are represented by t when numerical apertures required for information reproducing or information recording on the first, second and third optical discs are defined as NA the objective optical system including: an objective lens; and a diffraction structure formed on an optical surface in the objective optical system,