Patent ID: 8483033
Filing Date: 2013-07-09
Classification: G11B

Abstract:
1. An objective optical system for an optical information recording/reproducing apparatus configured to record information to and/or reproduce information from three types of optical discs including first, second and third optical discs differing in recording density, by selectively using light beams having first, second and third wavelengths emitted from light sources, the objective optical system comprising at least an objective lens, when λ when NA at least one surface of the objective optical system being configured to be a phase shift surface having a phase shift structure including a plurality of refractive surface zones concentrically divided so as to have steps giving different phase differences to an incident light beam at a boundary between adjacent ones of the plurality of refractive surface zones, wherein: the phase shift surface has a first area contributing to converging the first, second and third light beams onto recording surfaces of the first, second and third optical discs, respectively; the first area has an effective diameter larger than NA 0.3 at the first wavelength; in the first area, the phase shift surface has at least two types of phase shift structures including a first phase shift structure having first steps and a second phase shift structure having second steps; when P when Δφ