Patent ID: 6791932
Filing Date: 2004-09-14
Classification: G11B

Abstract:
An optical system of an optical pick-up that is capable of using a first optical disc having a first cover layer and a second optical disc having a second cover layer thicker than said first cover layer, said optical system comprising:a light source portion that emits a first laser beam having a first wavelength for said first optical disc and a second laser beam having a second wavelength longer than said first wavelength for said second optical disc; and an objective lens that converges said first laser beam from said light source portion onto an information layer of said first optical disc through said first cover layer and that converges said second laser beam from said light source portion onto an information layer of said second optical disc through said second cover layer, said objective lens being provided with a diffractive lens structure that has a plurality of concentric ring-shaped steps that have wavelength dependence such that spherical aberration varies in the overcorrected direction as wavelength of incident light increases, wherein divergence of said second laser beam incident on said objective lens is larger than divergence of said first laser beam, and wherein said objective lens converges said first and second laser beams of an identical diffractive order while keeping a constant distance between said objective lens and the surfaces of said first and second cover layers.