Patent ID: 6781764
Filing Date: 2004-08-24
Classification: G02B,G11B

Abstract:
An optical system for an optical pick-up comprising:a light source that selectively emits a first laser beam and a second laser beam, a wavelength of the second laser beam being longer than that of the first laser beam; and an objective lens that converges the first laser beam onto an information layer of a first optical disc through a cover layer thereof and converges the second laser beam onto an information layer of a second optical disc through a cover layer thereof, the second optical disc having lower recording density and a thicker cover layer than the first optical disc, wherein said light source is designed such that the divergence of the second laser beam incident on said objective lens is larger than that of the first laser beam thereby correcting part of spherical aberration caused by a difference in thicknesses of the cover layers, wherein at least one surface of said objective lens is divided into a central area, through which a laser beam of low NA that is necessary and sufficient for the second optical disc, passes and a peripheral area, through which a laser beam of high NA that is necessary only for the first optical disc, passes, and wherein a diffractive lens structure that changes spherical aberration in accordance with wavelength is formed within at least said central area so that the residual part of the spherical aberration caused by the difference in thicknesses of the cover layers is corrected for as the wavelength of the laser beam is switched; wherein said objective lens is configured to satisfy the condition: 0.01Â·(fl/&phgr;2)2<0.03 where, &Dgr;SA is the residual spherical aberration (unit: mm) at the edge of the effective diameter in said central area when a laser beam whose wavelength is equal to said first laser beam and whose divergence is equal to said second laser beam is incident on said second optical disc, fl is a focal length (unit: mm) of said objective lens at the wavelength of said first laser beam, and &phgr;2 is the maximum diameter (unit: mm) of said central area.