Patent ID: 6594222
Filing Date: 2003-07-15
Classification: G11B

Abstract:
An objective lens for an optical pick-up, comprising:a refractive lens having a positive refractive power; and a diffractive lens structure that is formed as a plurality of concentric ring areas having minute steps at the boundaries therebetween and is formed on at least one lens surface of said refractive lens, wherein said lens surface is divided into a high NA exclusive area through which a light beam of a first NA, which is necessary only for an optical disc having first recording density, passes, and a common area through which a light beam of a second NA, which is lower than said first NA and is necessary and sufficient for an optical disc having second recording density that is lower than said first recording density, passes, wherein the light beam of said first NA has a first wavelength and the light beam of said second NA has a second wavelength that is longer than said first wavelength; wherein said diffractive lens structure formed in said common area has such a wavelength dependence that spherical aberrations with different cover layers in thickness are corrected at said first and second wavelengths, respectively; and wherein said diffractive lens structure formed in said high NA exclusive area has a smaller wavelength dependence of a spherical aberration than that formed in said common area, and is designed such that a spherical aberration at said first wavelength is adequately corrected.