Patent ID: 6480344
Filing Date: 2002-11-12
Classification: G02B,G11B

Abstract:
An objective lens for an optical pick-up, comprising:a refractive lens having a positive refractive power; and a diffractive lens structure having a plurality of concentric ring-shaped steps that are formed on at least one lens surface of said refractive lens, wherein said diffractive lens structure has such a wavelength dependence that at least two diffracted beams having different wavelengths with the same diffraction order form appropriate wavefronts for at least two kinds of optical discs having cover layers of different thickness, respectively, wherein said diffractive lens structure has a predetermined power at any points in a middle ring area that is concentrically arranged about the optical axis at a middle range of the radius of said refractive lens, and wherein said middle ring area is defined as an area through which a light beam whose NA is 0.20 through 0.38 passes.