Objective lens for an optical pick up of an apparatus for optical disks

摘要

The objective lens (10) is a biconvex plastic lens with first and second aspheric surfaces. It has a refractive lens with a positive refractive effect. It also has a diffraction grating (11) formed on at least one lens surface of the refractive lens. The grating has several concentric annular steps. The wavelength dependence of the diffraction grating is such that the spherical aberration varies with increasing wavelength of incident light in the under corrected direction.