US Patent 7821906 Objective lens for optical pick-up

An objective lens for an optical pick-up which is used to record data to and/or to reproduce data from at least three types of optical discs by selectively using one of at least three light beams having different wavelengths is provided. When thicknesses of a first, second and optical discs are represented by t1, t2 and t3, respectively, a relationship t1≦t2<t3 is satisfied. When numerical apertures required for the first, second and third optical discs are represented by NA1, NA2 and NA3, respectively, a relationship NA1≧NA2>NA3 is satisfied. The first, second and third light beams are incident on the objective lens as substantially collimated light beams, respectively. In this configuration, at least one of surfaces of the objective lens includes a first area for attaining the numerical aperture required for recording data to and/or reproducing data from the third optical disc, and a second area located outside the first area. The first area includes an inner area including an optical axis of the objective lens, and an outer area located outside the inner area. The outer area is configured to converge the third light beam on a data recording layer of the third optical disc with an amount of an aberration being substantially zero. The objective lens satisfies a condition: 0.75<h1a/h1<0.87 . . . (1), where h1 represents an effective radius of the first area, and h1a represents an effective radius of the inner area.