Patent ID: 8199629

Claim:
An objective lens for use in an optical pickup device comprising a light source emitting a light flux with a wavelength of 450 nm or lower, and a light-converging optical system including an objective lens for converging the light flux emitted by the light source onto an information recording surface of an optical information recording medium, the optical pickup device being capable of recording and/or reproducing information when the light converging optical system converges the light flux from the light source onto the information recording surface of the optical information recording medium, the objective lens comprising: a single lens body made with a plastic, and a ring-shaped structure arranged on at least one optical surface of the objective lens and including ring-shaped zones which are three or more and thirty or less in number and are formed such that adjoining ring-shaped zones cause a predetermined optical path difference for an incident light flux, wherein the ring-shaped structure is formed as a turning structure comprising steps whose directions change at least once within an effective aperture of the objective lens, such that the depths of the steps become deeper in an area up to a predetermined height along a perpendicular direction to the optical axis, and the depths of the steps become shallower in an area away from the predetermined height, and the objective lens satisfies the following expressions: NA≧0.8 (1) 2.2 ≧f (mm)≧0.9 (2) 0≦δSAT/ f (λrms/° C.·mm)≦0.0015 (3) −0.012≦δSAλ/ f (λrms/nm·mm)≦−0.00804 (4) WD (mm)≧0.25 (5) where NA is an image side numerical aperture of the objective lens required for recording and/or reproducing information for the optical information recording medium, f (mm) is a focal length of the objective lens, δSAT (λrms/° C.) is a change rate of a third order spherical aberration of the objective lens caused by a temperature change (δSA3/δT), and δSAλ(λrms/nm) is a change rate of a third order spherical aberration of the objective lens caused by a wavelength change (δSA3/δλ, and WD (mm) is a working distance of the objective lens, and wherein the objective lens suppresses generation of a spherical aberration, even if a temperature change arises, by sacrificing wavelength characteristics to satisfy the expression (4) and by enhancing temperature characteristics to satisfy the expression (3), wherein δSAT is measured or calculated under a condition that a change rate of a wavelength of an incident light flux entering the objective lens caused by a temperature change is +0.05 nm/° C., and a sign of the third spherical aberration in δSAT and δSAλ is defined as “positive” for a change in an over-corrected direction and as “negative” for a change in an under-corrected direction.