Patent ID: 7333416

Claim:
An objective lens for an optical pickup apparatus comprising a first light source to emit a light flux having a wavelength λ 1 , a second light source to emit a light flux having a wavelength λ 2 (λ 1 <λ 2 ) and a light converging optical system, wherein the optical pickup apparatus conducts recording or reproducing information for a first optical information recording medium comprising transparent base board having a thickness t 1 by using the first light source and the light converging optical system, and the optical pickup apparatus conducts recording or reproducing information for a second optical information recording medium comprising a transparent base board having a thickness t 2 (t 1 <t 2 ) by using the second light source and the light converging optical system, the obiective lens comprising: at least two kinds of optically functional surfaces which are arranged in a direction perpendicular to an optical axis and have a different optical action from others; wherein light flux passing through at least an outermost optically functional surface among the optically functional surfaces is used for recording or reproducing information only when recording or reproducing information for the first optical information recording medium, wherein the objective lens is made of a uniform optical material, wherein a value of refractive index change dn/dT for a temperature change of the optical material satisfies the following conditional formula under the conditions of the wavelength of the first light source and the second light source and a room temperature environment | dn/dT |≦10.0×10 −6 (/° C.), and wherein at least one optically functional surface comprises a diffractive structure, wherein the optically functional surface closest to the optical axis is designed so that the spherical aberration in the course of conducting recording or reproducing of information for the first and second optical information recording media may be corrected by the use of a light flux passing through the optically functional surface closest to the optical axis, and wherein the outermost optically functional surface is designed so that the spherical aberration in the course of conducting recording or reproducing of information for the first optical information recording medium may be corrected by the use of a light flux passing through the outermost optically functional surface and over spherical aberration in the course of conducting recording or reproducing of information for the second optical information recording medium is generated on the second optical information recording medium.