Patent ID: 7639589

Claim:
An objective lens for use in an optical pickup apparatus, comprising: a first light source for emitting a light flux having a wavelength λ1; a second light source for emitting a light flux having a wavelength λ2, where (λ1≦λ2); a light converging optical system; a first optical information recording medium including a transparent base board having a thickness t 1 , the optical pickup apparatus recording or reproducing information for the first optical information recording medium by using the first light source and the light converging optical system; a second optical information recording medium including a transparent base board having a thickness t 2 , where (t 1 <t 2 ), the optical pickup apparatus recording or reproducing information for the second optical information recording medium by using the second light source and the light converging optical system; and at least two types of optically functional surfaces arranged in a direction perpendicular to an optical axis, the optically functional surfaces having a different optical action from others, wherein a 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 a pasted lens in which there are at least two types of optical elements each made of an optical material different from the other, wherein a value of refractive index change dn/dT for a temperature change of the optical material used for the optical elements having a power component stronger than that of other satisfies the following conditional formula under the conditions of the wavelengths of the first light source and the second light source and a room temperature environment | dn/dT|≦ 10.0×10 −6 (/° C.), wherein both of the outermost optically functional surface and the optically functional surface closest to the optical axis comprise a diffractive structure, and wherein a diffraction order n ot of a diffracted light ray having a highest intensity generated at the diffractive structure on the outermost optically functional surface and a diffraction order n in of a diffracted light ray having a highest intensity generated at the diffractive structure on the optically functional surface closest to the optical axis satisfy the following conditional formula when recording or reproducing information for the first optical information recording medium | n ot |>| n in |.