Patent ID: 7443779

Claim:
An objective optical system having a light source side and a recording media side for receiving light from the light source side and focusing the light on an optical recording medium in an optical pickup device, comprising: a diffractive optical element that consists of, arranged in order along an optical axis from the light source side, a first lens element and a second lens element that are made of different materials and that are cemented together at a diffractive surface; wherein the diffractive optical element is on the light source side of the objective optical system; the diffractive surface has a phase function that enables the objective optical system to focus light of a first wavelength λ 1 incident from the light source side of the objective optical system at a first numerical aperture NA1 onto a desired portion of a first optical recording medium having a substrate thickness T1, enables the objective optical system to focus light of a second wavelength λ 2 incident from the light source side of the objective optical system at a second numerical aperture NA2 onto a desired portion of a second optical recording medium having a substrate thickness T2, and that enables the objective optical system to focus light of a third wavelength λ 3 incident from the light source side of the objective optical system at a third numerical aperture NA3 onto a desired portion of a third optical recording medium having a substrate thickness T3; and the following conditions are satisfied: NA1≧NA2≧NA3 λ1<λ2<λ3 T1≦T2<T3.