Objective optical system for optical recording media and optical pickup device using it

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 λ1 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 λ2 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 λ3 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.