Patent ID: 7680014

Claim:
An objective optical system for focusing light from a light source onto optical recording media, the objective optical system comprising, in order from the light source side along an optical axis: a diffractive optical element having negative refractive power and a diffractive surface on one side; and an objective lens having positive refractive power and a rotationally symmetric aspheric surface on each side; wherein the distance along the optical axis between the diffractive optical element and the objective lens is variable; the objective optical system is configured to receive a collimated light beam of a first wavelength λ 1 on its light source side and focus second-order diffracted light diffracted by the diffractive surface of the diffractive optical element at a first numerical aperture NA 1 onto a desired portion of a first optical recording medium having a substrate thickness T 1 when the distance along the optical axis between the diffractive optical element and the objective lens is d 1 , to receive a collimated light beam of a second wavelength λ 2 on its light source side and focus first-order diffracted light diffracted by the diffractive surface of the diffractive optical element at a second numerical aperture NA 2 onto a desired portion of a second optical recording medium having a substrate thickness T 2 when the distance along the optical axis between the diffractive optical element and the objective lens is d 2 , and to receive a collimated light beam of a third wavelength λ 3 on its light source side and focus first-order diffracted light diffracted by the diffractive surface of the diffractive optical element at a third numerical aperture NA 3 onto a desired portion of a third optical recording medium having a substrate thickness T 3 when the distance along the optical axis between the diffractive optical element and the objective lens is d 3 ; and the following conditions are satisfied: λ1<λ2<λ3 NA1≧NA2≧NA3 T1≦T2<T3 d1<d3 d2<d3.