Patent ID: 7457223

Claim:
An objective lens for use with optical recording media, the objective lens consisting of two lens elements of different materials with surfaces that are cemented together, wherein: the lens surfaces that are cemented together include a phase function Φ so that the cemented surface becomes an optical diffractive surface that enables the objective lens to focus incident light of a first wavelength λ1 at a first numerical aperture NA1 onto a desired portion of a first optical recording medium having a thickness d1, that enables the objective lens to focus incident light of a second wavelength λ2 at a second numerical aperture NA2 onto a desired portion of a second optical recording medium having a thickness d2, and that enables the objective lens to focus incident light of a third wavelength λ3 at a third numerical aperture NA3 onto a desired portion of a third optical recording medium having a thickness d3; the optical diffractive surface is shaped so that the order of the diffracted light of the first wavelength λ1 having the largest diffracted intensity is different from the order of the diffracted light of the second wavelength λ2 having the largest diffracted intensity, the order of the diffracted light of the first wavelength λ1 having the largest diffracted intensity is also different from the order of the diffracted light of the third wavelength λ3 having the largest diffracted intensity, and the order of the diffracted light of the first wavelength λ1 having the largest diffracted intensity is the zero-order; and said phase function Φ is given by the following equation: Φ=Σ W i · Y 2i where Y is distance from the optical axis; and W i is a phase difference coefficient, and the following conditions are satisfied: NA1≧NA2>NA3 λ1<λ2<λ3 d1≦d2<d3.