Patent ID: 7742370

Claim:
An objective lens to be employed for an optical information read/write device that is configured to perform read/write operations for reading and/or writing information on each of a plurality of kinds of optical discs using a corresponding one of a plurality of kinds of laser beams having different wavelengths, the plurality of kinds of laser beams including three kinds of laser beams that have a first wavelength λ1 (nm), a second wavelength λ2 (nm), and a third wavelength λ3 (nm) according to a relationship λ1<λ2<λ3, respectively, the objective lens comprising a phase shift structure on at least one surface of the objective lens that has a plurality of ring-shaped refractive surface zones into which the at least one surface is concentrically divided, wherein the objective lens is made of material with an Abbe number νd satisfying a condition: 40≦νd≦80, and wherein the phase shift structure has a step between each couple of the adjacent refractive surface zones that gives an optical path difference to an incident laser beam, and wherein the phase shift structure is configured to satisfy a condition: 3.04<|ΔOPD/λ1|<3.29, where ΔOPD represents the optical path difference (nm) that the step gives to the laser beam with the first wavelength λ1.