Patent ID: 7889618

Claim:
An objective optical element for use in an optical pickup apparatus which conducts reproducing and/or recording information by forming a converged spot on an information recording surface of a first optical information recording medium having a protective substrate thickness t1 with a first light flux of a first wavelength λ1 emitted from a first light source, and conducts reproducing and/or recording information by forming a converged spot on an information recording surface of a second optical information recording medium having a protective substrate thickness t2 (t1≦t2) with a second light flux of a second wavelength λ2 (λ1<λ2) emitted from a second light source, the objective optical element being: an objective optical element which is commonly used for the first and the second optical information recording media, has 2 phase structures of a first phase structure and a second phase structure, and is a single lens whose at least one surface is aspheric surface, wherein the first phase structure corrects at least one of a spherical aberration due to a difference between the protective substrate thickness t1 and the protective substrate thickness t2 and a spherical aberration due to a difference between the first wavelength λ1 and the second wavelength λ2, and the second phase structure corrects at least one of a spherical aberration generated when the first wavelength λ1 is changed in a range of ±10 nm, a best image point movement generated when the first wavelength λ1 is changed in a range of ±10 nm, and a spherical aberration generated when environmental temperature is changed, wherein the first phase structure is a superimposed diffractive structure in which patterns whose sectional shape including the optical axis is a stair-shape, are concentric circularly arranged and steps are shifted by the height of the number of steps corresponding to the number of level surfaces for each of predetermined number of level surfaces, and the superimposed diffractive structure has a wavelength selectivity which does not diffract the first light flux, but diffracts the second light flux, and wherein a optical path difference added to the first light flux by a step difference of one of the superimposed diffractive structure is 2×λ1.