Patent ID: 8233370

Claim:
An objective lens configured to collect light beams with at least three wavelengths λ 1 , λ 2 , and λ 3 , which satisfy a relationship of λ 1 <λ 2 <λ 3 , on signal recording surfaces of compatible optical discs corresponding to respective wavelengths, the objective lens comprising: a diffractive portion that has a predetermined diffraction structure formed on an incident side surface, wherein the diffractive portion has a first region that is provided in an innermost peripheral portion so as to diffract the light beams, a second region that is provided outside the first region so as to diffract the light beams, and a third region that is provided outside the second region, wherein the first to third regions are formed to have diameters of apertures allowing a light beam with the wavelength λ 1 to correspond to the first to third regions, a light beam with the wavelength λ 2 to correspond to the first and second regions, and a light beam with the wavelength λ 3 to correspond to the first region, and wherein in the first region, a diffraction structure is formed in an orbicular zone shape with a predetermined height, wherein the diffraction structure of the first region is a periodic structure in which structures having a unit period are successively formed in a radial direction of the orbicular zone, and a structure in which a blazed structure with a height h is overlapped with a multi-step diffraction structure, wherein a slope direction of the blazed structure is an increasing direction opposite to a direction of increasing heights of steps of the multi-step diffraction structure, wherein the height h satisfies a relational expression of h<(λ 1 /(n 1 −1)), where n 1 is a refractive index of a constituent material of the objective lens at the wavelength λ 1 , and wherein the multi-step diffraction structure is a non-equidistant multi-step diffraction structure that is different from an equidistant multi-step diffraction structure, in which widths of the steps in the period are equivalent and heights of the steps in the period are equivalent, in that the widths thereof are changed; wherein the diffraction structure of the first region is a structure in which one blazed structure is overlapped with the multi-step diffraction structure and a binary structure is further overlapped therewith, and wherein the binary structure is a non-equidistant binary structure corresponding to the widths of the steps of the non-equidistant multi-step diffraction structure.