Optical pickup device, optical information recording and reproducing apparatus, expander lens, coupling lens and chromatic aberration correcting optical element

A coupling lens for an optical pickup apparatus comprising

• a first light source for emitting a first light flux with a first wavelength λ1 of 450 nm or less;
• a second light source for emitting a second light flux with a second wavelength λ2 which is in the range of 600 nm-700 nm;
• a third light source for emitting a third wavelength λ3 which is in the range of 730 nm-830 nm;
• an objective lens unit to converge the first light flux emitted by the first light source onto a first information recording surface of a first optical disk, to converge the second light flux emitted by the second light source onto a second information recording surface of a second optical disk, and to converge the third light flux emitted by the third light source onto a third information recording surface of a third obtical disk,
• wherein the second optical disk has a different recording density from that of the first optical disk,
• wherein the third optical disk has a different recording density from those of the first and the second optical disks,
• wherein the coupling lens is arranged between all of the first light source, the second light source, the third light source, and the objective lens unit, and
• wherein the coupling lens is in a common optical path of the first to third light fluxes,
• the coupling lens comprises a chromatic aberration correcting optical element which includes a diffractive surface on at least one optical surface of the chromatic aberration correcting optical element such that a diffractive structure which is constructed by a plurality of ring-shaped zones separated by fine steps is formed on the diffractive surface;
• the coupling lens is designed so that a diffraction order n2 for a diffracted ray having a largest diffraction efficiency among diffracted rays when the second light flux enters into the diffractive structure is a lower order than a diffraction order n1 for a diffracted ray having a largest diffraction efficiency among diffracted light rays when the first light flux enters into the diffractive structure;
• the coupling lens comprises a plurality of lens groups, and changes a slope angle of a marginal ray in an incident light flux to the objective lens unit by variably adjusting an interval between at least one lens group of the coupling lens and the objective lens unit;
• and
• the coupling lens satisfies one of the following combinations:
• (n1, n2, n3)=(2, 1, 1), (4, 2, 2), (6, 4, 3), (8, 5, 4), (10, 6, 5)
• where n1, n2 and n3 are diffraction orders of diffracted rays, wherein the diffracted rays have their largest diffraction efficiencies when the first, second, and third light fluxes enter into the chromatic aberration correcting optical element, respectively.