Optical disk device

An optical disk device comprising:

• a first emission light source;
• a second emission light source;
• a transparent substrate;
• and
• an objective lens,
• wherein a first light ray with a wavelength λ1 emanating from the first emission light source passes through the transparent substrate and is condensed by the objective lens on a signal surface of a first optical disk with a substrate thickness t1,
• wherein a second light ray with a wavelength λ2 (≠λ1) emanating from the second emission light source passes through the transparent substrate and is condensed by the objective lens on a signal surface of a second optical disk with a substrate thickness t2,
• wherein a step structure of at least one step is formed along a circle or an ellipse on a surface of the transparent substrate,
• wherein a phase of the first light ray is shifted by 2πn1+Δ1 (n1 is an integer, and Δ1 is not 0 but a value between −π and π) by the step structure and a phase of the second light ray is shifted by 2πn2+Δ2 (n2 is an integer, and Δ2 is not 0 but a value between −π and π) by the step structure,
• wherein a light ray incident with a substantially parallel wavefront on at least an area of the objective lens is condensed with substantially no aberration on a signal surface of a third optical disk with a substrate thickness t3 (a value different from t1 and t2), and
• wherein the substrate thickness t3 is a value between t1 and t2 and a relationship Δ1×Δ2<0 holds between the phase differences Δ1 and Δ2.