Patent attributes
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,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,a step structure of at least one step is formed along a circle or an ellipse on a surface of the transparent substrate, andand by the step of the step structure, a phase of the first light ray is shifted by 2πn1+Δ1 and a phase of the second light ray is shifted by 2πn2+Δ2.
