MULTI-FOCAL OBJECT LENS, OPTICAL PICKUP DEVICE AND OPTICAL INFORMATION RECORDING/REPRODUCING DEVICE

摘要

A multi-focal object lens used in an optical pickup device having a first diffraction structure on at least one optical plane and condensing a m-order diffraction light and an n-order diffraction light (m<>n) occurring when a light flux at wavelength lambda1 enters the first diffraction structure onto the information recording surfaces of two kinds of optical disks mutually different in protection layer thickness, wherein the absolute value of the difference between the spherical aberration value SAm of a wave front when the m-order diffraction light is condensed via the protection layer of an optical disk having a thicker protection layer and the spherical aberration value SAn of a wave front when the n-order diffraction light is condensed via the protection layer of an optical disk having a thicker protection layer and the absolute value of either one of the SAm and SAn are specified values, the above first diffraction structure is composed of a plurality of annular zones divided by fine step differences, and the above diffraction order m is an integer of one or larger and satisfies a specified expression.