LENS, AND OPTICAL SYSTEM, OPTICAL HEAD AND OPTICAL DISK DEVICE USING THE LENS

摘要

PROBLEM TO BE SOLVED: To provide a lens capable of condensing an optical beam on an information recording surface with a high efficiency for light utilization for each of plural kinds of light recording mediums having transparent substrates of different thicknesses, and to provide an optical system, an optical head and an optical disk device using the lens.

SOLUTION: The lens is designed so that the common use region for all monochromatic light in one side lens surface of a lens for multi-wavelength which condenses the plural kinds of monochromatic light of wavelength λiis divided into a plurality of aspheric parts having different refractive forces. When the amount of level difference in the direction parallel to the lens optical axis on the part where the aspheric parts are adjacent to each other among respective aspheric parts is Djin order near to the lens optical axis and, on the level differences corresponding to half or more of the level differences in the adjacent parts, the minimum value of Aijwith respect to respective wavelength λiis MIN(Aij) and the maximum value of Aijwith respect to respective wavelength λiis MAX(Aij), the following relation is satisfied; MAX(Aij)/MIN(Aij)<3, wherein Aij=¾Bij-mij¾, Bij=¾Dj¾×(ni-1)/λi-C, niis refractive index of lens at wavelength λi, mijis integer nearest to Bijand C is a correction term.

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