Patent ID: 8116187

Claim:
A method of designing an optical element having a diffraction function to be used for an optical disc drive in which each of a plurality of light beams having different design wavelengths passes through the optical element, the optical drive configured to record data to and/or reproduce data from each of first and second optical discs based on different standards, by selectively using a first light beam of the plurality of light beams having a first design wavelength for the first optical disc and a second light beam of the plurality of light beams having a second design wavelength, different from the first design wavelength, for the second optical disc, the method comprising: determining at least two types of optical path difference functions including first and second optical path difference functions in such a manner that: proportion, brought by the first optical path difference function, between diffraction orders at which diffraction efficiencies of the plurality of light beams are maximized is different from proportion, brought by the second optical path difference function, between diffraction orders at which diffraction efficiencies of the plurality of light beams are maximized, wherein the proportion brought by the first optical path difference function is determined such that each of the plurality of light beams is used as diffracted light, and the proportion brought by the second optical path difference function is determined such that each of the plurality of light beams is used as diffracted light, and a plurality of types of optical path difference functions including the at least two types of optical path difference functions having the same kind of function which is a function of correcting spherical aberration, are determined such that a sum of additional optical path lengths defined by the plurality of types of optical path difference functions corrects a spherical aberration caused when a corresponding one of the first and second optical discs is used; and obtaining a combined shape by combining shapes respectively defined by the determined plurality of types of optical path difference functions having the same kind of function which is a function of correcting spherical aberration, and the combined shape is applied to the at least one surface of surfaces of the optical element.