Patent ID: 6760295
Filing Date: 2004-07-06
Classification: G02B,G11B

Abstract:
An optical pick-up that is used for at least two kinds of optical discs respectively provided with cover layers having different thicknesses, said optical pick-up comprising:a plurality of light sources that emit light beams having different wavelengths, said plurality of light sources being switched with each other according to the kind of optical disc used; a refractive lens element that converges said light beams from said plurality of light sources onto recording layers of said optical discs; and a spherical aberration correcting element on which a concentric phase grating structure is formed, said phase grating structure altering a spherical aberration in response to a change of wavelength to correct a change of the spherical aberration due to a change of the thickness of said cover layer, wherein said spherical aberration correcting element is located between said plurality of light sources and said refractive lens element, said phase grating structure having a wavelength dependence such that said spherical aberration varies in an undercorrected direction as a wavelength of incident light increases, the following condition (1) being satisfied: âˆ’15h45)/&lgr;âˆ’P2Ã—(h45)2where h45 is the height from the optical axis of a point where a light ray of which NA is 0.45 intersects said phase grating structure, &lgr; is a design wavelength, and P2 is a second order coefficient when an additional optical path length added by said phase grating structure is expressed by the following optical path difference function &PHgr;(h):&PHgr;(h)=(P2h2+P4h4+P6h6+ . . . )Ã—&lgr;where P4 and P6 are coefficients of forth and sixth orders, and h is a height from the optical axis.