Patent ID: 7327663

Claim:
A recording reproducing optical system for conducting recording and/or reproducing information for an optical information recording medium, comprising: a light source; an objective lens to converge a light flux emitted from the light source onto an information recording plane of the optical information recording medium; and an aberration correcting optical element provided on an optical path between the light source and the objective lens; wherein the objective lens includes an aspherical surface on at least one surface thereof and at least one diffractive surface on which a diffractive structure structured by a plurality of concentric ring-shaped zonal steps is formed and the objective lens is a plastic lens structured in one element in one group, wherein the aberration correcting optical element comprises at least one plastic lens on which a diffractive structure structured by a plurality of concentric ring-shaped zonal steps is formed on at least one surface thereof, and wherein the following formulas are satisfied: PD1>0 PD2<0 PR2>0 where PD 1 is a diffractive power (mm −1 ) as a diffractive lens defined by the formula of PD 1 =Σ(−2·b 2i ·ni) when an optical path difference φ bi (mm) added to a wavefront having transmitted through a diffractive structure formed on an i-th surface of the objective lens is expressed as a function of a height hi (mm) from the optical axis by an optical path difference function defined by φ bi =ni·(b 2i ·hi 2 +b 4i ·hi 4 +b 6i ·hi 6 + . . . ) (herein, ni is a diffraction order of a diffracted ray having the maximum diffraction efficiency among diffracted rays generated by the diffractive structure formed on the i-th surface, and b 2i , b 4i , b 6i , . . . are optical path difference function coefficients of second order, fourth order, sixth order, . . . respectively), PD 2 is a diffractive power (mm −1 ) as a diffractive lens defined by the formula of PD 2 =Σ(−2·b 2j ·nj) when an optical path difference φ bj (mm) added to a wavefront having transmitted through a diffractive structure formed on an j-th surface of the aberration correcting optical element is expressed as a function of a height hi (mm) from the optical axis by an optical path difference function defined by φ bj =nj·(b 2j ·hj 2 +b 4j ·hj 4 +b 6j ·hj 6 + . . . ) (herein, nj is a diffraction order of a diffracted ray having the maximum diffraction efficiency among diffracted rays generated by the diffractive structure formed on the j-th surface, and b 2j , b 4j , b 6j , . . . are optical path difference function coefficients of second order, fourth order, sixth order, . . . respectively), and PR 2 is a refractive power (mm −1 ) as a refractive lens of the plastic lens of the aberration correcting optical element.