Patent ID: 7190658

Claim:
An optical element for use in an optical pickup apparatus for converging a light flux onto an information recording plane of an optical information recording medium, comprising: an optical path difference providing section having a function of providing an optical path difference, the optical path difference providing section being divided coaxially around an optical axis so as to form a plurality of ring-shaped zones each of which has a respective different thickness along the direction of the optical axis so that the optical path difference providing section makes the length of an optical path of a light ray having passed through a ring-shaped zone among the plurality of ring-shaped zones longer than the length of an optical path of a light ray having passed through a neighboring ring-shaped zone located next to an optical axis side of the ring-shaped zone by a length obtained by multiplying an operating reference wavelength λ 0 with an integer; and a refracting section having a refracting function of optimizing a spherical aberration of a converged light spot of a light flux which has the operating reference wavelength λ 0 and is converged onto the information recording plane; wherein the optical element converges an incident light flux having the operating reference wavelength λ 0 onto the information recording plane with almost no aberration, and the following formula is satisfied: |Δ f B /(λ−λ 0 )|<2000 where Δf B is a distance along the direction of the optical axis between a converged light spot formed by an incident light flux having an operating wavelength λ different from the operating reference wavelength λ 0 (|λ−λ 0 |<1 [nm]) and a converged light spot formed by an incident light flux having the operating reference wavelength λ 0 , and wherein in each of the plurality of ring-shaped zones, the radius (mm) of the innermost peripheral section and the radius (mm) of the outermost peripheral section of each zone are determined such that a wavefront error at the converged light spot formed by the incident light flux having the operating wavelength λ between a light ray having passed through the innermost peripheral section and a light ray having passed through the outermost peripheral section becomes ΔΨ(λ), ΔΨ(λ)=Ψ(λ 0 )(−1/λ 0 +δ/( n (λ 0 )−1))×(λ−λ 0 ) = k (−1/λ 0 +δ/( n (λ 0 )−1))Δλ (Δλ=λ−λ 0 ,δ=dn/d λ) wherein n(λ 0 ) is a refractive index, δ is a change of the refractive index due to variations of the wavelength, and k is an integer.