Patent ID: 7616549

Claim:
A chromatic aberration-correcting optical element comprising: a stepped structure having a concentric pattern on at least one side, wherein the stepped structure further comprises a step depth d, wherein d is defined by the formulas: d=m o ×λ o /( n o −1); d ( n 1 −1)/λ 1 =m 1 +α; and d ( n 2 −1)/λ 2 =m 2 +β where m o is an integer, λ o is a wavelength for a high-density recording medium, and n o is a refractive index of a diffractive optical element at the wavelength λ o ; m 1 is an integer, λ 1 is a wavelength for a first low-density recording medium, n 1 is a refractive index for a diffractive optical element, and −0.1<α<0.1; m 2 is an integer, λ 2 is a wavelength for a second low-density recording medium, and −0.1<β<0.1, and the first low-density recording medium has a higher recording density than the second low density recording medium, wherein a phase difference is generated between two adjacent steps of the stepped structure represented by d(n 0 −1)/(λ o +Δλ), when a light beam with a wavelength of (λ o +Δλ) is incident on the chromatic aberration-correcting element, wherein d ( n 0 −1)/(λ o +Δλ)≈{ d ( n o −1)/λ o }(1−Δλ/λ o )= m o (1−Δλ/λ o ) (λ o >>Δλ), and wherein steps of the concentric pattern increases by one order with regard to a radius r of the chromatic aberration-correcting element when a waterfront aberration increases by −m o (1−Δλ/λ o ).