Patent ID: 7408866

Claim:
An objective lens for converging a first light flux of a first wavelength λ 1 emitted from a first light source so as to conduct reproducing and/or recording information for a first optical information recording medium including a first protective substrate having a thickness t 1 (0 mm≦t 1 ≦0.2 mm) and for converging a second light flux of a second wavelength λ 2 (λ 1 <λ 2 ) emitted from a second light source so as to conduct reproducing and/or recording information for a second optical information recording medium including a second protective substrate having a thickness t 2 (t 2 >t 1 ), comprising: a first diffractive structure provided on at least one optical surface thereof and having plural concentric ring-shaped zones set such that n 2 , which is an order of a diffracted ray having the maximum light amount among diffracted rays generated when the second light flux comes into the first diffractive structure, is a lower order than n 1 , which is an order of a diffracted ray having the maximum light amount among diffracted rays generated when the first light flux comes into the first diffractive structure, where n 1 and n 2 are an integer other than 0, wherein the n 1 -th order diffracted ray is converged on an information recording surface of the first optical information recording medium through the first protective substrate in such a way that when a wavefront aberration is measured within a first numerical aperture NA 1 , the RMS value of the wavefront aberration becomes 0.07λ1 or less, and the n 2 -th order diffracted ray is converged on a information recording surface of the second optical information recording medium through the second protective substrate in such a way that when a wavefront aberration is measured within a second numerical aperture NA 2 (NA 2 <NA 1 ), the RMS value of the wavefront aberration becomes 0.07λ2 or less, and wherein the following formula is satisfied: 0.03<|(ΔSA/Δλ)/{(NA2) 4 ·f1}|<0.14 where (ΔSA/Δλ) represents a change ratio (λRMS/nm) of a spherical aberration on the first diffractive structure within the second numerical aperture NA 2 in the case that the first wavelength λ 1 change within a range of ±10 nm, and f 1 is a focal length (nm) of an entire system of the objective lens for the first wavelength λ 1 .