Patent ID: 8279738

Claim:
An objective lens used for an optical information recording/reproducing device for recording information to and/or reproducing information from three types of optical discs, by selectively using one of three types of light beams including a first light beam having a first wavelength λ 1 , a second light beam having a second wavelength λ 2 and a third light beam having a third wavelength λ 3 , the at least three types of optical discs including a first optical disc for which information recording or information reproducing is executed by using the first light beam, a second optical disc for which information recording or information reproducing is executed by using the second light beam, and a third optical disc for which information recording or information reproducing is executed by using the third light beam, the first, second and third wavelengths λ 1 , λ 2 and λ 3 satisfying a condition: λ 1 <λ 2 <λ 3 , the objective lens comprising a first area contributing to converging the third light beam onto a record surface of the third optical disc, the first area including a phase shift structure having a plurality of refractive surface zones concentrically formed about a predetermined axis, the phase shift structure including a first step group and a second step group, wherein: an optical path length difference given by the first step group to an incident light beam between adjacent refractive surface zones in the first step group is different from an optical path length difference given by the second step group to the incident light beam between adjacent refractive surface zones in the second step group; positions of steps formed in the first and second step groups are defined by first and second optical path difference functions, respectively; an i-th optical path difference function φi(h) is expressed by: φ i ( h )=( P i2 h 2 +P i4 h 4 +P 16 h 6 +P i8 h 8 +P i10 h 10 +P i12 h 12 ) m i λ where P i2 , P i4 , P i6 . . . (i: natural number) respectively represent coefficients of the 2 nd order, 4 th order, 6 th order, . . . of the i-th optical path difference function, h represents a height from an optical axis, m i represents a diffraction order at which diffraction efficiency is maximized, and λ represents a design wavelength of the incident light beam; the 2 nd order coefficient P 12 for a first optical path difference function φ 1 (h) is a positive value; the 2 nd order coefficient P 22 of a second optical path difference function φ 2 (h) is a negative value; the first step group is configured such that an optical path length difference ΔOPD 1 given by each step of the first step group to the first light beam satisfies a following condition: 2 N 1 +1.10 <|ΔOPD 1 /λ1|<2 N 1 +1.40 where N 1 is a natural number or zero; and the second step group is configured such that an optical path length difference ΔOPD 2 given by each step of the second step group to the first light beam satisfies a following condition: 2 N 2 −0.10 <|ΔOPD 2 /λ1|<2 N 2 +0.10 where N 2 is a natural number.