Patent ID: 7738345

Claim:
An objective lens used for recording information to and/or reproducing information from three types of optical discs including a first optical disc having a highest recording density, a second optical disc having a second highest recording density and a third optical disc having a lowest recording density by selectively using one of three types of light beams including a first light beam having a shortest wavelength, a second light beam having a second shortest wavelength and a third light beam having a longest wavelength, at least one of surfaces of the objective lens comprising a first region converging the third light beam on a recording surface of the third optical disc, the first region comprising a step structure configured to have a plurality of concentric refractive surface zones and to give an optical path length difference to an incident beam at each step formed between adjacent refractive surface zones, wherein the step structure is configured such that: the optical path length difference given by each step is substantially equal to an odd multiple of the wavelength of the first light beam; and a value of differentiation of an optical path difference function defining the step structure crosses zero in a height ranging from 30% to 70% of an effective diameter of the first region; wherein when the wavelength of the first light beam is represented by λ 1 (nm), an optical path length difference given by each step to the first light beam is represented by ΔOPD (nm), the step structure satisfies a condition: 2 N +0.70 <|ΔOPD /λ1|<2 N +1.30 (1) where N represents an integer, wherein when the optical path difference function is expressed by an equation: φ( h )=( P 2 h 2 +P 4 h 4 +P 6 h 6 +P 8 h 8 +P 10 h 10 +P 12 h 12 )mλ (2) where P 2 , P 4 and P 6 . . . are coefficients of second, fourth, sixth . . . orders, h represents a height from an optical axis, m represents a diffraction order at which diffraction efficiency is maximized, and λ represents a working wavelength of a light beam being used, the objective lens satisfies a condition: 2.50<( f 1 ×P 2 )/( t 3 −t 1)<13.00 (4) where f 1 represents a focal length of the objective lens defined when the first light beam is used, and t 1 and t 3 (where t 1 <t 3 ) respectively represent thicknesses of cover layers of the first and third optical discs.