| 摘要 |
There is provided an objective lens used for three types of optical discs including by selectively using one of three types of light beams. At least one of surfaces of the objective lens is provided with a first region converging the third light beam on a recoding surface of the third optical disc. The first region has a step structure configured to have 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. The step structure is configured such that the optical path length difference given by each step is substantially equal to an odd multiple of a wavelength of a 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.
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