Objective lens and optical pickup apparatus

An objective lens for an optical pickup apparatus conducting at least one of recording and reproducing information for a first optical information recording medium having a protective layer with a thickness t1 using a first light flux with a wavelength λ1, and conducting at least one of recording and reproducing information for a second optical information recording medium having a protective layer with a thickness t2 (t2≧t1) using a second light flux with a wavelength λ2 (λ1>λ2), the objective lens comprising:

• one optical surface in an aspherical shape including
• a central area for converging the first light flux and the second light flux and
• a peripheral area for converging the first light flux,
• wherein the central area comprises a first diffractive structure being a blaze type and including a plurality of ring-shaped zones around an optical axis,
• an optical path difference function of the first diffractive structure is Φ(h)=C2h2+ΣC2ih2i, where h is a height from an optical axis, i is an integer which is two or more, and C2 and C2i are coefficients,
• the optical path difference function Φ(h) has a local maximum value for a predefined height h1 which is smallest among heights h corresponding to local minimum or local maximum values of the optical path difference function Φ(h), and
• the peripheral area is an optical surface which makes a light flux passing through the peripheral area reach to a position apart from the optical axis on an information recording surface of the second optical information recording medium, when the optical pickup apparatus records and reproduces information on the second optical information recording medium using the second light flux.

An objective lens for an optical pickup apparatus conducting at least one of recording and reproducing information for a first optical information recording medium having a protective layer with a thickness t1 using a first light flux with a wavelength λ1, and conducting at least one of recording and reproducing information for a second optical information recording medium having a protective layer with a thickness t2 (t2≧t1) using a second light flux with a wavelength λ2 (λ1>λ2), the objective lens comprising:

• one optical surface in an aspherical shape including
• a central area for converging the first light flux and the second light flux and
• a peripheral area for converging the first light flux,
• wherein the central area comprises a first diffractive structure being a blaze type and including a plurality of ring-shaped zones around an optical axis,
• an optical path difference function of the first diffractive structure is Φ(h)=C2h2+ΣC2ih2i, where h is a height from an optical axis, i is an integer which is two or more, and C2 and C2i are coefficients, and
• the coefficients C2 and C2i satisfy a following expression:
• −ΣC2ihc2(i−1)−10 λ2 h−2≦C2≦−ΣC2ihc2(i−1)+9 λ2 h−2
• where hc is a height of a boundary between the central area and the peripheral area, and
• the peripheral area is an optical surface which makes a light flux passing through the peripheral area reach to a position apart from the optical axis on an information recording surface of the second optical information recording medium, when the optical pickup apparatus records and reproduces information on the second optical information recording medium using the second light flux.