| 摘要 |
There is provided an objective lens used for at least three types of optical discs. When thicknesses of a first, second and third optical discs are respectively represented by t1, t2 and t3, t1 is 0.6 mm, t2 is 0.6 mm, and t3 is 1.2 mm. When numerical apertures for the first, second and third optical discs are respectively represented by NA1, NA2 and NA3, a relationship NA1>=NA2>NA3 is satisfied. When the first and second optical discs are used, collimated light beams is incident on the objective lens. When the third optical disc is used, a diverging beam is incident on the objective lens. The following conditions are satisfied: -0.02<F1xM1<0.02 . . . (1), -0.02<F2xM2<0.02 . . . (2), and -0.29<F3xM3<-0.19 . . . (3). The objective lens includes a diffracting structure having a first region. The diffracting structure within the first region is configured such that a diffraction order at which diffraction efficiency of a first light beam is maximized is a second order and that diffraction orders at which diffraction efficiency of second and third light beams are maximized are first orders.
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