| 摘要 |
An optical scanning device is used for scanning first and second information layers with first and second radiation beams. The device includes a radiation source and an objective lens assembly (31). The lens assembly includes a doublet-lens system (61, 62) including a first objective lens (61) with a first cross-section having a first diameter (d1) and a second objective lens (62) with a second cross-section having a second diameter (d2). The first and second lenses are arranged for transforming the first radiation beam to a first focused radiation beam having a first numerical aperture. The objective lens assembly further includes a third objective lens (63) for transforming the second radiation beam to a second focused radiation beam having a second, smaller numerical aperture. The second and third objective lenses (62, 63) are integrally formed in one body (64). In a preferred embodiment of the objective lens assembly, the second diameter is smaller than the first diameter.
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