Compound objective lens having two focal points and apparatus using the lens

摘要

A compound optical lens (29) for focussing a light a transparent layer (22,24) on an information medium comprises a refractive lens (27);
the compound optical lens having a plurality of regions providing a plurality of numerical apertures (NA1,NA2) corresponding to transparent layers which have different thickness (T1,T2), where NA1 is lager than NA2 and T1 is thinner than T2;
the plurality of regions include first and second regions, wherein:
said first region (26b) is further from the optical axis of said refractive lens than the second region (26a);
wherein a light spot through a layer of thickness T1 with numerical aperture NA1 is converged from said first and second regions and;
a light spot through a layer of thickness T2 with numerical aperture NA2 is converged from said second region;
the compound optical lens further comprises a hologram lens (26) to correct the spherical aberration caused by the difference of layer thickness (T1,T2).