OBJECTIVE FOR OPTICAL DISK, OPTICAL HEAD DEVICE AND OPTICAL INFORMATION RECORDING/REPRODUCING APPARATUS BOTH COMPRISING THE SAME, MOLD FOR FORMING LENS, METHOD FOR MACHINING MOLD FOR FORMING LENS, AND SHAPE MEASURING INSTRUMENT

摘要

An objective lens for an optical disk including one lens and capable of satisfying an excellent focusing property on any of two kinds of optical disks each having a different thickness. The objective lens (2) is formed of a single lens having aspheric surfaces on both sides with the surface (3) at the side of the light source a rotationally symmetric aspheric surface. Furthermore, the surface at the side of the optical disk (6) of the objective lens (2) is divided into an inner circumference region (4) and an outer circumference region (5) and a surface (7) forming the difference in level of about 0.3 mu m that is provided in parallel to the optical axis (in the direction of the optical axis) at the boundary between the inner circumference region (4) and the outer circumference region (5). The spherical aberration is corrected by the inner circumference region (4) and outer circumference (5) of the objective lens (2) corresponding to the optical disks having a different thickness. IMAGE