| 摘要 |
PROBLEM TO BE SOLVED: To completely diffuse unnecessary light in the case of utilizing an optical disk whose recording density is low without providing a diaphragm and a filter as separate parts. SOLUTION: This objective 10 is a single lens whose both surfaces are asherical, and diffraction lens structure is formed on either lens surface 11. The diffraction lens structure of a shared area Rc has such wavelength dependency that the change of spherical aberration is compensated by switching the wavelength to be used for a DVD having a 0.6 mm protective layer and a CD having a 1.2 mm protective layer. The diffraction lens structure formed in an area Rh dedicated to high NA is constituted of a plurality of ring zones, which partially include at least one wide ring zone where the difference of optical path length has >=0.6 λ width between the end on an inner circumferential side and the end on an outer circumferential side with respect to 790 nm wavelength when a light condensing point corresponding to the CD is set as evaluation reference, so that luminous flux having 654 nm wavelength corresponding to the DVD whose recording density is high is condensed and luminous flux having 790 nm wavelength corresponding to the CD whose recording density is low is diffused. |
