COMPOSITE OBJECTIVE LENS, DIFFRACTION ELEMENT, OPTICAL HEAD DEVICE, OPTICAL INFORMATION DEVICE, OBJECTIVE LENS DRIVING METHOD AND CONTROL DEVICE

PROBLEM TO BE SOLVED: To use a serration diffraction element that records or reproduces conventional optical disks such as a DVD or CD in an optical head device that records or reproduces a high density optical disk using an objective lens with a large of NA (numerical aperture). SOLUTION: Steps, which are equal to or more than one wavelength in optical length for blue light but that are shorter than one wavelength in optical length for red and infrared lights, are used to provide the blue light with a function reverse to the one provided to the red and infrared lights. An effect that makes a working distance large for the CD and DVD achieves compatibility. The optical element is integrated with the objective lens so as to carry out a tracking servo sequence. COPYRIGHT: (C)2011,JPO&INPIT