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

摘要

An optical head device for recording or reproducing a high density optical disc by means of a large NA objective lens uses a saw-tooth like diffraction element to also record or reproduce a conventional type optical disc such as a DVD or a CD. 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 can make a working distance large for a CD and a DVD realizes compatibility. The optical element is integrated with an objective lens so as to carry out a tracking servo sequence.