Objective lens element with peripheral diffraction structure and central phase step structure for optical disks and optical head device incorporating the same

摘要

An objective lens records information on, or read information from, a first optical medium by utilizing a first light beam which convergences on the first optical medium at a first numerical aperture (hereinafter “NA1”). The objective lens records information on, or read information from, a second optical medium by utilizing a second light beam which convergences on the second optical medium at a second numerical aperture (hereinafter “NA2”). In the objective lens, NA1 is greater than NA2. The objective lens has an optical lens for receiving the first light beam and the second light beam. The optical lens has a peripheral diffraction structure disposed substantially outside an area of incidence of the second light beam for suppressing fluctuation in wavefront aberration of the first light beam, and a phase step structure disposed in a central region relative to the peripheral region for producing a phase difference in the second light beam.