Abstract
An optical head using a single objective lens for Blu-ray disc (BD), high-density digital versatile disc (HD DVD), digital versatile disc (DVD), and compact disc (CD) has been developed, which is suitable for high-speed recording and reading. A relay lens unit, which consists of two convex lenses and one concave lens, corrects the spherical aberration and realizes high light power efficiency for each disk. A numerical aperture (NA) control element, which contains a liquid crystal aperture and a wavelength selective aperture, controls the NA of the objective lens for each disk. A coma aberration caused by the objective lens shift is detected based on differential phase detection signals for read-only disks and push–pull signals for recordable or rewritable disks, and is corrected by moving one of the lenses in the relay lens unit by an actuator. A diffraction-limited focused spot and good readout signal characteristics were obtained for each disk, and the validity of this optical design has been demonstrated.