Non-periodic phase structures in optical disc systems

For DVD/CD compatibility, a non-periodic phase structure (NPS) has been developed, consisting of a limited number of wide zones. Sufficient correction of the spherical aberration due to the combination of a DVD objective lens and a CD cover layer thickness is achieved. Two options have been worked out for better definition of the numerical aperture for CD. Tests of these options with a separate phase plate in combination with a standard DVD lens showed good performance, with slightly larger tilt margins than for the conventional solution. The NPS (including an option for NA definition) has been implemented in a glass/2P objective lens, to obtain a lens compatible for DVD+RW and CD-RW. With this lens, read and write tests of both DVD and CD were successful.