Toru Yoshihara

New Spherical Aberration Compensator for Blu-Ray Disc

In high numerical aperture optical disc systems, a spherical aberration compensator is required to accomodate thickness deviations of the cover layer and the space layer. In some cases, a liquid crystal (LC) plate is employed as a spherical aberration compensator and it is generally attached to the objective lens actuator coaxial with the objective lens. It is useful to put the LC plate in a fixed part of the optical head to cope with high-speed disc rotation and to reduce the size of the objective lens actuator, but additional objective lens decentering against the LC plate is caused by the tracking operation. In this paper, a new spherical aberration compensator for Blu-ray Disc optical systems is introduced to decrease the wavefront aberration caused by objective lens decentering.

Focusing Error Detection Using Concentrically Separated Light Beams for Multilayer Optical Discs

We discuss focusing error detection achieving smooth and accurate layer-accessing operations in a multilayer optical disc system. In layer-accessing operations, focusing error signals both unchanged on all layers and detected on specific layers are ideally required. We have studied two types of focusing error detection that generate focusing error signals with different characteristics in regard to spherical aberrations. These focusing error signals are obtained simultaneously by detecting the individual light beams returned by an optical disc, which are separated concentrically using the phase distribution features of their spherical aberrations. We confirmed the effects of increasing and decreasing focusing error signal amplitude against spherical aberration in simulations and the effect of increasing it in experiments. The selective use of focusing error signals according to the layer-accessing operation enables us to achieve high accessibility and high assurance in layer-accessing operations in a multilayer optical disc system.