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.