OPTICAL SCANNING APPARATUS FOR A MULTI-LAYER RECORD CARRIER, INCLUDING A FOCUS CONTROL CIRCUIT

摘要

An optical beam (8) in a scanning device is focused on an information layer (3) of a multi-layer optical record carrier. A focus control circuit (22) keeps the focal point (9) of the beam on the information layer. When the focal point jumps to another information layer, the focus control circuit does not monitor the so-called central aperture signal in order not to depend on the presence of information in the information layers. During the jump the characteristics of the feedback loop of the focus control unit are modified to avoid instabilities in the movement of the focal point during the jump. The characteristics of the feedback loop are restored after a predetermined period of time, then the focal is near the designated layer and the restored control loop focuses the focal point of the designated layer.