Optical recording/playback apparatus having a focusing control system with reduced spot-offset sensitivity

摘要

The disclosed apparatus for recording and/or reading information in or from a track of a radiation-reflecting record carrier by means of a light beam includes an objective system for focusing the light beam so as to form a light spot on the record carrier in dependence on a control signal (Sf). The control signal is generated by a focusing-error detection system having an astigmatic element and four detectors (D1 to D4). The output signals S1 to S4 of the four detectors are combined in an electric circuit arranged to produce a control signal satisfying the formula: <IMAGE> This control signal drastically reduces the influence on the focusing of offset of the light spot on the four detectors.