| 摘要 |
PURPOSE:To attain excellent frequency response by allowing photodetector to detect a change in a beam pattern of photodetecting planes split into three and controlling the converging beam for the convergence on a recording medium so as to relax the setting accuracy of the detector. CONSTITUTION:A rectangular photodetector 10 is arranged to a focus of a cylinder lens 9 arranged for a return beam irradiated to a disc 7 from a laser light source 1. The detector 10 is divided into three divisions D1-D3, and the sum of the photodetecting areas D1, D2 is made equal to the D3. When focusing is attained, an output difference of the detector from the sum of outputs from the detectors D1, D2 is zero and a value of difference is generated if the focus is placed at the inside or outside of the detector 10. The position of a converging lens 6 is controlled so as to compensate the said difference. Thus, a small detector is enough for the detector 10 and the setting accuracy of the detector 10 is relaxed and the frequency response is improved. |
