| 摘要 |
PURPOSE:To detect defocusing due to displacement of an information recording surface, and also to simplify the constitution, by using a subtraction output of each output of a photodetector as a focus controlling signal. CONSTITUTION:As for a photodetector 10, the center of a light spot 15 irradiated on its photodetecting surface is on a center line X of the detector 10, and output signals (a), (b) and (c) corresponding to the photodetecting quantity of its each photodetecting surface are generated. Subsequently, a subtractor 12 executes an operation in accordance with the photodetecting quantity (a+b), (c) of each photodetecting surface, and a focus controlling signal corresponding to the diameter of a light beam irradiating the photodetecting surface of the detector 10 is derived as an output signal. This signal (d) is used as a signal for controlling the optical beam, and sets the diameter of the light spot to a desired value. |
