| 摘要 |
PURPOSE:To attain ease of focus position control without using any special optical element by the constitution that a photodetecting face of a beam detector is split into two detecting pieces so as to include the center of beam spot at focusing in the detecting piece having the smaller photodetecting face. CONSTITUTION:The detector 12 is installed so that the beam amount irradiated to the detecting piece 12a at the center of the detector 12 and the beam amount irradiated to the detecting piece 12b in the beam spot are made identical as shown in Fig. B-b at the focusing where a disc 7 is placed at a position (b) in Fig. A. When the disc 7 is placed at a position (a) in Fig. A, the diameter of the beam spot is smaller as shown in Fig. B-a and the amount of photodetection is increased more for the detecting piece 12a than the detecting piece 12b. When the disc 7 is placed conversely to a position (c) in Fig. A, the diameter of the beam spot is larger as shown in Fig. B-c, and the received light amount of the detecting piece 12a is increased more than that to the detecting piece 12a. A focus error signal is obtained by taking an output difference between the detecting pieces 12a and 12b. The focus position control is attained easily with simple constitution in this way. |
