| 摘要 |
PURPOSE:To detect exactly a focus error by dividing the first and the second photodetectors into two in the same shape, respectively, and placing them at a specified position. CONSTITUTION:The first photodetector 30 and the second photodetector 34 are installed at a position where the light quantity of optical beams L', L'' irradiated to the first and the second photodetectors 30, 34 becomes equal to each other, at the time of focusing, and also in case when they have been divided into two in a shape of a concentric circle, at a position where spot sizes of the optical beams L', L'' irradiated to the first and the second photodetectors 30, 34 are larger than an area of the first photosensitive areas 30a, 34a, respectively, and also become smaller than an area of photosensitive areas 30a+30b, 34a+34b of the first and the second photodetectors 30, 34. A focus error is detected by detecting a difference of between the sum of the irradiated light quantity of the first photosensitive area 30a of the first photodetector 30 and the second photosensitive area 34b of the second photodetector 34, and the sum of the irradiated light quantity of the second photosensitive area 30b of the first photodetector 30 and the first photosensitive area 34a of the second photodetector 34. |
