| 摘要 |
PURPOSE:To prevent the frequency characteristic of a reproduced signal from being deteriorated even if the width of an annular beam spot is narrow, by forming a blind area arranged between the 1st and 2nd detecting parts of a photodetector into a polygonal shape having a prescribed positional relation with the annular beam spot. CONSTITUTION:A separating blind area 18C formed like a square or the like is arranged between the outside and inside photodetecting parts 18A, 18B of a photodetector 18 detecting an annular beam spot S in order to detect focus status. The square has a fixed positional relation with the spot S so that the corners are arranged on the outside of the spot S and a part of sides are arranged on the inner circumferential side under the focused status and the length of arcs of the spot S is made coincide with each other on the outside and inside of the blind area 18C, so that the quantity of photodetection of the detecting parts 18A, 18B is uniform at the time of focusing. In said constitution, the part of the spot S made incident to the blind sector is made smaller than the case of a circular blind area even if the width of the spot S is narrow and prevent the frequency characteristic of the reproduced signal from being deteriorated without the reduction of S/N ratio. |
