| 摘要 |
PURPOSE:To make focusing control to a suitable position in following to a track change, by detecting an output difference of each split section of a photodetector shielded at a knife edge located at an image point of a detection system, in an optical disc device. CONSTITUTION:When a track of a disc 9 is coincident with an irradiated light focus of a pickup, since image is formed at a location where a knife edge 8 and an optical axis are in contact with each other, the amount of light incident to upper and lower two sets of photodetector faces of a detector 6' is equal, and the output difference from the detectors is zero. When the distance between the disc 9 and the pickup is shorter(longer), the position of image forming moves at the back(front) of the knife edge 8, the rays at the edge 8 are shielded with the edge 8, the amount of light to the photodetector at upper (lower) side is decreased and an output from the two photodetectors differs. Suitable read-in position can be assured by moving the entire pickup toward the direction where this difference is decreased. |
