| 摘要 |
PURPOSE:To drive focus servo to cancel an offset based on a variation in the optical path length of a reflecting beam from an optical disk even if the variation occurs by forming a chirped grating part with a first and a second chirped grating plates. CONSTITUTION:The chirped grating part 6 is formed with the first chirped grating plates 6a disposed for the part of a reflecting beam from the optical disk 100 above the optical axis and the second chirped grating plate 6d disposed for the part of the reflecting beams below the optical axis. Further, a photodetective part 7 is disposed for the respective spots from the first and the second chirped grating plates 6a, 6b of the part 6, and respective outputs from the part 7 are lead to a detection part 8 formed with three difference outputting circuits 8a, 8b, 8c that are comparators. An output from the part 8 is supplied to a focus control circuit 10 as a focus error detection signal F. As a result, the servo control can be achieved without any offset occurring. |
