| 摘要 |
PURPOSE:To decrease errors at the time of writing and reading data by obtaining a focal error signal which is not affected by means of noise mixture when a noise is mixed to reflected beams from a recording medium. CONSTITUTION:When the noise is mixed to reflected beams 5, the noise is included in respective branched beams 22 and 23, and dark parts 35 and 36 emerge near the center of respective spots 29-1 and 30-1. The dark part 35 in the spot 29-1 is a position on a photodetector 25, since a dark part 36 in the spot 30-1 is positioned on a photodetector 26, the output of the photodetector 25 slightly decreases, and the output of the photodetector 26 slightly decreases as well. That is, the both outputs of the photodetectors 25 and 26 decrease, a focal error signal hardly changes, and the output remains zero. In a defocusing condition, even when the noises are mixed to the reflected beams, the focal error signal hardly changes. Thus, the error is hardly generated at the time of writing and reading the data. |
