| 摘要 |
PURPOSE:To detect the extent of shift with a high precision by adapting the condensed spot shape of reproducing laser light to the width of a track as the scan object and irradiating a recording medium with this spot. CONSTITUTION:A phase shift plate having a function to change the phase of a half of a reproducing luminous flux, which is emitted from a semiconductor laser 1 through a collimator lens 2 and a shaping prism 3, (2n-1)lambda/2 times in comparison with the phase of the other half is inserted to and removed from an illuminating light optical path. The condensed spot shape of reproducing laser light is allowed to correspond to the width of the track as the scan object and is adapted to the recording medium. When a mark set is illuminated with a double-humped peak at the time of scanning, reflection diffracted light which has an interference pattern corresponding to mark intervals is generated. Since this condenser spot is thrown, the extent of shift is detected with a high precision if the condensed spot is shifted to another track. Thus, sure servo control and high-precision data reproducing are possible even in the case of the recording medium having various track width. |
