| 摘要 |
PURPOSE:To eliminate the offset of a dislocation information signal in a focus signal by executing the differential operation between differential signals in a tangential direction which are obtained with the aid of a fixing part having a second beam splitter and both photodetection means of the fixing part. CONSTITUTION:Light reflected on the recording layer of a disk 10 is made incident on the beam splitter 3 of the fixing part 13. The light beam which is reflected and conducted to the beam splitter of a detection system is reflected. Then, one of the light beams is made incident on a critical prism 6 and the reflected light beam detects a focus error signal by a split PD (photodiode) 8 being a photodetector. On the other hand, the light beam transmitted through the beam splitter 4 is condensed by a condensing lens 5 and received by a split PD 7 being the photodetector. Then, the differential operation is executed for two differential signals by the split PD 7 and the split PD 8, besides. Therefore, only the focus error signal whose offset caused by the deviation of an optical axis is eliminated is obtained. Thus, the offset of the focus is cancelled. |
