| 摘要 |
PROBLEM TO BE SOLVED: To make the offset adjustment of a focusing error signal(FES signal) small by arranging two diffracted lights so as to be received by two sets of respectively bisected photodetectors, making them to be diffracted to two points different in distances from a light source and making beam spreads on the photodetectors of the two diffracted lights so that the size of the beam spread distant from the light source is larger than the size of the beam spread near to the light source. SOLUTION: The beam spreads of lights to be diffracted by a hologram are made so that the beam spread 11A at the photodetectors 5A, 5D side of a side nearer to a light source 1 is smaller than the beam spread of 11b at the photodetector 5B, 5C side of a side farther from the light source 1. Since ratios in which photodetector parting lines are moved with respect to respective beam diameters are made roughly equal even when the hologram is rotated by making the beam diameter of a diffracted light (farther from the light source) whose radius of gyration is larger larger while changing two beam diameters, the generation of the offset of the FES signal is reduced. |
