| 摘要 |
PROBLEM TO BE SOLVED: To reduce groove crossing noise included in a focus error signal detected by astigmatism method. SOLUTION: A first photodetector 50 receives a first beam 70A which is given astigmatism and a second photodetector 51 receives a second beam 70B which is not given astigmatism. The photodetectors 50, 51 respectively have four receiving means 50A to 50D, 51A to 51D. After the signals output from the receiving means 50A and 50C, 50B and 50D located in the diagonal angle of the first photodetector 50 are added respectively, these are differentially operated to generate a focus error signal. After the signals output from the receiving means 51A and 51C, 51B and 51D located in the diagonal angle of the second photodetector 51, these are differentially operated. A couple of signal obtained by differential operations are further operated differentially to reduce groove-crossing noise of the focus error signal. |
