| 摘要 |
PURPOSE:To attain ease of position adjustment of a photodetector by providing the 4-split photodetector to a midpoint between two focal points of returned beams divided into two at an optical element and subtracting two output signals obtained through the addition of output signals in diagonal direction so as to form a focus error signal. CONSTITUTION:The optical element 6 divideds the returned beam from a half mirror 2 at a dividing line 11 orthogonal to an optical axis 13 and forms the image at points at both sides of the optical axis 13 within a plane including the optical axis 13 and the dividing line 11 and having a different distance from the optical element 6. The 4-split photodetector 7 is provided within a plane which is nearly vertical to the optical axis 13 and where two semicircular beams 15, 17 are projected as semi-circular beams 19, 20 of the same direction and size at nearly a midpoint between two focal points 16, 18. Then output signals of photodetection sections 7a, 7c in the diagonal direction are added by an adder 23 and output signals of photodetection sections 7b, 7d are added by an adder 24. Then respective output signals are subtracted by the subtractor 25, and the output signal of the subtractor 25 is fed to a drive coil 10 as a focus error signal 9. |
