| 摘要 |
PURPOSE:To reduce bad influence due to the generation of an offset in push-pull method. CONSTITUTION:A parallel plate 16 is arranged in front of a photodetector 4 and the detection face of the photodetector 4 is arranged so that the diffraction pattern of a reflected beam is focused on an ellipse rotated by 90 deg.. Since the oscillation of a reflected beam spot on the detection face of the photodetector 4 appears in a direction along a division line at the time of rotating a beam spliter/galvanometer 12 in order to execute tracking control, no variation is generated in the quantity of photodetection on each detection face. Thereby the bad influence of an offset in the pushpull method can be reduced. |
