| 摘要 |
PURPOSE:To increase the sensitivity of detection for a focus control signal, by projecting the control signal detecting beam only to a part of a lens aperture which is opposite to a recording medium. CONSTITUTION:The light given from a light source 1 is divided into three beams through a trapezoid prism 20, and the shape of the prism 20 is selected so as to project the beams from imaginary light sources 1R and 1L on only a partial region of a condenser lens 5. The beams from these light sources are reflected on a disk surface 6 and condensed at a detector 7 plus divided detectors 8 and 9 through a half-mirror 4. The cross-section of the condensed beam led to the divided detector is almost semicircular, and the right and left become adverse to each other before and after the focal point. Accordingly even a slight shift of focus increases the output of difference signal of component detectors 8o, 8i, 9o and 9i. Furthermore the shifting directions of beams caused by the shift of focus are reverse to each other on the divided detectors. Thus a large focus control signal is obtained at a terminal 30. |
