| 摘要 |
PURPOSE:To obtain a focus error signal without taking the trouble to use a knife edge, by installing a 2-split photodetector so that a side of at least one end of the photodetector is positioned in a reflected light beam from a surface to be detected. CONSTITUTION:2-split photodetectors 202, 203 are placed so that the side 205 of the end of said detectors is positioned in the reflected light beam 201. Therefore, only a part of the reflected light beam 201 is photodetected selectively, and a focus error signal FE is obtained as FE = (the output of a band-shaped photodetecting part 203) - (the output of a band-shaped photodetecting part 202) by a subtracter 204. Also, by installing a condenser lens 101 so that the center axis 107 of the reflected light beam, and an optical axis 106 of the condenser lens are eccentric in the direction orthogonal to the split axis 109 of 2-split photodetectors 103, 104, the position of a reflected light beam 105 moves on the photodetecting surface 102, therefore, it becomes difficult that the reflected light beam 105 gets on a band-shaped photodetecting part 104, and the leading range of a focus can be widened. |
