| 摘要 |
PURPOSE:To detect a focus with high reliability and sufficient sensitivity by constituting so that a laser beam goes toward a non-detecting area of a photodetector at the time of focusing and most of beam patterns are formed on its non-detecting area. CONSTITUTION:A photodetector 58 is provided with four rectangular detecting areas 78-1-78-4, also is provided with a cross-shaped non-detecting area 80, and its width W1 and W2 are set so as to have >=1/3 of diameters (a), (d) of a beam waist image 76. When changing to a pattern 82 at the time of non-focusing, which is slightly out-of-focus from the pattern 76 at the time of focusing, its diameter becomes large by DELTAh3 each in the direction 74 and its opposite direction, and is deflected as a whole by DELTAh3 along a direction 72 and its opposite direction, a signal level of an adder connected to the detecting areas 78-1, 78-2 increases, a signal level of an adder connected to the detecting areas 78-3, 78-4 decreases, and slight defocusing is also detected exactly. In this regard, 72 is a direction being parallel to a direction in which a tracking guide 16 extends. |
