| 摘要 |
PURPOSE:To set only one zero cross point and to perform stable focus servo, by forming the central part of an optical detector divided into three parts larger than both side parts substantially. CONSTITUTION:The optical detector 1 is divided into three parts A-C substantially by a parting line, and the outputs of the parts A and C are added by an adder circuit 3, and a differential amplifier circuit 2 as a generation circuit generates a focal point error signal from a difference between the output of the part B and that of the circuit 3. The constitution of the detector 1 is formed so that the length of the part B in a direction of the parting line can be set larger than those of the parts A and C substantially. Therefore, the output of the circuit 2 in which the sum of the outputs of the parts A and C is subtracted from the output of the part B goes to a zero level at a focused state. Also, when a focal point is deviated from the focused state to a side on one side, the level of the output of the circuit 2 keeps a negative level, and when it is deviated to the side on the other side, it keeps the positive level, then, the zero cross point is set only at a focused position. In such a way, it is possible to obtain the stable focus servo.
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