| 摘要 |
PURPOSE:To lead an objective lens into focus irrespective of the height of the objective lens at a focus position by adding height compensation so that the output of a position control circuit is nearly zero irrespective of the height of the objective lens. CONSTITUTION:Currents flowing in two PIN photodiodes 21 for focus error detection are converted by current-voltage converting circuits 75 into voltages, which are amplified by amplifiers 81 and led to a focus error generating circuit 22. A focus lead-in range needs to be searched first since it is much narrower than the variable range of an objective lens 17. For the purpose, an SW2 is positioned on a solid line first, an oscillator 76 is connected by an SW1, and the objective 71 is moved up and down to search for the focus lead-in range. Once the range is detected, the output of a height compensating circuit 70 is varied until the output of a position control circuit 78 approximates zero. consequently, a constant permissible range is entered, so the SW2 is switched to a broken line position and focus servo is initiated. |
