Focus servo control device for an optical system having an initial focus locking operation

摘要

A focus servo control device for an optical system having a focus servo loop including an objective lens movable between a nearest position and a farthest position relative to an object, and a focus error signal generator for producing a focus error signal including at least first and second in-focus signals, the first in-focus signal corresponding to a shorter one of two distance values between the objective lens and the object at which the first and second in-focus signals are generated and indicating a real in-focus state. The device is constructed to open the focus servo loop and move the objective lens from the farthest position to the nearest position, and then to move the objective lens from the nearest position to the farthest position, while monitoring the focus error signal, and closes the focus servo loop upon receipt of the first in-focus signal, thereby preventing a malfunction of the focus servo loop which might occur by locking the objective lens at a false in-focus position.