| 摘要 |
PURPOSE:To obtain a signal for correcting a deviation based on a temperature variation, etc. by a comparatively simple circuit constitution, by forming a correcting signal, based on a phase comparison of an ouput of an oscillator and an output of a frequency signal detecting circuit. CONSTITUTION:An output of an oscillator 64 and a detecting signal are brought to a phase comparison by a phase comparator 70 through a band pass filter 69 for making a frequency component of the oscillator pass through, by which an output corresponding to a focus shift can be obtained. On the other hand, when a temperature of an optical system starts to rise gradually, an optical path length is varied gradually, and even at a focusing time point, a reflected beam 23 is not made incident on a split line of a photodetector 25. When a slow variation of this interval and the optical path length is generated, a correcting signal corresponding to it is obtained in an output stage of an LPF 71, and applied to a correcting circuit 58. The correcting circuit 58 adjusts an input signal level of a differential amplifier 30 in response to the correcting signal. By this correction, even if the reflected beam 23 is not made incident on the split line of the split type photodetector 25, ann output of the differential amplifier 30 becomes zero at the time of focusing, and a normal focus control can be executed. |
