| 摘要 |
PURPOSE:To prevent the operation of a focus controlling system from being unstable even with the beam amount changed, by irradiating the beam of a prescribed wavelength by a laser light emitting device and irradiating the reflected light on a detector provided with plural photodetecting parts. CONSTITUTION:Even if the amount of light applied to a photodetector 17 without focus error is changed, the output of an adder 41 (i.e., focus error detecting signal) is unchanged. A difference signal of outputs of the adder 41 and a subtractor 22 outputted from a subtractor 43 corresponds to the change in the light amount change of the beam light applied to the photodetector 17 and is negatively fed back to a control input terminal of a laser driving circuit 44. As a result, the circuit 44 drives a laser light emitting device 15 so that the amount of beam light applied to the photodetector 17 is almost constant, thereby minimizing the output of the offset voltage to the output terminal of the subtrators 23 and 24. |
