PROBLEM TO BE SOLVED: To set the best focusing position corresponding to the maximum difference between a signal and a threshold by passing a beam through the surface through a focusing system, adjusting the focusing system, and monitoring the performance of a light source. ;SOLUTION: Part of diffused light 11 returns to a laser diode 1 through a focusing system 9, is received by a resonator 2, changes the performance of the laser diode 1. The changes in the performance of the laser diode 1 affects the shape of back emission beam 8 detected by a photodiode 7. The photodiode 7 generates an output signal for expressing the intensity of the beam 8 on a line 13, and the output signal is sent to a comparator system 14 together with an original drive signal for the laser diode 1, and thereby the drive signal can be removed from the signal on the line 13 and a difference between the two signals can be monitored. The difference between the two signals is sent to a comparator 17 (input with a threshold signal) through a band filter 16, thereby the focusing system can be set to the best focusing position corresponding to the maximum difference between the signal and the threshold.;COPYRIGHT: (C)1999,JPO
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