To accurately judge the deterioration of a laser diode.
A deterioration judgment circuit consists of a laser diode LD, a photodiode PDa for receiving its output light, a selector 20 for comparing the output of the photodiode with reference levels RFa and RFb which are set for each operation mode and for outputting the comparison output, and a driver 12 where the selector output is supplied. Further, a means 32 for judging the deterioration of the laser diode is provided. The judgment output is supplied to a control part 30. The level change in the pulse response at the emission output when the laser diode is turned on at a low-frequency pulse is detected. When the level difference before and after the emission output is equal to or more than a judgment level, it is judged that the emission characteristics of the diode deteriorated. Since the level difference is affected by scattering of an element and environmental temperature variation, the deterioration can be accurately judged.
OOURA SEIJI
MIHARA YOSHIZO
SATO YUKIO
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