JPH05299735

PURPOSE:To obtain an electric-to-optical conversion characteristic without fluctuation by a method wherein a laser diode is held at a definite temperature. CONSTITUTION:The following are detected by means of a peak-value detection circuit 7 and a mean-value detection circuit 10: the peak value and the mean value of output waveforms of a photodiode 4 which photodetects one part of the optical output of a laser diode 3. The difference DELTAD between the peak value and the mean value is operated by means of the mean-value detection circuit; it is inputted to a first comparator 12a and a second comparator 12b. The first comparator 12a compares the threshold value of a positive voltage with the difference DELTAD; the second comparator 12b compares the threshold value of a negative value with the difference DELTAD; their comparison results are outputted to a current driver 13. The current driver 13 controls a thermocooler 14 according to the comparison results of the first and second comparators 12a, 12b.