| 摘要 |
PURPOSE:To save the beam splitter or the optical detector as well as to facilitate an easy control by applying the difference of the average value signals between the output of the voltage comparator to give a comparison between the modulated optical signal and the reference voltage and its inverse output to the optical modulator in the form of the bias. CONSTITUTION:Output light 10 obtained by modulating input light 5 through optical modulator 3 and by the high frequency signal sent from input point 2 is supplied partially to output light detector 11 via beam splitter 9 and turned to modulated light intensity signal (i) to be then changed to binary voltage signal (e) according to the fact that is larger or smaller than the reference voltage (zero in this case) of voltage comparator 20. Signal (e) is then turned to averaged signals (g) and (h) through low-pass filters 23 and 24 along with binary signal (f) obtained by inverting signal (e). And the difference signal of signals (g) and (h) is supplied to modulator 3 via differential amplifier 14. Then the control is given so that the duty ratio may be 50% for output (e) and (f) and the output of amplifier 14 may be set to zero. As a result, the bias point drift caused the temperature change or the like can be prevented, and thus the bias point can be maintained at the optimum point of 1/2 the maximum output light. |
