OPTICAL MODULATION DEVICE BY EXTERNAL MODULATION SYSTEM

摘要

PROBLEM TO BE SOLVED: To cope with the speeding-up of a transmission rate with a relatively simple constitution, to shorten the rise time and the fall time of an optical transmission waveform and to automatically interrupt a light at the time of the interruption of an input electric signal with small dependency upon the amplitude of the input electric signal. SOLUTION: A low frequency signal is superposed to an optical signal by supplying the input electric signal to an optical modulator 21 after amplifying it to a limiting level in a driving amplifier 22 and by modulating the input voltage of the optical modulator 21 with the low frequency signal for bias control from an oscillator 24 by a modulation circuit 25 and the one part of the optical signal output of the optical modulator 21 is taken out in an optical branching circuit 28 to be detected in a photoelectric converter 29 and then the detected output and the low frequency signal are mixed in a multiplier 31. Then, the low frequency component of the mixed output is taken out in an LPF 33 and the level of the component is detected in a level comparator 34 and then the bias voltage of the optical modulator 21 is vaiably controlled according to the detected output.