Optical modulation/demodulation system

A synchronous type optical modulation-demodulation system connected by an optical fiber cable, comprising:

• a transmitting circuit having a leading edge and falling edge detection circuit for outputting pulses in response to a leading edge and falling edge of a timing pulse, a positive or negative polarity pulse generating circuit for generating positive or negative polarity pulse signals from the output pulses from said leading edge and falling edge detection circuit in such a manner that said positive polarity pulse signals and negative polarity pulse signals are respectively a number of an integer multiple of two in response to two levels of "0" and "1" of an input data signal, a light emitting element driving circuit for inputting and superimposing said positive polarity and negative polarity pulse signals and a signal having a predetermined flat level made from a ready-for sending signal and for outputting a driving signal, a light emitting element whose light intensity is modulated by said driving signal so as to output a carrier signal when said output signal from said light emitting element driving circuit is only at the predetermined flat level and so as to output a stronger light signal when said output signal is higher than said flat level signal and so as to output a weaker light signal when said output signal is lower than said flat level signal, optical coupling means for guiding output light signals of said light emitting element into an optical fiber cable, a receiving circuit having a light receiving element for converting the light signal transmitted through said optical fiber cable into an electric signal, a gain variable amplifying circuit for amplifying said electric signal to obtain an output signal of a predetermined level and whose gain is controlled to obtain said level, an averaging circuit for averaging the output signal from said gain variable amplifying circuit to obtain a predetermined flat level signal, a pair of binary circuits for inputting the output signal of said gain variable amplifying circuit and the output signal of flat level from said averaging circuit and for outputting individually binary signals when the former is higher or lower than the latter, a flip-flop circuit for inputting the binary signals from each binary circuit at the set and reset terminals of said flip-flop circuit and for outputting a receiving data signal, and a flip-flop circuit for inputting the binary signals from said pair of binary circuits and for outputting a receiving timing pulse.