To provide an optical communication system and an optical transmistter which can improve the receiving sensitivity to increase the non-relay transmission range and also to miniaturize the optical antennas of both transmitter and receiver sides.

The continuous beams which are outputted from a laser light source 1 are turned into the impulsive forms via the modulation of intensity undergone by the clock signal synchronous with the data signal and then inputted to an optical phase modulator 2. Meanwhile, the data signal is converted into a DPSK code by an encoder (NRZ/DPSK) 4 and applied to the modulator 2 as a drive signal. Thus, an optical signal of an impulsive form that undergone the phase modulation based on the data signal is generated. This optical signal is amplified by an EDFA 5 and radiated into a space through an optical antenna 6. The radiated optical signal is received an optical antenna 7, amplified and formed into a waveform. Then the phase change of the optical signal is converted into the intensity change by an optical interferer 10. The converted optical signal undergoes the photoelectric conversion and the data are reproduced by 8 receiving circuit 12.