An optical communication system for optically transmitting transmission data
from
a transmitting station to a transmitting device includes an adder for adding an
intermediate frequency subcarrier signal modulated with data to be transmitted
to a pilot carrier signal as a sinusoidal wave, and an electro-optical converter
for electro-optically converting the above sum signal to an optical signal by directly
modulating a semiconductor laser element having a resonant frequency fr in accordance
with the sum signal and transmitting the signal to an optical fiber for a down
link. The frequency fIF of the intermediate frequency subcarrier signal
and the frequency fLO of the pilot carrier signal satisfy
