Optical communication system and optical communication method

摘要

In an optical communication system, a transmitted signal is modulated with a subcarrier signal having a frequency higher than the frequency of said transmitted signal on the transmission side, the modulated signal light emitted to a transmission channel. On the reception side, the received signal is filtered and decoded by means of a band-pass filter having a center frequency approximately equal to the frequency of the subcarrier signal of the received signal. The above arrangement makes it possible to achieve an arrangement free from the influence of deterioration in the degree of modulation due to spontaneous emission generated from an optical amplifier disposed midway along the transmission channel. The optical amplifier may be an optical differential gain amplifier which has the differential-gain optical input/output characteristics of outputting spontaneous emission of low intensity according to a digital signal value "0" and output light of high intensity according to a digital signal value "1". The arrangement enables optical amplification of signal light without the risk of outputting high-level spontaneous emission due to direct current excitation. It is therefore possible to provide received signals of high quality by eliminating any deterioration in S/N ratio and any deterioration of the degree of modulation of signal light due to spontaneous emission from the optical amplifier disposed midway along the transmission channel.