In a long-distance optical amplifier chain, the waveform distortion is induced both by the Kerr nonlinearity and the group-velocity dispersion of optical fibers. To cope with this problem, the optical communication system using mid-span optical phase conjugation has been proposed and experimented with. In this paper, after reviewing the methods for combating with such waveform distortion, we describe the principle of the system using optical phase conjugation. The proper design for the amplifier spacing and the dispersion value of transmission fibers is necessary to improve the distortion compensation efficiency. Some examples of system design are shown, and the feasibility of practical optical phase conjugators is briefly discussed.
