Transmission limitations due to self-phase modulation in optical PSK heterodyne detection systems employing chromatic dispersion equalization

Even if the amplitude of a phase-modulated optical signal is constant before transmission, amplitude modulation is caused by fiber chromatic dispersion. As a result, self-phase modulation (SPM) is induced. In optical heterodyne detection, SPM cannot be compensated for by the delay equalizer (electrical domain) used to compensate fiber chromatic dispersion. However, the transmission distance limitation of multi-repeatered coherent transmission systems has not been investigated in the presence of SPM. This paper theoretically and experimentally investigates the transmission distance achievable with a phase-shift-keying (PSK) heterodyne detection system employing in-line optical amplifiers and delay equalization. The calculated results show that equalization is effective when γP₀/2B²|β₂|<10 in the normal dispersion regime, and γP₀/2B²|β ₂|<15 in the anomalous dispersion regime. Furthermore, the increase in transmission distance achieved by using equalization is experimentally shown in an 8 Gb/s PSK heterodyne transmission experiment using a conventional single-mode (SM) fiber and in-line fiber amplifiers