Performance limitations of amplified coherent optical transmission systems due to interfering echoes

Interfering echoes produced in semiconductor amplifier cascades are known to cause bit error rate (BER) degradations in optical transmission systems. An experimental investigation of the effect of interfering echoes in a coherent differential phase-shift keying (DPSK) system is described and results in good agreement with theoretical predictions are reported. It is concluded that interfering echoes limit the minimum allowable inter-amplifier attenuation, which in turn severely limits the number of semiconductor laser amplifiers that can be cascaded without optical isolation.<
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