Effect of interfering echoes in a DPSK coherent optical transmission system with optical amplifiers

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Abstract
Interfering echoes in semiconductor amplifier cascades can cause severe BER degradations in optical transmission systems. We describe an experimental investigation of the effect in a coherent system, and report results in good agreement with theoretical predictions. It is concluded that interfering echoes will severely limit the minimum interamplifier attenuation, which will in turn limit the number of semiconductor laser amplifiers that can be cascaded without optical isolation.