System degradation by the SPM-induced mean nonlinear phase shift in optical QAM transmission

Optical QAM transmission inherently suffers from performance degradation caused by different mean nonlinear phase shifts experienced by symbols with different power levels. This paper illustrates this effect and shows possible compensation schemes and their efficiency.

1. Introduction

Recently, high-order optical modulation formats have been extensively investigated, because their potential for increasing spectral efficiency is of high interest for network operators. Thereby, coherent demodulation - becoming more feasible using high-speed digital signal processing now available - facilitates the detection of advanced modulation signals and offers enhanced capabilities for electronic equalization. Transmission over long distances using higher-order modulation formats and coherent homodyne detection was already shown for high-order phase modulation formats such as quadrature phase shift keying (QPSK) in [1] and 8-ary phase shift keying (8PSK) in [2],[3]. Very recently, optical QAM transmission was demonstrated in [4]. However, it is not yet clear which distances can be bridged using optical QAM formats.