The impact of the extinction ratio (ER) on the performance of a 40-Gb/s return-to-zero transmission system over standard single-mode fiber (SSMF) is presented. Several dispersion maps are analyzed in order to minimize the distortion due to the intrachannel nonlinear effects, namely, intrachannel cross-phase modulation and intrachannel four wave mixing (IFWM). The decrease of the ER until a specific value leads to an increase of the intensity distortion, which is mainly due to IFWM. As a consequence, two distinct transmission regimes are identified, depending on the input average power of each section and the ER of the optical signals. The first regime has previously been called the pseudolinear regime in the literature and occurs when high ERs are considered. The optimum dispersion map of this regime has a given optical precompensation and a total residual dispersion near zero. The second regime occurs with the decrease of the ER. Under such a circumstance, the optimum dispersion map obtained in the pseudolinear regime leads to significant degradation, which is mainly due to ghost pulses appearing in the symbol “0.” This effect can be reduced by a system with residual dispersion that is significantly different from zero, leading to a detected eye pattern with low degradation in the symbol “0” but high timing jitter, which limits the use of such signals in feasible transmission systems. We call this new regime pseudosolitonic as the intrachannel nonlinear effects are apparently reduced by the residual group velocity dispersion (as it is observed in the solitonic regime occurring at lower bit rates), but strong waveform degradation occurs along the SSMF transmission. The exact value of ER for which the change between the two transmission regimes is observed depends on the optical average power at the input of each section. A simple expression to predict the system conditions (namely, ER, input average power of each section, and number of sections) for which the transition between the two regimes occurs is proposed, and its validity is analyzed.
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