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The accuracy of both GN and Enhanced GN (EGN) models are investigated for a legacy dispersion-managed ultra-long-haul optical link by comparing its predictions with split-step simulations. The EGN model shows good accuracy in system performance prediction, when signal power depletion and ASE contribution to nonlinear interference (NLI) are considered.

We derived an approximate non-linear interference (NLI) closed-form model (CFM), capable of handling a very broad range of optical WDM system scenarios. We tested the CFM over 8500 randomized C-band WDM systems, of which 6250 were fully-loaded and 2250 were partially loaded. The systems had highly diversified channel formats, symbol rates, fibers, as well as other parameters. We improved the CFM accuracy by augmenting the formula with simple machine-learning factors, optimized by leveraging the system test-set. We further improvedthe CFM by adding a term which models special situations where NLI has high self-coherence. In the end, we obtained a very good match with the results found using the numerically-integrated Enhanced GN-model (or EGN-model). We also checked the CFM accuracy by comparing its predictions with full-C-Band split-step simulations of 300 randomized systems. The combined high accuracy and very fast computation time (milliseconds) of the CFM potentially make it an effective tool for real-time physical-layer-aware optical network management and control., Author's version of the manuscript accepted for publication on the Journal of Lightwave Technology, May 2020

To effectively operate multivendor disaggregated networks, the performance of the physical layer needs to be assessed by a quality-of transmission estimator (QoT-E) delivering quick results with a given reliability range. Current state-of-the-art wavelength-division-multiplexing channels are based on multilevel modulation formats relying on DSP-operated coherent receivers, propagating on uncompensated and amplified optical links. In this transmission scenario, beside amplified spontaneous emission noise accumulation, nonlinear propagation impairments are well summarized by the accumulation of a Gaussian-distributed disturbance: the nonlinear interference (NLI). When exploiting a transmission bandwidth exceeding the C-band, the interaction of NLI generation with the stimulated Raman scattering (SRS) must be properly considered. We present the derivation of the generalized Gaussian noise (GGN) model for NLI generation, including the SRS and, in general, a spectral and spatial variation of gain/loss. We validate its accuracy by comparing performances predicted by a QoT-E based on the GGN model with measurements on a testbed exploiting commercial equipment, including 100 Gb/s transponders. Considering that operational parameters of the commercial equipment are known with a large range of uncertainty, an excellent agreement with errors within 0.5 dB on the generalized Signal-to-noise ratio ( $\text{SNR}$ ) is shown, demonstrating that the GGN-model can be used for the QoT-E in multivendor network scenarios. Moreover, the GGN model has shown the capability to predict the spectral tilting due to SRS in $\text{SNR}$ performances, enabling its application to evaluate the impact of linear pretilting for SRS precompensation and NLI generation.

This chapter aims to provide a comprehensive picture of the impact of fiber nonlinear effects on modern coherent wavelength division multiplexing () systems' performance. First, the main nonlinearity models currently available are introduced and discussed in depth. Then, various specific aspects are addressed, such as the interplay of polarization mode dispersion ()/polarization dependent loss () and nonlinearity, or the dependence of nonlinear effects on modulation format. The important topic of nonlinear effects mitigation is then dealt with. Finally, system performance metrics and capacity are discussed extensively, as to how they are fundamentally influenced and limited by fiber nonlinearity.

We tested the accuracy of a fully-closed-form approximate GN-model formula over 3000 different $C$ -band fully-loaded WDM-QAM system scenarios and 1200 partially-loaded ones. By leveraging the large system test-set, we modified the formula to obtain a closed-form formula approximating the enhanced GN-model (EGN-model). The combined high accuracy and very fast computation time (ms) of such formula potentially make it an effective tool for real-time physical-layer-aware optical network management and control.

We propose an accurate nonlinearity closed-form model (CFM) for forward Raman-amplified WDM links which enables fast system optimization. We show a detailed study of a single-span link SNR maximization and flattening.