Nonlinear noise in WDM systems: study of classical and quantum channel interaction and capacity

The thesis is a research-oriented work that aims at expanding the knowledge of nonlinear interaction between channels copropagating in the same optical fiber, classical and quantum, along with the derivation of metrics of interest for novel optical communication systems. Wavelength Division Multiplexing (WDM) is a major enabling technology for wideband optical communications, however, aspects belonging to nonlinear effects are not yet completely understood and modeled. In the scenario of WDM systems, many intrinsic subtleties arise, such as increased effort required to compensate for chromatic dispersion, channel-dependent Raman on-off gain in Raman fiber amplifiers, Four Wave Mixing (FWM), and Cross Phase Modulation (XPM). In particular, a major nonlinear penalty is Nonlinear Interference Noise (NLIN), originated from XPM-related pulse phase perturbation due to other channels which propagate in the same fiber medium. This is particularly interesting in the case of coherent optical transmission. On a separate research track, quantum channels are gaining interest, as Quantum Key Distribution (QKD) is a possible disruptive technology in the secure communication field, and it is enabled by physical phenomena which can take place in optical fibers. QKD over dark optical fibers is reaching its technological maturity but models for interaction with classical channels are somehow still lacking. An experimental study has been done for On-Off Keying. Existing models for interaction between classical and quantum channels focus on Raman scattering and optimal spectral placement for avoiding Stokes and anti-Stokes bands. Since the study topic encompasses various fields, such as quantum and classical optical communications, nonlinear optics, optimal resource allocation, etc. an especially thorough review of literature is necessary to find suitable models. The connection between different fundamental approaches, such as modal analysis and field quantization in optical fibers, may shed light on system performance estimation and design. Many numerical routines to calculate key propagation behaviors are already available from previous works. Available routines include: Split Step Fourier Method for nonlinear propagation of a single channel Nonlinear Schrödinger Equation (NLSE); and collision integral evaluation for pulse collision magnitude with noise variance evaluation for flexible scenarios. They can be easily extended and will be used to validate analytical results. First results in Nonlinear Interference Noise estimation have already been obtained and a conference paper have been sent to ECOC 2022. This contribution summarizes the effort to include wavelength and position-dependent attenuation and Raman gain in the model. Its main theoretical feature is the introduction of coupled NLSE model for the channel of interest and any interferent channel, which succeed in describing the NLIN effect. The remaining part of the thesis has two objectives. The first objective is to develop novel noise estimation methods to tackle classical WDM transmission. This model can be eventually integrated into an optimal resource allocation routine for the design of various fiber optics components. The second objective is to generalize the method and its results also for quantum channels, to study the feasibility of co-propagation of channels for various purposes inside the same fiber, linking it to key fiber and channel properties and parameters.