Modelling And Optimisation Of Ultra-Wide Band Long-Haul Raman-Amplified Coherent Optical Transmission Systems

As data transmission demands grow with increasing users and complex applications, long-haul optical transmission links face greater pressure. Researchers have explored two approaches to address this: increasing bandwidth efficiency by transmitting more bits per unit frequency and expanding the exploitable bandwidth within fibre optic links. The first method, which involves using more complex modulation formats, has reached limitations due to stricter signal-to-noise ratio (SNR) thresholds. Consequently, bandwidth expansion has become the primary strategy for increasing data transmission capacity. This thesis investigates the use of Ultra-Wide Band (UWB) systems, specifically utilizing the C, L, and S bands, to expand usable bandwidth to around 20 THz, compared to the 5 THz available in the C band. UWB systems, however, present significant challenges. The S band lacks mature amplification technologies and relies on backward Raman amplification. Additionally, inter-channel stimulated Raman scattering (ISRS) complicates propagation, as higher-frequency channels amplify lower-frequency ones, depleting themselves. Effects like loss, dispersion, and non-linearity, relatively constant in the C band, vary across wider bands, complicating physical modelling. Accurate physical models that account for these complexities are essential for commercial UWB systems, which also require computational efficiency for real-time input power optimization. This thesis builds on the Gaussian noise (GN) model, a widely used physical model for optical systems, to better understand UWB transmission. A key contribution of this thesis is the development of a more efficient algorithm for solving the power profile equations of signals and pumps, reducing the bottleneck caused by current numerical integration methods. This thesis developed a new method to compute the power profile more efficiently. Traditional methods, which solve Raman differential equations using forward-backward approaches, are slow and cumbersome. The new method, which uses an integral form and approximates the solution via matrix operations, allows for faster, forward-only propagation. This method achieved up to a tenfold speed increase with an error margin under 0.05 dBm. In conclusion, this thesis presents a significant step toward optimizing UWB systems and reducing computation time for power profile calculations, enabling further studies in multi-band optical transmission. Future work will focus on increasing the operability range of the method, rendering it more robust for a wide range of transmission scenarios.