Study of Nonlinear Optical Fibre Communications

• Main Author: Yangzhang, Xianhe
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2020
• Subjects: Fourier transforms

The research presented in this thesis explores the use of non-linear Fourier transform (NFT) in long-haul optical fibre communication systems with digital coherent receivers. It aims to improve the data rate limit caused by the strong non-linear inter-channel interference at high signal powers. The non-linear frequency-division multiplexing (NFDM) system using the continuous spectrum of NFT was systematically assessed and compared with the conventional WDM system. The per-sample capacity of NFDM was estimated via the Blahut–Arimoto algorithm using statistics from a large number of system simulations of an EDFA-amplified optical fibre link of 1600 km. NFDM shows only a marginal gain of 0.5 bit/symbol in achievable rate in comparison with the conventional WDM system in the per-sample capacity sense. However, the spectral efficiency (SE) of NFDM is not even comparable with SE of the current WDM system due to the inefficient modulation approach. In this context, efforts were made to find a better signal modulation scheme using NFT. A dual-polarisation b-modulation scheme was proposed instead of the conventional qc-modulation scheme. At the same time, a large number of sub-carriers up to 1050 and the dispersion pre-compensation technique were also adopted to improve the SE of the NFT-based system. The above modifications resulted in the highest net rates among existing NFT-based systems of 400 and 220 Gbit/s (7.2 and 4 bit/s/Hz) on an EDFA-amplified fibre link of 960 km in single-channel simulation and experiment, respectively. In addition, progress was made on the modelling of spectral broadening of a Gaussian process in optical fibre using the iterative Kolmogorov-Zakharov model. The bandwidth expansion is observed to be related to the signal power, distance and initial signal bandwidth in a certain form. The relation can be used to calculate a tighter upper bound on the information-theoretic capacity of the optical fibre channel.