Advanced modelling and signal processing in nonlinear coherent optical fibre systems

The importance of optical fibres in the global information society has increased significantly over the past four decades. However, the ever-growing demand for high-capacity data transmission poses a significant challenge, as the fibre channel’s nonlinear properties limit the achievable capacities, spectral efficiencies, and distances. This thesis aims to address this challenge by investigating advanced modelling and signal processing in nonlinear coherent optical fibre systems to predict and improve overall system performance. The first part of the thesis examines the effectiveness of nonlinear compensation (NLC) techniques, such as digital back-propagation (DBP) and optical phase conjugation (OPC), in enhancing achievable information rates (AIRs) in C-band systems that use both EDFA and distributed Raman amplification. Results indicate that the effectiveness of NLC techniques in enhancing AIRs depends heavily on the signal modulation formats and target transmission distances, with NLC being more effective for higher-order modulation formats at shorter system distances. The second part investigates the performance of long-haul Nyquist-spaced wavelength division multiplexing (WDM) optical communication systems with electronic dispersion compensation (EDC) and digital NLC with significant laser linewidths, and presents an analytical model based on the Gaussian noise model to predict the system performance considering the impact of equalisation enhanced phase noise (EEPN). A reduction up to 1.41 dB in SNR was observed in a 32-GBd 2000- km 5-channel system using NLC due to EEPN. This thesis also conducts a comprehensive analysis to study the performance of Kalman filter (KF) under realistic long-haul optical link conditions. The effectiveness of the KF in mitigating phase distortions has been thoroughly analysed. Numerical simulations were conducted on both dispersion-unmanaged and dispersion-managed nonlinear long-haul transmission systems. The joint application of KF and NLC significantly improved system performance, achieving approximately 4 dB higher SNRs than pilot-aided CPE. The findings of this thesis could help advance the design of nonlinear coherent optical fibre systems influenced by laser phase noise for high-capacity data transmission

Machine learning for fiber nonlinearity mitigation in long-haul coherent optical transmission systems

Fiber nonlinearities from Kerr effect are considered as major constraints for enhancing the transmission capacity in current optical transmission systems. Digital nonlinearity compensation techniques such as digital backpropagation can perform well but require high computing resources. Machine learning can provide a low complexity capability especially for high-dimensional classification problems. Recently several supervised and unsupervised machine learning techniques have been investigated in the field of fiber nonlinearity mitigation. This paper offers a brief review of the principles, performance and complexity of these machine learning approaches in the application of nonlinearity mitigation

Imperfect Digital Fibre Optic Link Based Cooperative Distributed Antennas with Fractional Frequency Reuse in Multicell Multiuser Networks

The achievable throughput of the entire cellular area is investigated, when employing fractional frequency reuse techniques in conjunction with realistically modelled imperfect optical fibre aided distributed antenna systems (DAS) operating in a multicell multiuser scenario. Given a fixed total transmit power, a substantial improvement of the cell-edge area's throughput can be achieved without reducing the cell-centre's throughput. The cell-edge's throughput supported in the worst-case direction is significantly enhanced by the cooperative linear transmit processing technique advocated. Explicitly, a cell-edge throughput of $\eta=5$ bits/s/Hz may be maintained for an imperfect optical fibre model, regardless of the specific geographic distribution of the users

Photonic integration enabling new multiplexing concepts in optical board-to-board and rack-to-rack interconnects

New broadband applications are causing the datacenters to proliferate, raising the bar for higher interconnection speeds. So far, optical board-to-board and rack-to-rack interconnects relied primarily on low-cost commodity optical components assembled in a single package. Although this concept proved successful in the first generations of optical-interconnect modules, scalability is a daunting issue as signaling rates extend beyond 25 Gb/s. In this paper we present our work towards the development of two technology platforms for migration beyond Infiniband enhanced data rate (EDR), introducing new concepts in board-to-board and rack-to-rack interconnects. The first platform is developed in the framework of MIRAGE European project and relies on proven VCSEL technology, exploiting the inherent cost, yield, reliability and power consumption advantages of VCSELs. Wavelength multiplexing, PAM-4 modulation and multi-core fiber (MCF) multiplexing are introduced by combining VCSELs with integrated Si and glass photonics as well as BiCMOS electronics. An in-plane MCF-to-SOI interface is demonstrated, allowing coupling from the MCF cores to 340x400 nm Si waveguides. Development of a low-power VCSEL driver with integrated feed-forward equalizer is reported, allowing PAM-4 modulation of a bandwidth-limited VCSEL beyond 25 Gbaud. The second platform, developed within the frames of the European project PHOXTROT, considers the use of modulation formats of increased complexity in the context of optical interconnects. Powered by the evolution of DSP technology and towards an integration path between inter and intra datacenter traffic, this platform investigates optical interconnection system concepts capable to support 16QAM 40GBd data traffic, exploiting the advancements of silicon and polymer technologies