Volterra-assisted Optical Phase Conjugation: a Hybrid Optical-Digital Scheme For Fiber Nonlinearity Compensation

Mitigation of optical fiber nonlinearity is an active research field in the area of optical communications, due to the resulting marked improvement in transmission performance. Following the resurgence of optical coherent detection, digital nonlinearity compensation (NLC) schemes such as digital backpropagation (DBP) and Volterra equalization have received much attention. Alternatively, optical NLC, and specifically optical phase conjugation (OPC), has been proposed to relax the digital signal processing complexity. In this work, a novel hybrid optical-digital NLC scheme combining OPC and a Volterra equalizer is proposed, termed Volterra-Assisted OPC (VAO). It has a twofold advantage: it overcomes the OPC limitation in asymmetric links and substantially enhances the performance of Volterra equalizers. The proposed scheme is shown to outperform both OPC and Volterra equalization alone by up to 4.2 dB in a 1000 km EDFA-amplified fiber link. Moreover, VAO is also demonstrated to be very robust when applied to long-transmission distances, with a 2.5 dB gain over OPC-only systems at 3000 km. VAO combines the advantages of both optical and digital NLC offering a promising trade-off between performance and complexity for future high-speed optical communication systems

Overview and Comparison of Nonlinear Interference Modelling Approaches in Ultra-Wideband Optical Transmission Systems

The recent advances in modelling nonlinear interference of systems operating beyond the C-band are discussed. Estimation accuracy as well as computational complexity of current approaches are compared and addressed

Stimulated Brillouin scattering in single mode optical fibre ring resonators

This thesis describes the theoretical and experimental investigation of stimulated Brillouin scattering (SBS) in single mode optical fibres and all-fibre ring resonators. SBS is a nonlinear threshold effect which occurs in single mode fibres at low optical pump powers, and can be a limitation in optical fibre coherent transmission systems. High finesse optical fibre ring resonators are attractive for the study of SBS as their geometry offers a very low round-trip loss coupled with a significant enhancement of the circulating power over the input power resulting in low SBS thresholds. After a review of the SBS theory and an overview of the research work carried out in the field of SBS in single mode optical fibres, the system of coupled equations which described the generation of the SBS process is solved numerically. The numerical results are compared with existing analytical solutions and published experimental data, and the implications and applications of the results to optical fibre transmission systems and Brillouin amplifiers are discussed. The evaluation of high finesse single-mode ring resonator operation carried out, including the fabrication, alignment, and characterisation of devices fabricated from ordinary and polarisation maintaining fibre, at 633 nm and 830 nm is described. The theoretical and experimental investigation of SBS generation in all-fibre single mode ring resonators is presented. An analytical theory describing the operation and performance parameters of all-fibre Brillouin lasers is derived, and a numerical modelling of the transient operation of the Brillouin laser is carried out. The transient and steady-state characteristics of a Brillouin amplifier based on an all-fibre ring resonator are considered. The experimental investigation of the operation of all-fibre Brillouin lasers is described. Particular attention focuses on the semiconductor laser pumped Brillouin laser and the first demonstration of this device operating at a submilliwatt threshold is presented. The processes of four wave mixing (FWM) and a related process of Brillouin enhanced four wave mixing (BEFWM) are observed for the first time in all-fibre ring resonators, and the physics of these processes is considered. The applications of the work in this thesis including local oscillators, generation and distribution of high frequency microwave components, amplification, and phase conjugation are also discussed

On Achievable Rates for Long-Haul Fiber-Optic Communications

Lower bounds on mutual information (MI) of long-haul optical fiber systems for hard-decision and soft-decision decoding are studied. Ready-to-use expressions to calculate the MI are presented. Extensive numerical simulations are used to quantify how changes in the optical transmitter, receiver, and channel affect the achievable transmission rates of the system. Special emphasis is put to the use of different quadrature amplitude modulation formats, channel spacings, digital back-propagation schemes and probabilistic shaping. The advantages of using MI over the prevailing $Q$-factor as a figure of merit of coded optical systems are also highlighted.Comment: Hard decision mutual information analysis added, two typos correcte

The ISRS GN Model, an Efficient Tool in Modeling Ultra-Wideband Transmission in Point-to-Point and Network Scenarios

An analytical model to estimate nonlinear performance in ultra-wideband optical transmission networks is presented. The model accurately accounts for inter-channel stimulated Raman scattering, variably loaded fibre spans and is validated through C+L band simulations for uniform and probabilistically shaped 64-QAM

On the Impact of Optimal Modulation and FEC Overhead on Future Optical Networks

The potential of optimum selection of modulation and forward error correction (FEC) overhead (OH) in future transparent nonlinear optical mesh networks is studied from an information theory perspective. Different network topologies are studied as well as both ideal soft-decision (SD) and hard-decision (HD) FEC based on demap-and-decode (bit-wise) receivers. When compared to the de-facto QPSK with 7% OH, our results show large gains in network throughput. When compared to SD-FEC, HD-FEC is shown to cause network throughput losses of 12%, 15%, and 20% for a country, continental, and global network topology, respectively. Furthermore, it is shown that most of the theoretically possible gains can be achieved by using one modulation format and only two OHs. This is in contrast to the infinite number of OHs required in the ideal case. The obtained optimal OHs are between 5% and 80%, which highlights the potential advantage of using FEC with high OHs.Comment: Some minor typos were correcte

Replacing the Soft FEC Limit Paradigm in the Design of Optical Communication Systems

The FEC limit paradigm is the prevalent practice for designing optical communication systems to attain a certain bit-error rate (BER) without forward error correction (FEC). This practice assumes that there is an FEC code that will reduce the BER after decoding to the desired level. In this paper, we challenge this practice and show that the concept of a channel-independent FEC limit is invalid for soft-decision bit-wise decoding. It is shown that for low code rates and high order modulation formats, the use of the soft FEC limit paradigm can underestimate the spectral efficiencies by up to 20%. A better predictor for the BER after decoding is the generalized mutual information, which is shown to give consistent post-FEC BER predictions across different channel conditions and modulation formats. Extensive optical full-field simulations and experiments are carried out in both the linear and nonlinear transmission regimes to confirm the theoretical analysis

Sensitivity Gains by Mismatched Probabilistic Shaping for Optical Communication Systems

Probabilistic shaping of quadrature amplitude modulation (QAM) is used to enhance the sensitivity of an optical communication system. Sensitivity gains of 0.43 dB and 0.8 dB are demonstrated in back-to-back experiments by shaping of 16QAM and 64QAM, respectively. Further, numerical simulations are used to prove the robustness of probabilistic shaping to a mismatch between the constellation used and the signal-to-noise ratio (SNR) of the channel. It is found that, accepting a 0.1 dB SNR penalty, only four shaping distributions are required to support these gains for 64QAM.Comment: Title and introduction were updated and the discussion of Section IV-B was extended. Additionally, some minor modifications were made to the manuscrip