Optical Fibre Communication Systems in the Nonlinear Regime

This thesis studies solutions to increase the capacity of optical communication systems in the presence of nonlinear effects. Extending the optical bandwidth and mitigating nonlinear distortions were identified as promising ways to increase the throughput in transmission system. Raman amplification was investigated as a potential replacement of the conventional erbium-doped fibre amplifier (EDFA). In this context, the performance of discrete and distributed Raman amplifiers was studied in the linear and nonlinear regimes. Despite the bandwidth benefits, discrete Raman amplifiers were shown to exhibit an increased noise figure and nonlinear distortions, compared to EDFA. Additionally, for the first time, a thorough study of digital back-propagation for distributed Raman amplified links was performed, allowing for higher transmission rates at the expense of an increase of 25% in the algorithm complexity. A major focus of this work was to investigate the growth of nonlinear distortions in optical communication systems as the bandwidth is expanded. This work was the first to experimentally validate the Gaussian noise model (and variations accounting for inter-channel Raman scattering) in a wideband transmission regime up to 9~THz. Using these models, the merit of increasing the optical bandwidth was addressed, showing a beneficial sublinear increase in throughput despite the growth of nonlinear effects. An alternative nonlinear compensation method is optical phase conjugation (OPC). The performance of OPC was experimentally evaluated over an installed fibre link, showing limited improvements when OPC is used with practical transmission constraints. To overcome this limitation, a new method combining OPC and Volterra equalisation was developed. This method was shown to enhance the performance of two limited nonlinear compensation techniques, offering an attractive trade-off between performance and complexity. The results obtained in this research allow for higher information throughput to be transmitted, and can be used to plan and design future communication system and networks around the world

Experimental Analysis of Nonlinear Impairments in Fibre Optic Transmission Systems up to 7.3 THz

An effective way of increasing the overall optical fibre capacity is by expanding the bandwidth used to transmit signals. In this paper, the impact of expanding the transmission bandwidth on the optical communication system is experimentally studied using the achievable rates as a performance metric. The trade-offs between the use of larger bandwidths and higher nonlinear interference (NLI) noise is experimentally and theoretically analysed. The growth of NLI noise is investigated for spectral bandwidths from 40 GHz up to 7.3 THz using 64-QAM and Nyquist pulse-shaping. Experimental results are shown to be in line with the predictions from the Gaussian- Noise model showing a logarithmic growth in NLI noise as the signal bandwidth is extended. A reduction of the information rate of only 10% was found between linear and non-linear transmission across several transmission bandwidths, all the way up to 7.3 THz. Finally, the power transfer between channels due to stimulated Raman scattering effect is analysed showing up to 2 dB power tilt at optimum power for the largest transmitted bandwidth of 7.3 THz

4 Tb/s transmission reach enhancement using 10 × 400 Gb/s super-channels and polarization insensitive dual band optical phase conjugation

In this paper, we experimentally demonstrate the benefit of polarization insensitive dual-band optical phase conjugation for up to ten 400 Gb/s optical super-channels using a Raman amplified transmission link with a realistic span length of 75 km. We demonstrate that the resultant increase in transmission distance may be predicted analytically if the detrimental impacts of power asymmetry and polarization mode dispersion are taken into account