Fibre nonlinearity mitigation techniques in optical communication systems

Fibre nonlinearity, more specifically Kerr, or third-order χiii, nonlinearity, is an impairment associated with optical fibres. The effects of fibre nonlinearity must be properly managed in optical communication systems in order to minimise signal distortion when the transmitted power is high. For two decades, carefully designed dispersion maps have been used to minimise fibre nonlinearity induced impairments. A well designed dispersion map allows classic systems using on-off keying (OOK) with non-return-to-zero (NRZ) modulation at 10 Gb/s to achieve transmission distances in excess of 10,000 km. This is adequate for all point-to-point links around the world, including submarine links. However, optical links are becoming more ‘meshed’ to form optical networks containing optical switches. This increases the complexity of dispersion map design, as they have to accommodate different optical paths across the network. Recent developments in digital signal processing (DSP) allow chromatic dispersion (CD) to be compensated digitally. This has greatly simplified link design because it removes the need for a dispersion map. Recent advances in optical modulators and receivers have also allowed higher-order modulation formats to be used to increase the data rate carried on each wavelength to above 100 Gb/s. However, these systems are more sensitive to the phase shifts induced by fibre nonlinearity, which is unavoidable if the operating power is high. Thus fibre nonlinearity and amplified spontaneous emission are the key limitations of reach in current-generation optical systems. This project aims to improve the nonlinearity-limited performance of optical communication systems. The modulation formats considered include direct detection optical orthogonal frequency division multiplexing (DDO-OFDM), coherent optical OFDM (CO-OFDM) and single-carrier coherent optical systems using phase-shift keying (PSK) or quadrature amplitude modulation (QAM). The techniques discussed in this thesis are self-phase modulation (SPM) compensation, cross-phase modulation (XPM) compensation and optimising the subcarrier granularity of multi-carrier systems. The SPM compensation techniques investigated in this project are based on intensity-driven phase modulation. Improvements are proposed for both single-step and split-step SPM compensation. Firstly, the single-step SPM precompensation technique originally proposed for coherent systems was adapted and optimised for DDO-OFDM. Secondly, single-step SPM compensation is improved by band-limiting the intensity waveform. Results showed that this increased the benefit of single-step SPM compensation in high-bandwidth coherent systems. Finally, methods of reducing the number of steps needed in split-step nonlinearity compensation were proposed. Numerical simulation results showed that the proposed improvements allow the number of steps to be decreased by a factor of ten, compared to previously proposed split-step methods, for a similar performance improvement. Two methods of XPM compensation are proposed and demonstrated in this thesis. The first uses a photodiode before the optical demultiplexer to detect the intensity fluctuations of several wavelength channels. This method is effective for compensating for the XPM generated by intensity fluctuations in neighbouring wavelength channels for any coherently detected system. The second proposed method transmits an unmodulated pilot tone along with the signal. The results show that the XPM induced onto the pilot is a good estimate of the XPM induced onto the signal. This method is predominantly designed for CO-OFDM systems, where a pilot tone can be placed in the centre of the OFDM signal. Finally, the dependence of nonlinearity-limited performance on the subcarrier granularity in a super-channel with orthogonal subcarriers was investigated. Results showed that subcarrier granularity has a significant effect on the nonlinearity-limited performance. The optimal granularity is dependent on the dispersion map and the length of the link. For links without dispersion compensation, the optimal granularity was found to be ~6 GHz for an 800-km link or ~3 GHz for a 3200-km link. A self-tuning receiver capable of separating multiple subcarriers within the receiver’s analogue bandwidth, without additional complexity, was also proposed. Fractionally-spaced equalisers (FSE) separate and equalise the linear impairments of every received subcarrier. An initial unique training sequence can be used to initialise the equaliser taps to remove the need for a pre-filter before the FSE. After initialisation, blind channel estimation techniques can be used to track time varying effects such as polarisation mode dispersion (PMD). The generation of multiple subcarriers with a single optical modulator was also demonstrated in this paper. The methods of improving the nonlinearity-limited performance presented in this thesis, along with the theoretical explanation of their operation, can be used to improve the nonlinearity limited performance of future long-haul optical communication systems. Concepts developed during this project will be useful in improving future optical systems.