Appathurai, V.S.D.; (2005) Investigation of the limiting fibre nonlinearities and their suppression in 40Gbit/s optical transmission systems. Doctoral thesis , University of London.

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Abstract

This thesis investigates the fundamental limitations to optical transmission at a bit-rate of 40Gbit/s. The signal distortion due to nonlinear effects, noise and dispersion are analysed and techniques for their suppression through dispersion management and optimum choice of modulation format are demonstrated. The high launch powers required for overcoming noise from the amplifiers result in an increase in fibre nonlinearities. Transmission at 40Gbit/s favours the RZ modulation format. However, RZ signals were found to be limited by intra-channel cross phase modulation (IXPM) and intra-channel four-wave-mixing (IFWM). These intra-channel nonlinear effects take place as a result of nonlinear interaction between overlapping pulses of the same wavelength channel. Minimising such pulse overlap by controlling the dispersion-induced pulse broadening during propagation in the fibre was investigated by reducing the fibre local dispersion and by pre-compensating the signal at the transmitter. Dispersion compensation using higher-order-mode devices with high nonlinear tolerance was also investigated, enabling transmission over in-line pre-compensated amplifier spans. In the second part of this thesis, the nonlinear tolerance of the RZ modulation format was increased by use of alternate-polarisation and alternate-phase between adjacent pulses. These techniques were found to improve the transmission performance by approximately 50% and required simple modifications to the transmitter only. These advanced RZ signals were found to be compatible with dispersion management techniques. However, the optimum pre-compensation at the transmitter was found to be dependent on the modulation format and dominant intra-channel effect. A novel modulation format combining alternate-polarisation and phase simultaneously was demonstrated for maximum nonlinear suppression without the use of dispersion management. Finally, a new experimental technique was demonstrated for the investigation of dispersion tolerance. It was found that the choice of optimum modulation format requires a trade-off between nonlinear tolerance and dispersion tolerance. The results of this work can be applied to optimise the design rules of future optical networks.

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