Dispersion compensation in high bit rate transmission systems using midspan spectral inversion
This thesis presents experimental and theoretical work on optical fibre communication systems covering dispersion compensation, short pulse generation and high speed transmission. The focus of this thesis is on dispersion compensation using the midspan spectral inversion (MSSI) technique. Experimental investigation and theoretical studies on an MSSI system suggest potential compensation of both fibre chromatic dispersion as well as fibre nonlinearity. The performance of MSSI systems is found to be limited by a few effects that cannot be compensated for by phase conjugation and a different approach has to be employed in order to overcome these factors. Optical phase conjugators are essential elements of MSSI transmission systems. Dispersion shifted fibre (DSF) and semiconductor optical amplifiers (SOA) are two of the most important nonlinear media for phase conjugation. A comparison of these two types of phase conjugator has been carried out. By using fibre Bragg grating based filters, the noise performance of the conjugators could be optimised. Computer numerical models of various optical components have been developed which could simulate the basic function of erbium-doped fibre amplifiers, optical band pass filters, optical fibres and fibre gratings. Signal transmission in single mode fibre is modelled using the split-step Fourier method to solve the nonlinear Schrödinger equation (NLSE). These numerical models can be programmed to suit different experimental setups and allow the study of various system experiment configurations. The theoretical results help to explain and provide estimated limitations to the experimental results. Experiments carried out in the laboratories have finally culminated in a 10 Gbit/s MSSI field trial in Italy. Successful transmission of 6 picosecond pulses at 10 Gbit/s over 140 km of standard installed fibre was demonstrated. The field trial proved not only the feasibility of 40 Gbit/s MSSI transmission in the final field trial but also an implication on upgradability of currently installed links around the world. A novel polarisation insensitive optical phase conjugation configuration is proposed and demonstrated. With the use of all-fibre DFB lasers as FWM pumps, the first all fibre optical phase conjugator is realised. This configuration can achieve polarisation insensitive operation with great simplicity compared to conventional methods, which are complex and of high component count. Moreover, the same configuration can be applied to an SOA with equally good performance. We have demonstrated wideband and polarisation independent phase conjugation using a highly nonlinear DSF and a long SOA. High bit-rate operation up to 40 Gbit/s and MSSI transmission over 204 km of standard fibre using these devices has also been experimentally investigated.