The consequence of random birefringence in soliton communication systems
Authors: Boardman A. D. , Mehta H. , Putnam R. , Sangarpaul A. , Arnold J. 
Published: February 2023

The progress towards an optical soliton telecommunication system is now gathering momentum. This means that design criteria for real fibre deployments must be addressed, urgently. Among these is the question of just how birefringence1-4 places limits upon the system performance, in terms of its influence on realistic bit patterns and, therefore, upon soliton interactions. A fibre with a constant birefringence can lead to pulse splitting, and it is possible for a fibre to have a distributed birefringence, either intrinsically or perhaps, through the way the fibre is laid. The role of randomly distributed birefringence over randomly distributed sections will, therefore, occupy most of this presentation. Some new work on fibres with a uniform birefringence will also be included, however. Birefringence has been considered, in principle, in the past, but its detailed influence on soliton interactions and a full, generic simulation involving bit patterns has not been attempted before. Mainly, uniform and Gaussian polarisation distributions have been investigated here, but other, more novel, distributions have also been assessed. The fibre has been modelled as randomly distributed sections, with different properties and randomly distributed lengths. We will show that, for a Gaussian distribution, for example, there is a dramatic influence upon solitonic behaviour. In fact, increasing the variance rate significantly reduces interaction between solitons. Two forms of encoding for the multibit patterns have been used, namely pulse code modulation and pulse position modulation, for which it will be demonstrated that realistic birefringence can cause quite a rapid pattern degeneration. Previous work has not introduced soliton interactions in a realistic model of a fibre, which is imperfect due to unexpected birefringence, so in this report we will delineate the restrictions that this type of defect will place on a model of a real communication system. To do this we will use bright solitons and introduce loss and amplification, with both discrete and distributed gain. We permit noise to be added and consider the bounds of the Gordon-Haus effect by calculating jitter and by producing eye diagrams. Techniques for reducing noise are considered, and the progress of periodic trains of bright or dark solitons is monitored. Bit trains with unequal amplitudes and varying degrees of antiphasing are also followed by computer. The propagation of very short pulses, liable to experience two nonlinear time scales (instantaneous and delayed Raman-type), is also simulated. It will be emphasised that all the work will be presented as generic plots, based upon many hours of computation. They will include depolarisation figures and pulse collapse length in interacting systems as a function of average birefringence parameter and initial electric field vector orientation. Mathematical work will be presented that will demonstrate that the generic plots can be underestood in an elegant, and reasonably quantitative, way. Finally, both space and time effects, leading to optical bullets in birefringence fibres, are analysed for their potential use in future systems.