Nonlinear Pulse Propagation in Optical Fibres
Pulse propagation in optical fibre communication systems is affected by the fibre nonlinearity even at relatively low power levels (< 1 mW). One example of the action of the nonlinearity is the formation of solitons in anomalous dispersion fibres. We analyse two topics in this context: compression of pulses into short fundamental solitons, and periodical dispersion-compensation in order to reduce the nonlinear effects that are detrimental while, at the same time, the soliton character of the pulse is retained. This type of stationary pulse is called a disperion-managed soliton.
In order to increase transmission speed there is a need for increasingly shorter pulses. In soliton systems fundamental solitons propagate without changes in the pulse form due to a perfect balance between nonlinear and dispersive effects. It is important that the launched pulses are close to ideal fundamental solitons in order to avoid build-up of dispersive radiation inthe fibre. We show that it is possible, by passing the pulse through a fibre section of suitable dispersion, to transform higher order solitons and radiation into almost ideal fundamental solitons. We also demonstrate that fundamental solitons can be adiabatically amplified and compressed with high efficiency in a distance shorter than the soliton period.
The disperion-managed soliton has several advantages compared to standard solitons, their power is enhanced, leading to increased noise margins, and most remarkably, the the dispersion-managed soliton can propagate in fibre links with zero and normal path-average dispersion, reducing the jitter in pulse arrival time. We analysed soliton propagation around zero path-average dispersion, and identified regimes for stable propgation. In particular we have found that there is a critical, strength of the dispersion map, above which propagation in net zero and normal dispersion is possible. We have also studied the influence of loss and that of optical filtering. In the case of filtering the critical srength is removed and replaced by a critical power.