Nonlinear wave propagation in optical fibres

A generalised mathematical theory leading to the evolution equation of nonlinear pulses propagating in an optical fibre waveguide is presented. The magnitudes of the optical self-steepening and third-order dispersion coefficients are calculated and their relative importance is assessed. The possibility of self-steepening is carefully examined and represented on a pulse-displacement-distance parameter diagram. It is concluded, for specimen single mode fibre data, that, even for a 1% deviation from zero group dispersion, the shock term interacts with the group dispersion to produce a pulse distortion in the form of a velocity change limited by the group dispersion rather than selfsteepening. Also, for such a fibre, self-steepening is so small that vast runs down the fibre are required for it to be seen. Any observation of self-steepening in fibres will require much closer tuning to zero group dispersion and will need the suppression of the third-order term.