4. Combined effects of linear and non-linear distortions: solitons

4.1 Basic phenomenon

The principle of soliton transmission is based on the existence of a stable regime of isolated pulse propagation, of particular shape and power, chosen so that the effects of chromatic dispersion and phase self-modulation (Kerr effect) compensate each other. This equilibrium guarantees invariance of the pulse characteristics along the supposedly lossless fiber, whatever its length. The fundamental idea is therefore to use non-linear effects to stabilize propagation, whereas in conventional transmission we seek to minimize them.

If β ₂ is positive, the group propagation time increases with frequency and the leading (resp. trailing) edge propagates faster (resp. slower), so that the impulse expands...