Ultra-long distance soliton transmission: putting fiber nonlinearity to work

The author discusses how the soliton owes its very existence to fiber nonlinearity. As a pulse self-trapped in time, it is able to resist a great many effects, such as chromatic dispersion and polarization dispersion, that are highly destructive of ordinary ("linear") pulses. It is extremely robust in this regard. The transmission line picks the soliton out of whatever reasonable pulse is launched into it, and discards the residue as dispersive wave radiation. In a line with amplifiers, the soliton tends to propagate stably over an indefinitely long distance. For the soliton in a broad-band transmission line, however, one optical frequency is as good as another. This indifference to optical frequency makes it relatively easy for spontaneous emission, by way of the nonlinear term, to change the soliton\´s frequency, and hence to change its velocity. The resultant random spread in pulse arrival times, known as the Gordon-Haus effect, tends to limit the maximum allowable bit-rate for error-free transmission