Solitons in optical fibres

The soliton is an impulse with the remarkable property of being able to propagate unaltered over extremely long distances (theoretically infinite) thanks to the mutual compensation of linear and non-linear effects. Solitons have been observed since the 19th century in the field of fluid mechanics, in the form of waves advancing without deforming on rivers, and physicists have modeled them.

This concept was applied to optical telecommunications in the 1990s, thanks to the low losses achieved by single-mode optical fibers and the advent of optical amplifiers capable of compensating for them: the maintenance of the pulse during its propagation in the fiber is then ensured by mutual compensation of chromatic dispersion (linear) and the Kerr effect (non-linear): each of these effects taken separately distorts the signal, but their combination enables it to maintain its initial shape. What's more, given that the very concept of a soliton assumes that we're in a nonlinear regime, it's possible to work at high power, which is forbidden in the non-solitonic regime, where nonlinearities constitute a negative effect that degrades the signal. What's more, unlike the usual transmission regime, the soliton presents some strange characteristics, such as the ability to separate signal and noise occupying the same frequency band. The idea naturally arose to exploit these properties to transmit signals over extremely long distances, beyond the limits permitted at the time by "conventional" techniques; an interesting potential application was undersea transmission, where distances reach several thousand kilometers (10,000 km for a transpacific link).

Propagation over almost infinite distances (millions of kilometers), far beyond that required for telecommunications networks, has been achieved in the laboratory on recirculating loops.

However, several phenomena limit the use of this technique. The property of conservation of the soliton's shape during propagation is theoretically only verified if it is alone on the fiber, and as soon as several solitons propagate on the same fiber, they interact with each other. However, this interaction can be controlled within certain limits. In addition, optical amplifier noise introduces jitter, and these factors limit transmission throughput.

With the progress of "conventional" transmission techniques, and in particular the emergence of coherent systems with enormous power to compensate for transmission faults, thanks to electronics, the prospects for applying solitons to telecommunications systems have vanished.

This article explains the basics of optical soliton theory, presents the state of the art at the end of the 1990s and the results obtained at that time....