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Device physics

Electrical solitons come of age

Individual packets of light energy, known as optical solitons, have long been the darlings of communications engineers. Finally, their electrical siblings are getting a look in — and could become the new favourites.

Digital communication systems seem so robust that many of us take their reliability as an axiom of modern life. That reassuring façade, however, belies a surprising fragility: in practical communication systems, impairments of a distinctly analogue nature can affect the propagation of digital pulses. Nonlinear effects, for instance, amplify the low- and high-amplitude portions of a signal differently, and so distort the signal's shape. And even without amplitude nonlinearity, the signal shape can still be distorted if the elements of different frequency of a signal wave — its so-called Fourier components — propagate at different velocities (as they do in most practical media). This kind of dispersion limits signalling to rates at which the smeared-out trailing edge of one pulse only negligibly perturbs the leading edge of the next.

The quest to mitigate effects that distort the shape of digital signals has been an obsession for communications engineers. Their efforts have paid off handsomely: modern communication channels using optical fibres can sustain data rates greater than 50 gigabits a second over a distance exceeding that between Earth's poles. That's fast enough to transmit the entire print collection of the US Library of Congress in about half an hour.

As two recently published papers1,2 remind us, such achievements are the result of a rare suspension of Murphy's law that anything that can go wrong will go wrong. Specifically — and remarkably — it is possible for the shape distortion produced by nonlinearity to cancel that produced by dispersion. The result is a single pulse of stable shape, dubbed a soliton. Writing in IEEE Transactions on Microwave Theory and Techniques, Ricketts, Li and Ham1 show that this fortuitous cancellation does more than simply allow the faithful propagation of digital pulses: in fact, it can be used as part of an electrical oscillator to generate pulses in the first place. And in a paper in Physical Review Letters, Wu, Kalinikos and Patton2 describe a related system that deliberately provokes inherently nonlinear dynamics to produce chaotic soliton oscillations. Both of these systems are purely electronic; their relative ease of manufacture gives them many advantages over the ‘photonic’ devices, involving light waves, that currently dominate soliton research.

These modern developments ultimately trace their origins to an observation made in the early nineteenth century, when, two decades before Louis Pasteur uttered the words, the Scottish naval engineer John Scott Russell found that chance does indeed favour the prepared mind. As Scott Russell watched a barge being pulled along Edinburgh's Union Canal one August day in 1834, the rope from the barge horses suddenly snapped. He noticed that the wave produced by the prow's rapid drop onto the water's surface propagated quickly down the canal with negligible change in shape over a distance of several kilometres — as he convinced himself by pursuing the wave crest along the side of the canal on horseback.

Scott Russell was certain that the abnormally low attenuation and dispersion of this ‘wave of translation’ revealed principles of fundamental importance, and constructed a large water tank in his back garden to prove it. Alas, he was virtually alone in this belief. Despite their appearance in various guises in nature (Fig. 1), the lack of any practical significance of such ‘solitary waves’ — solitons — meant that the subject would remain largely ignored for more than a century. Not until the 1960s did theoretical studies resume in earnest and reveal the conditions under which amplitude nonlinearity counteracts dispersion to permit the creation and propagation of solitons. These studies coincided with the develop- ment of lasers, whose high power density provides a practical means of teasing nonlinear behaviour out of optical fibres. It is these photonic descendants of Scott Russell's fluid solitons that have revolutionized long-distance digital communications in the past decades.

Figure 1: Keep on waving.
figure1

This image of solitons propagating out from the Straits of Gibraltar was taken by NASA's STS-41G space shuttle mission in October 1984. Lighter, fresher Atlantic sea water (which accelerates through the narrow straits from top left) compresses and upwells as it makes contact with the denser, more saline water of the Mediterranean Sea. The resulting sets of solitary waves propagate without distortion on the open sea over great distances.

Optical soliton systems exploit the nonlinearity of an optical fibre's refractive index, under high electric fields. In contrast, electronic soliton systems use a transmission line in which the amplitude of the response to an applied pulse is itself nonlinear. Purely electronic soliton systems offer the same theoretical advantages enjoyed by their optical cousins, but have the added appeal that they are simpler to produce, as they can be manufactured using standard integrated-circuit technology.

Ricketts and colleagues1, for example, use the voltage-dependent capacitance of conventional semiconductor junction diodes to create a discrete nonlinear transmission line. They connect this transmission line around an amplifier to make a closed feedback loop that produces an oscillating electrical signal. Ensuring the stability of an oscillating circuit requires careful control of some appropriate system parameter. In this case, the authors adjust the amplifier's gain dynamically both to guarantee that oscillations start and to avoid the onset of chaos. The result is a stable, periodic train of self-generated solitons. The short-duration, periodic soliton train produced by the oscillator could be widely deployed in communication and instrumentation technologies.

Wu and colleagues2 use a topologically similar arrangement, but bring about nonlinear feedback using a ferromagnetic film made of yttrium–iron–garnet. The complex properties of this material allow a rich variety of nonlinear behaviour, ranging from stable-amplitude oscillations to chaos. Rather than ensuring stable oscillations, Wu and colleagues operate their oscillator in a chaotic regime. The chaotic nature of the signal readily scrambles a message over a large bandwidth, and so reduces the probability that it can be detected or interfered with. Meanwhile, synchronizing a chaotic transmitter with its intended receiver permits unambiguous decoding of the original message3. Much effort is currently going into such chaotic, covert communication schemes. The convenience of having such systems in a flexible electronic form using electrical solitons will do much to accelerate their development.

From chance observation of a summer canal-side ride, to linchpin of new communications technologies: John Scott Russell would no doubt have been delighted to see his belief in the importance of solitons so vindicated.

References

  1. 1

    Ricketts, D. S., Li, X. & Ham, D. IEEE Trans. Microwave Theor. Tech. 54, 373–382 (2006).

  2. 2

    Wu, M., Kalinikos, B. A. & Patton, C. E. Phys. Rev. Lett. 95, 237202 (2005).

  3. 3

    Roy, R. Nature 438, 298–299 (2005).

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