A chance sighting by a Scotsman out riding his horse more than 150 years ago may prove the key to a revolution in the communications technology of tomorrow.
In 1834, John Scott Russell was riding along the banks of a narrow canal when he noticed a large, well-defined wave rolling along without any apparent change in shape or size. Intrigued, Russell spurred his horse on and followed the wave for more than a kilometer.
That sighting marked the first known observation of a curious physical phenomenon since termed the soliton. Solitons are ripples, waves or pulses that travel, sometimes for great distances, without any distortion in shape or size (see box.) Interest in solitons has taken off in recent years with the remarkable finding that not only waves of water but, in particular circumstances, also pulses of light can form solitons. Light or optical solitons are now widely regarded as "the wave of the future" in the dynamic field of communications.
Prof. Yaron Silberberg of the Weizmann Institute's Physics of Complex Systems Department is riding that wave. A physicist specializing in ultrafast optics, he believes light solitons could be the way to make the best use of the new fiber-optic cable networks now being laid down around the world. These cables have a much greater capacity than existing electric telephone lines, and Silberberg believes optical solitons could be the way to bring that capacity to its maximum.
But what do light pulses have to do with communication lines? When telephone calls or computer data travel along telephone lines, the sound waves or images are translated into a pattern of fluctuations in an electric current, which are converted back into sound or visual images at the receiving end. The Internet explosion of this decade has stretched existing telephone line capacities to their limits, mainly because transmitting a computer image takes up massively more "space" on a line than does converting simple sound waves.
"To make a telephone call, you need only 64,000 bits [the binary units of data -- Ed.] per second," says Silberberg, "but to transmit one screen image from your computer you may need a million bits."
Light was long ago envisaged as a replacement for electric currents because, in the right circumstances, it can travel over much larger distances at many more bits, or pulses, per second. But until recently there were two major problems in working with light: first, it is absorbed by many substances and therefore quickly lost; second, over a distance, light pulses tend to spread out and break up into their component colors, or wavelengths, leading to a distortion of the signal.
The development of fiber-optics several decades ago provided the solution to the first problem: optical fibers are narrow cables made of pure glass that do not absorb much light but keep it bouncing along until it reaches its destination. Fiber-optic cables using light pulses are now replacing traditional lines using electric currents. The fastest commercial fiber-optic system in use today attains a capacity of 2.5 billion bits per second, hundreds of times faster than the fastest electric lines. In laboratories, experimental fiber-optic systems have achieved capacities of up to a trillion bits per second.
Scientists and communications companies are now hoping that optical solitons will prove to be the solution to the second problem, that of light pulses breaking up. Scientists have found that a laser-generated light pulse of a particular wavelength, 1.5 micrometers, can create a soliton in a fiber-optic cable. Such solitons remain stable over thousands of kilometers. Moreover, they retain their integrity: send two solitons toward each other down a fiber and they will cross paths and separate again without merging -- a finding that holds great promise for increasing the capacity of any single line.
"Once created, the optical soliton is a beautiful thing," says Silberberg. He is focusing his research on understanding the basic properties of optical solitons and how they can be manipulated. In particular, he wants to learn how to manipulate one pulse of light with another. This would make it possible to build optical circuits that use optical switches, rather than the electronic switches of today, to relay data in the form of light pulses.
"It is a far-off goal, but it is my dream: to learn how to control light with light," says Silberberg.
Forming a Soliton:
When a group of people race along a standard track, they normally disperse over a distance, with the faster runners pulling ahead and the slower ones falling behind. But imagine runners placed on a mattress that gives under their weight. The bulk of the average runners in the middle would create a valley, so the faster runners at the front would find themselves on an upward slope that slows them down, while the slower runners at the back would find themselves on a downward slope that speeds them up. This would have the effect of preventing dispersion and keeping the group together as it runs along, forming a "soliton."
Like the runners on a standard track, light normally disperses over a distance because its different colors travel at different speeds. But for light pulses in certain conditions ? in particular, those around a wavelength of 1.5 micrometers -- an optical fiber acts as the equivalent of the runners' mattress, holding the light together and preventing it from dispersing. Such a pulse of light can form an optical soliton.