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Solitons and the erbium gain factor.
When network designers talk about big pipe, they mean optical fiber - strands of glass drawn as fine as human hair and as transparent as the purest crystal. The fiber allows huge amounts of data to be sent quickly and reliably between continents and enables new applications such as full-motion videoconferencing and high-speed computer networking. Right now, transoceanic fiber systems have a transmission speed of around 2.5 Gbits per second - to put this into perspective, they can send about one and a half copies of the entire Oxford English Dictionary every second. Damn fast, you're saying, but even zippier lightwave systems are in the works. The combination of two technological developments - erbium-doped fiber and solitons - is likely to allow a transmission of more than 100 Gbits per second. At this speed, Wired could deliver to you 20 uncompressed electronic copies of this issue - text and graphics - per second.
Since 1975, the transmission capacity of fiber had increased by a factor of 10 every four years. But engineers hit a wall in the mid-1980s: it seemed that optical fiber had been fully optimized, and speeds were stalled at 0.15 Gbits per second. The problem stemmed from the fact that light pulses decline in strength as they travel down an optical pipe. Many nonoptical communications systems resolve this by using repeaters that grab the signal, amplify it, and send it on its way. All kinds of ideas for building optical repeaters on chips were tried but tossed out - the chips had to convert the light pulses into electricity and then back into light, a process that proved to be a bottleneck.
The breakthrough came in 1987, when Emmanuel Desurvire (now with Alcatel France) and his colleagues at AT&T Bell Labs amplified optical pulses with a piece of fiber impregnated with the rare element erbium. The erbium ions in the glass are optically potent: when pumped up with infrared light, they want desperately to emit their stored power. So, when an optical data pulse comes along, the erbium dumps its energy by amplifying the pulse. And rather than pulling the pulses off at one end, stuffing them into an electronic box marked "amplifier," and squirting them out again, the group at Bell Labs built the amplifier into the fiber. Desurvire was able to do this by building on the work of groups at Southampton University in the UK (which had already discovered the joys of erbium doping) and earlier Bell Labs teams that had built amplifiers with elements other than erbium. Desurvire put the pieces together and in one step created the enabling technology to jack up fiber-optic transmission capacity by a factor of a hundred.
Another promising technique involves the generation and propagation of optical solitons, or solitary waves - strange pulse shapes that could theoretically travel down a fiber forever. In another form, these curious beasts date back to 1834, when an engineer named John Scott Russell was slacking off near the Edinburgh-to-Glasgow canal, watching a barge pull up and stop. He noticed a water pulse leave the prow and begin traveling down the waterway. The sight of this hump of water moving along the surface, wonderfully maintaining its shape, caused Russell to leap on his horse and track the solitary wave. Weird waves come and go all the time in a busy water channel, but Russell followed this one, moving about 9 mph, for a distance of about 2 miles. Being a proper protonerd, Russell rushed home and began playing with water tanks to study his findings.
Russell's solitary waves differ from ocean waves, the walls of water that draw surfers to the beach. As an ocean wave heads to shore, different pieces of the wave begin traveling at different speeds. The top of the wave overtakes the lower part, and the wave breaks, creating a great tunnel ride for the intrepid surfer. Solitons, on the other hand, are waves in perfect balance. Their natural tendency to fall to pieces, either by spreading out or by breaking, is canceled by the nonlinear effect of the shallow water channel.
This same effect was seen with optical pulses by Linn Mollenauer and his co-workers at Bell Labs. Because the optical power in a fiber is so confined, the strong electromagnetic fields interact with the material to produce nonlinear effects. So, just like waves traveling in shallow water, very-short-duration light pulses of the right shape can propagate as solitons. Thoughts of really high bit-rate, long-haul communications danced in the scientists' minds. While normal waves quickly crumble when they are sent at too high a rate, solitons retain their shape. But because of light's natural tendency to lose intensity when travelling through any medium, solitons also eventually break up and turn into noisy mush. Still no joy.
The turnaround came with the successful mating of erbium amplifiers and solitons. In 1989, Masataka Nakazawa at NTT Labs in Japan successfully generated soliton pulses in his erbium-doped-fiber amplifiers, and the race to speed up and lengthen the transmission capacity was on.
The battle for first place has bounced back and forth between NTT and AT&T ever since; currently, NTT leads with its soliton-fiber experiments. Nakazawa has recently shown he can send soliton data down a fiber at 10 Gbits per second. Fancy multiplexing techniques and production-quality equipment should increase that by at least an order of magnitude, perhaps more. What is remarkable is that even after it had traveled 180 million km (or about 4,500 times around the Earth), NTT researchers saw no data degradation. Now that is one long and furious ride. n
David Voss (dvoss@aaas.org), a former optics jock, gave up aligning lasers by eye to become a senior editor of the journal Science.
