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Light Conversation

Although photonic fibers are a next-generation technology in 2001, the history of conveying voice data using light extends back more than a century. After inventing the telephone in 1876, Alexander Graham Bell didn’t rest on his laurels. In 1880 he showed that light, rather than electricity, can carry a person’s words to a distant ear. Bell’s “photophone” used vibrating mirrors to transmit sound via sunlight. But it was an idea long before its time. Sending electrical signals down copper cables proved much more reliable, and the photophone was largely forgotten as telephone lines enmeshed the world.

After eight decades of the supremacy of copper wire, the invention of the ruby laser in 1960 put light back on the communications agenda. Here was a source bright enough to really put light to work. Just as the transistor ushered in the age of microelectronics, the laser sparked the age of photonics. In 1970 Corning proudly announced that it had sent a laser beam down a glass fiber and recovered as much as one percent of the light at the other end, a kilometer away (today’s glass fibers are so efficient that 80 percent of the light will survive that distance). By the 1980s, telephone companies began replacing copper cables with optical fibers.

An optical fiber can carry thousands of times more data than a copper cable: in principle, a single fiber can transmit up to 25 trillion bits per second. That’s enough capacity to carry all the telephone conversations taking place at any instant in the United States-with room to spare. Small wonder that the worldwide web of information technology is being woven from light-bearing glass.

In a conventional optical fiber, light is confined in a silica inner rod by a “cladding” of glass with a slightly different composition than that of the core. Typically, small amounts of germanium or phosphorus are added to the core (a process called “doping”), giving it a different refractive index from the cladding. Light striking the interface between core and cladding is reflected, so the signal bounces back and forth and remains within the core. Information is encoded in a series of pulses from electronically controlled lasers and fired down the fiber to a photodetector at the other end, which converts the signal back into electrical form for processing in a telephone, computer or routing device.

Sounds great. So, where’s the catch? It’s a matter of limits. As communications networks get bigger, busier and more ambitious, the drawbacks of conventional glass fibers are becoming evident, and existing optical-fiber networks will eventually be unable to cope. One factor that limits performance is the fading of the light signal over distance. A certain amount of the light is “scattered”-impurities in the silica disrupt the transmission of some of the signal-as it travels through the glass core; other light simply escapes from the fiber altogether, because the interface between glass core and cladding is not a perfect mirror.

Unremedied, these losses would cripple long-distance fiber-optic communications: eighty percent transmission over a kilometer would leave less than a ghost of a signal at the far end of a transatlantic cable. The answer is to amplify the light every 70 kilometers or so. But amplifiers are expensive, and they require their own power sources (see “5 Patents to Watch: Booster Shots). Each amplifier typically adds a million dollars to the price of a long-distance transmission line. For a cable thousands of kilometers long, that begins to add up to real money. And when an amplifier breaks down mid-Atlantic, there’s no option but to send out a ship to dredge up the cable. “It costs a fortune to fix them at the bottom of the ocean,” says Bath’s Russell.

This daunting economic reality is the spur for developing the new generation of fibers. Cambridge, MA-based OmniGuide Communications, founded last year by several MIT professors, claims its new fibers will be able to squeeze losses so low there would be no need for any amplification. What’s more, the company says, the usable bandwidth will be substantially larger than in existing optical fibers. The trick is to strip out the fiber’s glass core and replace it with-well, nothing at all.

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Tagged: Communications

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