Though the day when a fiber optic cable hangs between the street and your house is still a few years off, communication companies are scrambling to find a way to replace the rat's nest of copper wiring that carries phone, Internet and television with a network of super-fast fiber optics. But how do you maintain such a complicated network of delicate, hair-thin glass? Let the glass keep an eye on itself, say optical engineers at the University of Rochester.
Turan Erdogan and researchers in his laboratory at the Institute of Optics have developed a device smaller than an inchworm that clings to a single optical fiber and precisely measures how efficiently it's performing. If trouble strikes and the fiber loses a few bits of data, the device could instantly detect the problem and tell headquarters to reroute the signal. Traditionally, such devices are filled with lenses and motors and take up as much space as a toaster, severely limiting where they can operate, but the new device has no moving parts and could fit anywhere the fiber goes.
"It's like a line tester for the fiber optic age," says Erdogan, "except instead of pulling out an oscilloscope whenever you think there's a problem, each fiber would monitor itself and let you know when there's trouble."
Trouble on a network of fiber optics is much different than trouble on a conventional network. Copper lines transmit one signal at a time while an optical fiber may transmit over a hundred wavelengths of light all at once, each one carrying more information than an entire copper line. Most fiber optic lines are strung between phone company "central offices" while the slower copper lines extend to the customers. Though faster, optical fibers are glass and are more prone to breaking, which could cut off your phone conversation or demolish your download.
Over the next several years, communication companies will replace the copper wires connecting businesses and eventually homes with fast fiber optics, demanding a complete change in how engineers sustain the network.
"Network monitoring is more important for fiber optics than it is for copper lines because there's not much you can do with copper besides send an electric signal down it," explains Erdogan, professor of optics. "It either works or it doesn't. But in four or five years when the next generation of fiber optic communications hits, the complexity of the networks will make it much more important to know the exact condition of every line, everywhere, instantly."
By exploiting some quirks of light, Erdogan and his team of scientists found a more efficient way to verify that every wavelength is passing through a fiber intact. The scientists etch tiny groovesactually density variations of the glassinto a fiber, diverting a little of the light in the fiber like logs might divert water in a stream. As the light is diverted it splits into a rainbow, showing off all the wavelengths being carried by the fiber.
Traditionally, scientists have used bulky devices to sort out each wavelength's color in the rainbow, but Erdogan has discovered a way to slim down much of the detector's size. Just downstream from the grooves, Erdogan builds a mirror, thinner than a human hair, that sends some of the light back upstream. Just as before, the reflected light hits the grooves and diverts from the stream, creating another rainbow. When the two rainbows overlap, they produce a series of shadows. The inchworm-sized device, actually a photodetector on a chip, counts the shadows, does some math, and reports exactly which wavelengths are moving through the fiber.
Lucent Technologies has developed a similar device, but Erdogan's is smaller and can measure at a higher resolution, he says. "Our idea is much simpler, more elegant, and easier to implement," says Erdogan. His millimeter-wide detector is about as long as a thumbnail and fits snuggly within the insulation that surrounds a typical fiber optic line. Erdogan believes it's the smallest such device capable of accurately monitoring fiber optic transmissions.
"Ultimately, new ways of controlling light can lead to the ability to transport more information more quickly through optical fibers, whether it's voice, faxes, computer data, or even images," offers Erdogan, whose research centers on finding unique ways of controlling how light travels through optical fibers. "Given the increased complexity of these systems, we're always thinking about ways that simple devices could help monitor the fidelity of the signals passing through such systems."
Erdogan and graduate student Mark Froggatt presented the design for the device at the 1999 Optical Fiber Communications Conference in San Diego and have published the results in the journal Optics Letters (vol. 24, pg. 942,1999).