So just how small are the losses of light in such a next-generation fiber? Because the company is still in its early stages, the founders are keeping that information close to their chests. “All I am free to say at this stage,” says Joannopoulos, “is that with a hollow-tube OmniGuide [fiber] we could in principle achieve losses less than optical fiber.” But for a telecom industry looking to push more and more light through optical networks-and eventually facing the limits of current-generation fibers-even such carefully worded pronouncements are tantalizing.
The company is developing a series of fiber products based on the OmniGuide concept. These fibers are, in theory, far more efficient in transmitting light than a standard optical fiber. Indeed, they should be able to overcome the current limitations of glass fibers, achieving, among other things, less signal loss as the light travels down the fiber. Such heightened performance is possible, says Fink, now an assistant professor of materials science at MIT, “because we can achieve an unrivaled degree of confinement.”
The OmniGuide fibers should be able to convey much more intense signals than normal optical fibers. High-intensity light traveling in glass fibers suffers from distortions that can disrupt the transmission of signals at different wavelengths, causing cross talk between channels unless they are widely separated in frequency. This effect limits the number of different wavelengths you can stuff in a conventional glass fiber, and also how bright they can be. Because signals in air don’t suffer these effects, Fink explains, the OmniGuide fiber can convey signals at higher powers, with channels spaced closer together. That is great news for telecom companies, since stronger signals travel farther before losses begin to compromise them, and closer channels mean that more data can be packed within a given wavelength range.
The MIT approach, however, is only one way to make a photonic fiber. Other researchers have produced photonic-band-gap materials that, in cross section, are like a honeycomb in which the holes form structures that refuse entry to light of certain wavelengths. These kinds of photonic crystals, first made in the late 1980s, also nearly totally block out light. The glass fibers made at Bath, for example, are penetrated by an orderly array of holes running parallel to the thread along its entire length; at the center is an empty core in which light can be nearly perfectly confined. To give some indication of the precision involved in making the fibers, if the long, parallel holes were the diameter of the Chunnel connecting England and France, the experimental fibers made at Bath would reach Jupiter. How does one drill such perfect tunnels through a glass strand thinner than a human hair?
Fortunately, the holes don’t have to be drilled at all. They are ingeniously constructed by drawing the glass fibers from a bundle of hollow capillary tubes. The tubes are packed together in a hexagonal array a few centimeters in width, and the bundle is heated to soften the glass. As the array is pulled out into a fine fiber, its cross section gets shrunk by a factor of a thousand or so but remains laced with holes.
Initially, the Bath physicists made a light-conducting channel at the core of the fiber by substituting a solid glass rod for the central glass capillary. But still better than carrying the light in a solid core would be to send it through a hollow core-through air, with the very low losses and absence of distortion that entails. In collaboration with Douglas Allan, a researcher at Corning, the Bath team succeeded in achieving light confinement in a hollow-core photonic crystal fiber in 1999. Recently they have formed optical fibers many meters long out of their novel materials.