May 2001

The Next Generation of Optical Fibers

Continued from page 2

By Philip Ball

Pure Air

It sounds so obvious. Light travels through air with little scattering. So why not just send laser light down a hollow glass tube? The answer lies in physics. To achieve the internal reflection necessary to keep light confined in the center of a conventional optical fiber, the cladding has to have a lower refractive index than the inner medium. But all known materials have a higher refractive index than air. So the conventional arrangement doesn't work in making a hollow fiber.

Which means an unconventional approach is needed. Enter photonic crystal fibers. Researchers worldwide are busy making materials that act as "light insulators," which are impassable to light just as most plastics are impassable to electrical currents. In the jargon of physics, these light insulators have a "photonic band gap" corresponding to specific wavelengths of light; those wavelengths simply cannot enter the material. If made correctly, these materials-unlike the cladding in glass fibers-should permit virtually no light to escape from an empty core wrapped in them.

Of course, many substances will stop light from passing through; but this is generally because the materials simply absorb the light rather than reflecting it. And while you might think of metallic mirrors-silvered glass-as good light reflectors, the truth is that they are not nearly reflective enough to work in fiber optics; they absorb and dissipate a small but significant part of an incoming beam. A light signal traveling down a silver-lined glass tube would travel only a short distance before dispersing entirely. Photonic-band-gap materials, on the other hand, block all photons of particular wavelengths; the oncoming light is reflected almost perfectly. In other words, they are just the thing for confining light inside a hollow tube.

In 1998, Yoel Fink, then an MIT graduate student, fabricated a "perfect mirror" out of a photonic-band-gap material. Others had previously made specialized mirrors from thin layers of dielectric materials (materials that contain electrically charged particles but have insulating properties). These mirrors have photonic band gaps, and can be extremely efficient reflectors, but they have a major flaw: they work only with light striking absolutely face-on, limiting their use to specialized applications. Fink figured out how to make a version of a dielectric mirror that reflects light coming at it from all angles, as the material would have to in the core of a fiber-optic thread.

Once you have such a mirror, seeing the commercial potential is (for photonics researchers, at least) obvious. Fink and a pair of his MIT professors, physicist John Joannopoulos and materials scientist Edwin Thomas, along with Uri Kolodny, cofounded OmniGuide. The company's goal is to use the perfect mirror as cladding for an optical fiber. Imagine taking a flat mirror and bending it around the inside of a tube, and you have a crude picture of an OmniGuide fiber.