Switching to glass makes device ultrafast.

Switching to glass makes device ultrafast

Electrons are the best foot soldiers for doing computations, and photons are the speediest messengers for communications. Taking this technological tenet to heart, communications companies over the last few years have laid thousands of miles of optical fiber cables. These pathways for light promise to transmit unprecedented volumes of information - from telephone conversations to computer data and high-definition television signals-much faster and more efficiently than their conventional copper counter-parts.

But as the optical fiber networks grow, companies will be faced with the increasingly difficult problem of policing all that photon traffic. And someday they will need devices that can almost instantaneously switch or route light sinals between optical fibers.

While that day is not yet here, the first- generation ultrafast optical switch is-at least in its laboratory version. Scientists at Bell Communications Research in Red Bank, N.J., will soon unveil an all-optical, fused-quartz device that can repetitively shuttle a light beam into different optical fibers in less than a picosecond (10 -12 second). That, says physicist Peter W. Smith, head of the group that designed the switch, is thousands of times faster than existing electro-optic switches, which use electrical signals to control the routing of light beams.

"This is really the first demonstration of such fas repetitive switching in any type of device," he notes. "It's the world's fastest switch."

Behind this technological benchmark stands a new realization about the optical properties of glass and, in particular, their nonlinear qualities. Nonlinearity means that what goes in does not necessarily come out in the same manner; put another way, not all light signals passing through a nonlinear device are treated equally. The kind of nonlinearity that most interests Smith's group is the way glass refracts a beam of light: The more intense a light beam is, the more it will change the glass's index of refraction, which is related to the light wave's velocity and, in this case, its ability to be routed into a switch's optical pathways.

Other scientists have been aware that they could exploit this nonlinearity to route beams of light by changing their intensities. But they had largely dismissed glasses as useful candidates for optical switches because their nonlinearities are very small-meaning that relatively large increases in intensity would be required to change the glass's refractive index. Instead, most research has focused on semiconductors and other materials that have much greater nonlinearities, and hence lower intensity requirements.

But the problem with these materials is that they absorb energy from the light beam. Not only can this degrade the light signal, but the resulting heating also changes the material's refractive index, causing the beam to act in undesired ways. "Although these semiconductor materials can switch fast once, they can't switch again for a very long time-that is, until they cool off," says Smith.

A switch made of optical glasses, on the other hand, is so transparent that it absorbs essentially no energy as the light passes through, thereby avoiding the heating effects that have plagued the development of optical switches, says Smith. In the Dec. 12 IEEE JOURNAL OF QUANTUM ELECTRONICS, he and Stephen R. Friberg defined a "figure of merit" that quantitatively balances a material's intrinsic nonlinearity against its adverse heating effects. After comparing the figures of merit for a number of different materials, the researchers concluded that optical glasses are the clear choice for optical switches. Smith plans to present the details of his work in April at the Conference on Lasers and Electro-Optics, to be held in Anaheim, Calif.

Photo: The switch's fast speed is due to the use of

         both a novel material and a novel
         geometry: It is a fused-quartz fiber that
         contain two closely spaced cores, (1) and
         (2). When the light intensity is low, the
         refractive index of both cores is equal
         and light traveling in (1) leaks into and
         gradually builds up in (2). Higher
         intensities change the refractive index of
         (1), altering the beam's speed in a way
         that prevents light from coupling to (2).