The Science of Light

When it comes to promoting optical networking technology, companies - and the writers who report on them - just can't resist tossing in references to the speed of light.

When it comes to promoting optical networking technology, companies — and the writers who report on them — just cant resist tossing in references to the speed of light.

Do a quick Google search on "optical networks" and "speed of light" and up pop a bunch of press releases and news articles loaded with phrases such as "data transfer at light-speeds," "information technology at the speed of light" and "speed-of-light communications."

"Its the next-generation Internet — one on which the once separate worlds of telephony and Internet come together at the speed of light," brags a job page on Nortel Networks Web site. "Imagine an information superhighway thats even faster than it currently is, able to transmit data, quite literally, at the speed of light," ponders an article on the North American Precis Syndicates Web site.

But in truth, the efficiency of optical networks has little to do with the speed at which photons travel. In fact, in a sense, optics are slower than electronics, says Karen Liu, director of the optical components service at telecom analyst firm RHK.

"If you compare a coaxial cable to a fiber, the signal is actually moving faster in the cable," Liu says. "The bit duration is much shorter in a fiber: They move more slowly, but more are arriving every second."

But the main reason optical networks send data so quickly is that signals of light traveling through strands of silicon decay far more slowly than signals sent by moving electrons along copper wires. Both light and electric signals lose energy and get distorted over distances. "You can only carry that bit so far before you have to amplify it," says Darcy M. Lorincz, vice president of technology and engineering at Global Crossing. A light signal retains its integrity over far greater distances.

" Why cant I transport 1 gigabit per second in copper? The answer is you can, but it will only be a few hundred feet, rather than a few hundred miles," says Kamran Sistanizadeh, chief technology officer at Yipes Communications.

Another advantage of optics is that since the bits of information are sent as beams of light that can be turned on or off, separate channels of information can be sent over different wavelengths. Dense Wavelength Division Multiplexing, as this technique is known, enables a single strand of fiber, narrower than a human hair, to become the thoroughfare for many lanes of traffic. Thus, more data can move over a single channel each second. By utilizing more wavelengths, the capacity of the fiber can be increased, limited only by the technology that amplifies or switches the signal.

Since electrical signals are sent as charged particles moving through a copper wire, however, additional channels require more wires.

That glass fibers can carry light waves so fast and far is a property of the material and the manufacturing process, Sistanizadeh says, noting that at wavelengths outside the band used for optical transport, light cannot travel efficiently through silicon.

New production techniques remove trace amounts of water from the silicon in the manufacturing process, Lorincz says, making the capacity even greater.

Thus, the design of new materials and improvements in manufacturing will enable optical fibers to carry even more data for longer distances.