Статьи 6 семестр / All-optical networks (1)

INFORMATION TECHNOLOGIES

All-Optical Networks

Fiber optics will become more efficient

as light waves replace electrons for processing

signals in communications networks

by Vincent W. S. Chan

Contemporary fiber-optic net-works transmit voice, video and data at speeds 10 to 100 times faster than the standard copper wiring that has been used in telecommunica­tions for over a century. They have, nonetheless, realized only a small frac­tion of the promise of the technology.

To fulfill its potential, fiber optics must do more than simply replace cop­per telephone wiring with thin, cylindrical conduits of glass that guide light. Optical transmission must in fact go beyond the limitations imposed by the electronics technology that preceded it. In contemporary fiber-optic networks, each time a light pulse is amplified, switched, inserted into or removed from the network, it must be changed into a stream of electrons for processing. This optoelectronic conversion can become an impediment in very high speed com­munications. A network must be saddled with more expensive and complex electronics, and it becomes more difficult to process the small­er pulses of light needed to trans­mit tens of gigabits (a gigabit is a billion bits) of digital information in a second's time. Above a certain transmission speed—about 50 giga­bits per second—electronic equip­ment will find it hard to handle this constant back-and-forth trans­formation between electrons and light waves.

It would be simpler, faster and more economical to transfer an op­tical signal from one end of a net-

work to the other by using the properties of the light wave itself to route the transmission along different pathway through the network. The signal would become electronic only when it move< into the circuits of the computer for which it is destined or else into a lower-speed network that still employs electronic processing of signals.

This all-optical network would build on the successes of fiber-optics net­works currently deployed commercially that rely on optoelectronic components for signal processing. Commercial fiber­optic cables owned by long-distance telecommunications companies, for ex­ample, transfer telephone calls and vid­eo images as digital bits, as many as 2.5 gigabits each second per fiber. This multigigabit transport of information is fast enough to move an edition of the Ency­clopedia Britannica from coast to coast in a second's time. But if communica­tions traffic grows dramatically, reliance on optoelectronics will eventually limit the ability of these networks to carry more information.

With such huge communications pipes already in place, one might even ques­tion the necessity of pursuing the de­velopment of technologies that can in­crease network capacity 100-fold. In fact, during the early 1990s, corporate finan­cial officers at major U.S. telecommu­nications companies cited an apparent surfeit of communications capacity and an absence of market demand to justify budget reductions that led to the virtu­al disappearance of fiber-communica­tions research departments in the U.S.

Despite such dire predictions, the rev­olution in high-speed fiber communica­tions may have only just begun. The ad­vent of a market for digital video could overwhelm the fastest commercial op­tical networks. Digital video will require up to 500 times the communications capacity, or bandwidth, needed for the

ALL-OPTICAL NETWORKS operate without converting the optical signal to an electronic format. Each computer or video camera in a segment of the net­work, or subnetwork, such as the one at the left, uses a transmitter to send a single wavelength—represented by the colored lines. Each wavelength can carry several gigabits per second. The three signals are multiplexed, or merged, into an optical fiber and are sent through an optical amplifier that boosts the signals. Then each wavelength moves into a router, from which it is sent to other subnetworks. If two signals share the same wavelength within the same fiber—as happens in the subnetwork at the upper right—a wavelength converter changes one of the signals to a different wavelength to avoid a conflict; the signal from the router is switched from red to blue, for example.

72 Scientific American September 1995