Статьи 6 семестр / All-optical networks (2)
.docroutine telephone calls transmitted over fiber-optic networks.
Such a network would also accommodate the flow of enormous quantities of digital data. Proposals abound to create on-line libraries large enough to hold all the text, images and audio archives from the entire Library of Congress. A communications network that could transmit terabits (trillions of bits) of information could open the full resources of a research library to every home, school, office and laboratory.
For these scenarios to materialize, physicists and engineers must devise methods to use more of an optical fiber's capacity. A single fiber could, in theory, transport 25 terabits each second, an amount sufficient to carry simultaneously all the telephone calls in the U.S. on Mother's Day., But the practical information-carrying rate of a fiber is much more limited: it is checked by the tendency of a pulse representing a digital 0 or 1 to lose its shape over long distances, as well as the absence of optical components that can process information at these blazing speeds. Recent research advances hold promise for tapping much more of a network's unused bandwidth.
Technical Milestones
Many existing research efforts at-tempt to build on the most important development in optical communications of the past decade: the optical amplifier. The device allows the power of a signal to be restored to its original strength without the usual optical-to-electronic conversion. In an optical amplifier, erbium ions are embedded in the glass of a fiber. When stimulated with a laser, the excited ions revivify an optical signal that has weakened after
a journey of tens of miles. Optical amplifiers, which have recently been deployed in commercial networks, demonstrate superior performance for very high speed networking: unlike electronic amplifiers, they can amplify a signal carrying data at transmission speeds greater than 50 gigabits per second, and they can boost the power of many wavelengths simultaneously.
Optical multiplexing technology, for one, allows a fiber to be used more efficiently because separate data-carrying signals can be sent over the same fiber. Multiplexing is important because of the demands on capacity that could be put on future networks. A television program with high-resolution images, for instance, could consume up to a gigabit per second of bandwidth if the data in the image are not compressed. The simplest form, called wavelength-division multiplexing, is analogous to radio broadcasting [see illustration on these two pages]. Each transmitter on this network contains a laser that can be adjusted to dispatch a signal at a certain wavelength, or color, of light.
Just as a transistor radio can be set to pick up a certain frequency, an optoelectronic receiver can be tuned to the desired light wavelength. In the labora-
Scientific American September 1995 73