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Now Playing at the Multiplex Odeum
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Now Playing at the Multiplex Odeum: Communicating at Light Speed, Starring WDM, Gradium Glass, and Soliton Propogation

 

by Douglas PageŠ1999

The explosive growth of computer internetworking and the demands of multimedia, high-speed LANs, telemedicine, supercomputer-to-supercomputer interconnections, digital libraries and virtual reality applications are driving data transfer requirements beyond the Megabit/sec into the Gigabit/sec range and beyond.

Traditional networking techniques cannot handle the traffic demands of emerging broadband communications. Over the past decade, multiwave optical networks have become the new backbone of the communications infrastructure. Using time division multiplexing (TDM) technology, carriers now routinely transfer data at 2.4 Gb/s (gigabits per second) on a single fiber, with some deploying equipment that quadruples that rate to 10 Gb/s.

But even this technology, widely recognized for its capacity and flexibility, has struggled to keep pace with a demand that doubles every five weeks, according to WorldCom CEO John Sidgmore. Sidgmore predicts by the year 2000 half the bandwidth use will be for the Internet, while half will be voice calls. By 2003 or 2004, he says, the Internet will account for 99 percent of bandwidth usage.

Vint Cerf, senior vice president of Internet Technology at MCI and co-inventor of the TCP/IP - the language of Internet communication, says his fiber optic network is growing 400 percent a year. He expects to need 80 to 160 Gb/s capability by the year 2000. He thinks major carriers like MCI could have 700 Gb/s technology three years after that, and Terabit/sec (one trillion bits per second) technology by about 2010.

Researchers are trying all sorts of things to gain more bandwidth, from increasing the data rate to wavelength division multiplexing (WDM). (It should be noted that capacity and bandwidth are not the same thing. WDM technologies provide greater capacity by creating more 'wires', not higher bandwidth by making faster ones.) WDM technology is generally considered to be a key to satisfying the escalating bandwidth requirements of the Information Age. WDM devices serve as traffic cops, allowing a wide range of data, at varying speeds, to move simultaneously through an optical fiber network. An enormous amount of unused bandwidth (approximately 24x1012 hertz) exists in the low-attenuation communications bands (in the 1.3 mm and 1.5 mm optical transmission bands) of a optical fiber. One fiber thread is capable of carrying more than 2,000 times as much information as all the current radio and microwave frequencies.

WDM allows the large bandwidth of the optical fiber to be more fully exploited. With WDM, optical fiber becomes more than a simple one-to-one replacement for copper wires. WDM divides the optical transmission spectrum into non-overlapping wavelength bands, or channels, where each channel may operate at peak speed, allowing multi-Gb/s data rates. One example of a WDM technology emerging, called Dense Wavelength Division Multiplexing, multiplies the capacity of a single fiber sixteen fold, to a throughput of 40 Gp/s.

Once electrical "bottleneck" is overcome (by removing electronics from the network the requirement for a complex central switch is eliminated), optically multiplexed WDM data travels at light speed over the fiber optic cable, supporting multi-Gb/s or even Tb/s data transfer. Transmission capacity of signals via fiber optic links is virtually unlimited and can be achieved over huge distances. Optical erbium fiber amplifiers replace electronic switches by regenerating and amplifying the optical signal without conversion into electrical signals and feed it back into the fiber cable. In contrast to optoelectronic signal processing, which is less flexible and more complex and expensive, the new technology allows network operators to make more economical use of their fiber links.

Still, many carriers are worried about where all the bandwidth will come from in next few years.

One new development that may help, from LightPath Technologies, is Gradium Glass, a high performance glass said to be capable of the work of multiple pieces of glass by virtue of its precise, internal light-bending characteristics. Fiber optic products rely on lens systems to harness light's various properties. According to physics, the perfect path for light to travel in most contemporary applications is curved, not straight. Science has sought for years for a means to produce optical conduits that have light bending power within the material. According to LightPath, Gradium Glass technology integrates more and more functions within a single lens element or optical component. It is so powerful a paradigm shift, the company believes the technology will have the same impact on the optics, optoelectronics and photonics industries as the invention of the integrated circuit had on the electronics and computing industries.

In addition to high-speed lightwave systems, the concept of using optical nonlinear solitons as a transmission scheme is emerging as a solution to data communications concerns. Soliton propagation is a phenomenon observed in nonlinear systems whereby energy is propagated by solitary waves called solitons rather than by a continuous wave train. The effect can be used for efficient pulse transmission in optical fiber networks. According to Carolyn Davenport, a network engineer at Los Alamos National Laboratory's Computing, Information and Communications Division, solitons are very short time-division optical pulses not easily destroyed by small propagation inhomogeneities in fibers. Since pulse distortion due to dispersion does not occur, the time duration of a bit can be very short (100-200 fsec) so that it is possible to achieve a very high transmission rate.

Revolutionary experiments at Bell Labs by L.F. Mollenauer and his group have demonstrated soliton WDM transmission at 80 Gb/s over transoceanic distances.

-end-

Article appeared in the December 1999 issue of High Technology Careers Magazine.

Comments? Questions? Assignments? douglaspage@earthlink.net
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