No mixed signals
If sending an optical signal down a strand of fiber were a simple task, we wouldn't have so many companies with so many technologies competing for the honor—and the reward. The trouble with the task is that a lightwave traveling down a waveguide is an unruly creature. It's given to stretching and twisting, changing strength, jumping around, and mixing it up with its neighbors. As a result, an optical signal can become very hard to read by the time it reaches its destination.
This month, we're focusing on some of the more challenging causes of signal problems—polarization, chromatic- and polarization-mode dispersion, insertion loss, and other sources of signal degradation. Dispersion has been a major source of concern as networks have sped up, and the typical solution has been to add dispersion compensating fiber or, in some advanced designs, to add tunable optical dispersion compensators. John Trail and his colleagues at Big Bear Networks argue that electronic equalization, sometimes in concert with optical compensation, will do the trick at considerable cost savings. In particular, deployment of electronic equalization in metro networks will allow high-speed transmission over an older, installed base of optical fiber.
Zhizhong Zhuang and Barry Zhang at Optellios show how measurement and control of polarization are critical not only to address problems such as dispersion, but also to test polarization dependence of various WDM components. Some sources of trouble can be alleviated during manufacture, and that is the point of an article by Matthew Adams at JDS Uniphase Instrumentation. In two case studies he shows the advantages of modular testing for insertion loss in passive DWDM components and for bit-error rate in 10-Gbit/s transponders.
As optical networks become more dynamic and sophisticated, it's a safe bet that sending signals won't become easier, and it's a good bet that companies that respond to the challenge with innovative, cost-effective technology will be rewarded.