TABLE OF CONTENTS Performance requirements for WDM system applications are slightly different from DWDM applications. Custom-tailored optical fibers are required for high-performance WDM systems. In addition, adjustment of chromatic dispersion, dispersion slope, and fiber core effective area is critical. A 1310-nm WDM communication system using standard single-mode, custom-tailored optical fiber is widely deployed even now due to its high reliability and superb performance [6]. SM-fibers used in systems have impressive low attenuation of 0.2 to 0.3 dB/km at a wavelength of 1550 nm, and a dispersion close to 17ps/nm-km. This dispersion affects the performance of a high-speed transmission system. Research studies performed on chromatic dispersion indicate that dispersion can be improved by varying the refractive-index profile of the fiber. Even the zero-dispersion-shifted optical fibers are not suitable for WDM systems because they allow nonlinear effects to buildup.
As stated earlier, large dispersion can limit transmission speed, while a little dispersion is necessary to combat the four-wave mixing problem. A nonlinear effect arises when three channels are combined to generate a weak signal at a fourth optical frequency. Since the channels are spaced uniformly, the fourth optical frequency interferes with another channel and generates optical noise. With low dispersion, the signals stay in phase over long distances, which result in four-wave mixing. Furthermore, a little dispersion spreads the signals, thereby reducing their interaction and damping four-wave mixing. Too much dispersion limits transmission span and requires extensive dispersion compensation.
