OPTICAL FIBERS AND AMPLIFIERS FOR WDM SYSTEMS

The development of optical-fiber amplifiers allowed a dramatic increas e in the capacity of optical transmission systems while reducing syste m costs. Capacity increases are possible because the high output power s afforded by optical-fiber amplifiers support higher bit rates, while their broad bandwidth and slow gain dynamics allow multichannel opera tion. This benefit comes at the expense of having to manage signal-to- noise ratio degradations due to the accumulation of amplifier noise an d dispersion distortions accumulated over the total system link. Furth ermore, nonlinear optical effects become significant with the use of h igh power signals over long lengths of fiber, causing cross talk among the different channels and increasing signal distortions. To fully ex ploit the potential capacity of wavelength division multiplexing syste ms, the optical characteristics of the fibers and optical-fiber amplif iers must be optimized. The optical amplifiers should have low noise a nd flat gain, which can be achieved by using 980-nm pump lasers, optim ized fiber glass composition, and gain-flattening filters. The optical fibers should have a small nonzero dispersion and large effective are a. Both features can be achieved by optimizing the fiber index profile . This paper summarizes the state of the art in these components and p oints to directions for future exploration.