n designers was to narrow the linewidth of the lasers employed in these systems and to make them more powerful. Unfortu- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Transport Technologies nately, increasing the power and reducing the laser linewidth is expen- sive, so another solution emerged. Dispersion-Shifted Fiber (DSF) One solution that emerged was dispersion-shifted fiber. With DSF, the minimum dispersion point is mechanically shifted from 1,310 nm to 1,550 nm by modifying the design of the actual fiber so that waveguide dispersion is increased. The reader will recall that waveguide dispersion is a form of chromatic dispersion that occurs because the light travels at different speeds in the core and cladding. One technique for building DSF (sometimes called "zero dispersion- shifted fiber") is to actually build a fiber of multiple layers. In this design, the core has the highest index of refraction and changes gradually from the center outward until it equals the refractive index of the outer cladding. The inner core is surrounded by an inner cladding layer, which in turn is surrounded by an outer core. This design works well for single- wavelength systems but experiences serious signal degradation when multiple wavelengths are transmitted, such as when it is used with DWDM systems. Four-wave mixing, described earlier, becomes a serious impediment to clean transmission in these systems. Given that multiple wavelength systems are fast becoming the norm today, the single- wavelength limit is a show stopper. The result was a relatively simple and elegant set of solutions. The second technique was to eliminate or at least substantially reduce the absorption peaks in the fiber performance graph so that the second and third transmission windows merge into a single larger window, thus allowing for the creation of th