ive difference in channel powers when optical amplifiers with unequalized gains are used in WDM systems. Launch Unequal Powers Unequal amplifier gains combine to give constant powers at the end of the system Schematic of pre-emphasis to obtain equal channel powers at the end of a WDM system with carefully selected launch powers. Chapter 6 Optical Fiber Transmission Target Profile Synthesized Profile Tantalum (needle method) pentoxide (n = 2.065) Wavelength (nm) Several very fine layers Silica (n = 1.465) Film Position (um) Fig. 6.8 Computed thin-film GEF to be used in transmission with a C-band erbium-doped fiber amplifier. Top: the filter reflectivity profile superposed on the gain profile of the EDFA. Bottom: the refractive index structure of the thin films. (Results produced by Miguel Zamora [2005].) ticularly challenging for pre-emphasized networks because the channel powers can differ greatly after reconfiguration. Gain-Equalizing Filters (GEFs) are normally used to achieve flattened spectra in discrete amplifiers (Othonos et al., 2006). Because of the nature of the semiconductor band gaps, SOAs have parabolic gain profiles (Dutta, 2003; Zimmerman & Spiekman, 2004). However, the semiconductor material has a high refractive index, causing reflections, and SO low- finesse Fabry-Pérot modes can be superposed. Even with measures such as buried and angled waveguides and antireflection coatings, the ripple can be ~0.5 dB or more. Neverthe- less, the overall parabolic shape can be straightforward to equalize with suitable filters. C-band EDFAs have a more problematic gain profile with a rather narrow peak at ~1532 nm, a minimum at ~1540 nm, and a broader plateau around 1555-1565 nm (Srivistava & Sun, 2002). Consequently, carefully designed GEFs are required. An example based on thin-film technology (Othonos et al., 2006; Miguel Zamorra, 2005) is shown in Figure 6.8. GEFs should have low excess losses and, for fear of deteriorating overall noise perfor- mance, they should not be located at the amplifier's input. (Th