distance can be used between regenerators. This prop- erty explains why hybrid amplifiers are mostly used in ultra-long-haul systems, where the reach is key in the system value proposition. Telecommunication Networks Raman Raman Output WDM Input WDM EDFA-C band EDFA-L band FIGURE 4.41 Block scheme of a wide band amplifier using EDFAs for C and L bands and Raman for S band. Flattening filter Transmission EDF 1 NZD fiber EDF 2 000000 000000 Raman Circulator Isolator EDFA 1 EDFA 2 FIGURE 4.42 One of the possible schemes of hybrid EDFA and Raman amplifier. A second advantage is a more uniform gain along the length of the fiber. This gives rise to better signal to noise ratio performance and reduced nonlinear penalty. Finally, the gain equalization, gain level correction, Add-Drop multiplexers, and disper- sion compensation can all be placed in between the two stages of the EDFA, thus requiring no modification to the standard architecture and design of such subsystems. On the other hand, there are also challenges in using hybrid amplifiers. A first challenge is constituted by the high Raman pump powers propagating in the transmission fiber. Pumps on the order of 500 mW or even 1 W are common for Raman-distributed amplifiers if using an SSMF transmission fiber. At these power levels, connectors are highly vulner- able to damage. In addition, there are issues associated with higher sensitivity to spurious reflections and to environmental and mechanical changes. Careful optimization and engineering are needed to design practical hybrid amplifiers; however, despite all these difficulties, many ultra-long-haul systems adopt them. The potential performances of hybrid amplifiers are illustrated in Figure 4.44 where SN0 and NF are shown for a high-performance hybrid amplifier [74]. Here SN0 is the signal to Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices Concentrated amplifiers power distribution average value 8 mW Hybrid amplifiers power distribution average value 3 mW Raman gain FIGURE 4.4