all-signal fiber Gon-off: Raman amplifier. Pump powers (contradirectional): 823 mW 18 dB (top), 686 mW (center), and 548 mW (bottom). Raman gain coefficient is I = 0.35W- km-1; Unpumped Fiber pump and signal wavelength losses are 0.3 and 0.2 dB/km, respectively; Gon-off and Gnet are defined in equation 6.10. Length Coordinate (km) System Design and Implementation should a fiber end be accidentally exposed. An increasing worry is fiber damage through what has come to be known as the "fiber fuse effect" (Sikora, 2006), in which many kilome- ters of transmission fiber could be rendered irreparable. Given these issues, there is a need for rapid automatic laser shutdown in the event of unforeseen fiber damage. Although the relatively large effective area of G.652 fiber is beneficial in lowering the potential for nonlinear crosstalk, by equation 6.35 it also reduces Raman amplifier pump efficiency. (The high pump powers reported in Figure 6.32 are evidence for this.) We could resort to other kinds of transmission fibers, such as NZ-DSF, which have sufficient chromatic dispersion to frustrate XPM and FWM, together with values of Aeff in the 50- to 70-um2 range. Unfortunately, such fibers often have their 20 in the S-band. When we use multi- wavelength Raman pumping to obtain spectral equalization, it is possible to obtain four- wave mixing between the pump wavelengths (Neuhauser et al., 2001). To overcome this problem, the ITU-T has proposed a relatively new fiber standard, G.656 (ITU-T G.656), according to which D is nonzero in the C- and L-bands and 20 no longer lies in the range of wavelengths used for pumping. Fiber Dispersion Maps Normally the single most important measure to frustrate nonlinear crosstalk is the appropri- ate management of the system's chromatic dispersion, as described in the relevant earlier section. Even though the dispersion causes unwanted pulse broadening, it can be beneficial because it is very effective in reducing FWM and XPM via the pulse walk-off effects explained previously. Most operat