104 CHAPTER 4 EDFAs, Raman amplifiers and hybrid Raman/EDFAs Alternatively, the channel power adjustment capability of ROADMs can be used to rebalance channel powers to mitigate the effects of SHB. In transoceanic systems traversing very large numbers of EDFAs, the cumulative effect can be many dB and must be taken into account in determining the order in which wavelength channels will be populated as utilization of the system's capacity increases through its life. Site dependent pumping (SDP), also known as pump wiggle or pump induced inhomogeneity, is another manifestation of inhomogeneous effects in EDFAs. Pump wiggle is the name given to the dependence the gain spectrum has on the precise wavelength of pump light in the 980 nm pump band due to the preferential excitation of different groups of ions by different pump wavelengths [36,37,38]. Because of this effect, for DWDM applications very tight specifications on the wavelengths of 980 nm pump lasers are required to meet the gain flatness requirements. Because of the relative populations of the different sublevels in the Stark-shift broadened manifolds of the electronic energy levels, the gain spectrum can vary significantly over temperature, increasing the gain ripple by 1 dB or more over the specified range of ambient operating temperatures, typically on the order of -5 C to 55 C [39]. For demanding DWDM applications it is common to reduce the temperature dependence of the EDFA gain spectrum using heaters to maintain the