t the gain flatness can be improved by working on the doped fiber Fluoride fiber Standard erbium doped fiber FIGURE 4.28 Gain bandwidth of a standard EDFA amplifier and an EDFA realized with a fluorine doped fiber. A single-pump scheme is considered. (After Ohishi, Y. Wavelength (nm) et al., Opt. Lett., 22(16), 1235, 1997.) Telecommunication Networks Fluoride fiber Standard erbium doped fiber FIGURE 4.29 Noise figure versus wavelength for a standard EDF and a fluorine doped EDF. (After Ohishi, Y. et al., Opt. Lett., 22(16), 1235, 1997.) Wavelength (nm) material. For example in Figure 4.28, the spectrum relative to a fluoride-doped fiber is also reported, whose uniformity is by far better with respect to that of an EDFA using a standard doped fiber. Several studies have been carried out along this direction, achieving good results [51-53]. However, changing the material composing the doped fiber also causes changes in the amplifier noise factor NF. In Figure 4.29, the spectral behavior of NF is shown for the same amplifier of Figure 2.28. The noise degradation due to the fluoride glass is evident from the figure. Better results have been obtained with improved fabrication techniques and materials, but in any case an NF degradation has to be taken into account. Another technique to obtain an intrinsically gain-flattened erbium-doped fiber is to manage suitably the fiber index profile [54]. For example, using a staircase index profile [55], good flattening results can be achieved. Also in this case however, the price to pay is an increase of the noise factor due to the lower field confinement that reduces the superpo- sition between pump and signal with the erbium ions distribution lowering the gain for a given pump power and increasing ASE noise generation. Even if working on the doped fiber itself is interesting under several aspects, the most used method to obtain an almost flat gain bandwidth is to shape suitably the transfer function of the filter that always exists at the amplifier output to limit the amount o