ed fiber parameters, the number of stages and the pumping scheme for each stage and finally the doped fiber length for each stage, the EDFA amplifier is dimensioned for what regards the optical part. Such amplifiers exhibit all the phenomena we have introduced in the previous section: gain saturation with signal power, ASE noise, and SO on. Naturally, since the lifetime of the excited state is on the order of milliseconds, no dynamic distortion is created at every bit rate of interest in systems using EDFAs. Another important characteristic of the EDFAs is their wide amplification bandwidth, which seems to be in contrast with the nature of the mechanism causing amplification, that is, stimulated emission in a discrete energy levels spectrum. This is due to the fact that the electrons energy levels of the erbium ion population in glass matrix are composed more of small energy bands [50]. This splitting among electrons levels is due essentially to two effects: Stark splitting and homogeneous line broadening. Stark splitting is due to the interaction of the electron orbitals of each erbium ion with the electrical field due to the surrounding glass molecules. Since the microscopic position of each ion is different, this influence generates a level splitting. Homogeneous line broadening is due to the interaction with the ions electrons with local phonons of the glass. This mechanism, which is called homogeneous since it influences every ion in the same mode, causes the typical Lorenz type line widening. Inhomogeneous mechanisms exist too in the interaction between erbium ions and the host glass, but gener- ally homogeneous broadening is dominant. From the earlier discussion, it is clear that in general the amplification bandwidth of an EDFA cannot be uniform, but on the contrary EDFA spectral gain will have a complex behavior. Just for an example, the gain spectrum of a practical single-stage amplifier is reported in Figure 4.28. Considering the nature of the phenomena causing bandwidth shaping, it is also evident tha