t is also the stimulated emission level. In this condi- tion, the intraband transition happening after the ion excitation is substituted by a similar interband transition and real two-level operation is never reached. On the other hand, pumping at 980 nm attains lower noise factor and can be operated with more powerful lasers. Thus, in modern low-noise EDFAs almost only 980 nm pump- ing is used. Once the pumping scheme is individuated, the overall stage architecture is fixed and it has to be optimized for the required characteristics. The first parameters to be optimized are the doped fiber length and the pump power. In order to do this optimization, a complete model of the amplifier has to be developed, that in its simpler form is a three-level version of the model developed in Section 4.3.2; thus, the main parameters have been already introduced and the main qualitative phenomena are already known. We do reporthere a detailed model for the EDFA for sake of simplicity and also because practical EDFAs are really near to the ideal working. The interested reader is encouraged to start from the model reported in Appendix D and from the related bibliography. In order to make an optimization example, we have also to detail a set of realistic param- eters for a doped fiber. In Table 4.9, microscopic parameters of a doped fiber optimized for in-line amplifiers are reported. Using these parameters, the single-stage amplifier linear gain Go is reported versus the pump power in Figure 4.25 for single copropagating pump- ing at 980 nm. Different fiber lengths are considered. The effect of pump saturation on the linear gain is clear, in this case, if this pumping scheme is chosen, going beyond 6mW of injected pump is practically useless. The situation is better if the pump is divided by two and a double counter-propagating pumping scheme is used: in this case, the result is the family of dashed curves of Figure 4.25. The reason for the improvement is clear: if the pump is splitted and injected in the two directions in the do