ped fiber, the pump distribution along the fiber is much more TABLE 4.9 Parameters of an Erbium-Doped Fiber Optimized for In-Line Amplifiers Core Area 13 um ² Er+ concentration 5.4x1024m3 Signal-Er+ overlap integral Pump-Er+ overlap integral Signal emission cross section 5.3x10-25m2 Signal absorption cross section 3.5x10-25m2 Pump absorption cross section 3.2 X 10-25 m² Signal local saturation power 1.3 mW Pump local saturation power 1.6mW Telecommunication Networks FIGURE 4.25 Gain saturation with pump power in EDFA ampli- fiers for different lengths of the active fiber. Total pump power (dBm) Single pump Two pumps FIGURE 4.26 Pump power profile along the active fiber in a sin- gle-pump and double counter-propagating pump schemes. Position along the fiber (m) uniform and the pump local power smaller. This situation is shown in Figure 4.26, where the pump local power is shown for the two pumping schemes versus the position along the doped fiber. Once a pumping scheme is chosen, it is easy to demonstrate that an optimum fiber length exists, as is shown in Figure 4.27, where the linear gain behavior is represented versus the fiber length for single copropagating pump at 980nm. Pump (mW) FIGURE 4.27 Gain saturation with the length of the active fiber in EDFA amplifiers for different values of the pump in a single-pump configuration. The presence of an optimum value of the active fiber length is evident. Doped fiber length (m) Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices In deciding the doped fiber length however, it is to consider the fact that the noise factor also increases while increasing the fiber length, as it is quite intuitive, and this trend is fast while going near the optimum length. This makes the choice of the fiber length a delicate problem, and it is not rare that a shorter length is chosen with respect to the optimum gain value because having a lower noise factor is more important than having the maximum possible linear gain. Fixed the pump wavelength and the dop