f ASE power propagating along the transmission line. This is the so-called gain flattening filter [56,57]. A typical transfer function of a realistic gain flattening filter is represented in Figure 4.30. This transfer function can be realized by the thin films dielectric technology, a con- solidated technology that is able to shape almost every wideband optical transfer function with a good approximation. Another technology that is often used to manufacture gain flattening filters is fiber Bragg gratings technology. Flattening filters allow very good performances to be achieved in terms of uniformity, as shown in Figure 4.31 where the flattened gain of a commercial single-stage EDFA is shown in all the amplifier useful bandwidth. Such results are achieved at the expense of a relevant reduction of the gain maximum value. As a matter of fact, a filter is a passive device and gain flattening is practically achieved through the introduction of the filter wavelength selective loss. Performing a selective wavelength suppression of the peak gain causes the noise to be more dangerous, simply due to the smaller signal exiting from the amplifier end. Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices FIGURE 4.30 Shape of the transfer function of a gain flattening fil- Wavelength (nm) ter for a single-pump EDFA. Input power -15 dBm -12 dBm -9 dBm -5 dBm Wavelength (nm) FIGURE 4.31 Spectral gain behavior of an EFDA using a gain flattening filter for different values of the signal input power. The parameters of a few real EDFAs that have been optimized for use in communication systems are compared in Table 4.10. The first thing that emerges from the table is the extreme variety of performance an EDFA can exhibit depending on the type of doped fiber, on the pump scheme, and on the inner architecture. This variety demonstrates the flexibility of EDFA technology and somehow explains its great success. All the considered amplifiers are designed to work in deep saturation and in stationary state mod