e. The first condition is assured by the prescription of a minimum input power: the amplifiers performances are not guaranteed below this power since deep saturation is no more verified. The second condition is assured for all the signals of interest in telecommunications by the long lifetime of the 4/13/2 state (compare Figure 4.21). The amplifier design depends on the application and ranges from a simple single-stage single pump for single-channel amplification to a very complex three stages, two pumps design allowing to accommodate into the amplifier a great interstage loss (13 dB) while retaining a saturated output power as high as 25 dB. This kind of amplifier can be used as in-line amplifier to accommodate both a DCF and an optical add drop. In this last case, the complex multistage design also allows a low noise factor to be main- tained during standard saturated working, a fundamental condition for the use of the amplifier in long-haul and ultra-long-haul transmission systems. Telecommunication Networks TABLE 4.10 Characteristics of Practical EDFAs Optimized for Different Applications From One to Four Channels In-Line In-Line and Application Preamplifier Booster Booster Amplifier Preamplifier Architecture Single stage Single stage Two stages Two stages Three stages Pump scheme Two pump lasers at Copropagating Pump sharing Two pumps Two pump 980 nm in opposite at 980 nm among the at 980 nm lasers at directions stages 980 nm Electronic control AGC-APC-NM AGC-APC AGC, NM interface interface Gain bandwidth (nm) Saturated output power (dBm) Average input saturat. power (dBm) Minimum power in (dBm) Maximum mid-stage loss (dB) Maximum noise figure (dB) Gain flatness (dB) AGC, automatic gain control; APC, automatic power control; MN, separately manageable by network manager. All the described amplifiers are naturally equipped with electronic control that allows the user to drive them via digital signals after the insertion into the transmission system. Specific functions required in high-performance amplifiers ar