**6. Gain flatness of EDFA for a WDM FSO system**

An amplifier does not have a flat gain curve, that is, it does not generally provide equal amplification for all wavelengths of signals transmitted over the same


**Table 2.**

*Comparison of EYDWA post- and pre-amplifier.*

**Figure 12.** *Quality factor as a function of FSO range for EYDWA post- amplifier under clear (light), medium and heavy rain.*

**Figure 13.** *Quality factor as a function of FSO range for EYDWA preamplifier under clear (light), heavy and medium rain.*

transmission line (WDM). This disparity is an important limitation for wavelength division multiplexed systems. Good gain flatness requires continuous control of the input power to adjust it to its optimum value [15]. There are several methods for the design of a flat spectral EDFA gain e.g. by combining in-line amplifiers with gain equalizing filters or by controlling some internal EDFA parameters such as the length of the doped fiber, the concentration and the pump power [16].
