# Cladding-Pumped Erbium/Ytterbium Co-Doped Fiber Amplifier for C-Band Operation in Optical Networks
## 3 Results and Discussion  
In this section, we reveal how the EYDFA's configuration parameters, such as the Er\({}^{3+}\)/Yb\({}^{3+}\) co-doped fiber's length, its absorption and emission cross-sections, and the pump signal direction, impact its wavelength-dependent characteristics, namely, gain uniformity, noise figure, and maximum output power. Before the evaluation of an EYDFA-induced power penalty, we characterize its performance for a multiwavelength scenario by varying the number of DWDM channels and their power levels. The goal of these simulations is to find the most appropriate amplifier configuration that introduces the least distortion while ensuring the most uniform gain spectrum possible. Throughout the analysis, we use an optical pump source operating at \(\lambda\)p = 975 nm and 3 W of output power. These values are selected based on the specifications of our high-power light source in the laboratory. We consider both the co-propagation and counter-propagation directions for the pump signal. To select the length of the EYDF and the direction of the pump signal, we use the curves obtained for a 40-channel WDM system showing how the amplifier gain, noise figure, and the maximum output power change with the EYDF length (see Figure 3). The results show that the maximum gain is reached for an 8 m long EYDF regardless of the pump direction (Figure 3a). A longer EYDF does not result in a higher gain, which is explained by the depletion of the pump radiation. A further increase in doped fiber length not only cannot produce additional gain, but the amplified signal power also starts to decrease due to the attenuation of the EYDF itself. Furthermore, the amplifier becomes noisier, especially for the configuration where the signal and the pump are launched in the counter-propagating directions (Figure 3b). Otherwise, the noise figure is not larger than 4.5 dB (co-propagation) and 6 dB (counter-propagation). Finally, Figure 3c shows the maximum gain difference detected for Channels 1 to 40 (Ch1-Ch40) in the 40-channel DWDM system with the input optical power of \(-20\) dBm/channel. The gain uniformity is an important characteristic, especially for systems with several amplification spans. Unless all DWDM channels are amplified equally, the power difference increases with every span, limiting the maximum transmission distance. The smallest gain difference (\(\Delta\)G \(<\) 9 dB) is obtained for a 7 m long EYDF for both the co-propagation and counter-propagation of the 975 nm pump signal. It is significantly larger for shorter and longer EYDF segments, which indicates that an appropriate level of ion population inversion is achieved for this specific combination of the pump power (3 W) and the EYDF length (7 m). Therefore, we keep these parameters unchanged. Finally, we choose to use the pump signal in the co-propagation direction. Although the counter-propagation ensures a 0.9-1 dB higher gain, its cost is a higher noise figure, which is almost 1 dB higher compared to the co-propagation case. Therefore, we choose a lower noise figure over a higher gain. Figure 4a shows the output (parametrized) spectrum, whereas Figure 4b shows the individual gain and noise figure of each EYDFA-amplified DWDM channel. Due to the wavelength-dependent gain and noise figure of the amplifier, the output spectrum is not uniform. Specifically, the amplifier's output power levels change from 0.1 to 8.3 dBm per channel (dBm/channel, see Figure 4a), resulting in a gain difference of 19.7-28.3 dB, and the noise figure changes from 3.7 dB to 4.2 dB (Figure 4b). The input optical power (\(\text{p}_{\text{IN}}\)) was set to \(-\)20 dBm/channel in all 40 WDM channels considered. Figure 4: (**a**) Optical spectrum at the output of the EYDFA (7 m EYDF, 3 W, 975 nm, co-propagation), (**b**) individual gain and noise figure of each amplified DWDM channel with \(\text{p}_{\text{IN}}=-\)20 dBm/channel.