# Cladding-Pumped Erbium/Ytterbium Co-Doped Fiber Amplifier for C-Band Operation in Optical Networks
## 3 Results and Discussion
The increase in the DWDM channel count to 40 in the case of a \(-\)25 dBm input signal changes the average level of population inversion throughout the EYDF to a value that provides more equal gain in the transmission system frequency band. Therefore, we observe a more uniform gain (amplification) of all 40 DWDM channels with \(\mathrm{p_{IN}}=-\)25 dBm/channel, whereas for \(-\)10 dBm/channel, the population inversion is drained much faster before the similar uniformity is achieved. A similar tendency is observed for the EYDFA's noise figure (Figure 5c). For channels with \(\mathrm{p_{IN}}\geq-\)20 dBm/channel, the noise figure first decreases with every additional DWDM channel until the number of channels (and their combined power level) reaches a certain optimum point, exceeding which the noise figure starts increasing. For fewer power channels, the noise figure first increases by 0.5-1 dB and then starts decreasing, reaching 4-4.5 dB for 32-40 DWDM channels. The insets in Figure 5b,c show that the gain difference and the noise figure increasing dramatically when the number of DWDM channels exceeds 40, which confirms the bandwidth limitations of the amplifier. Signals outside the operation band get absorbed by the EYDF. In \(\mathrm{Er^{3+}}\)/\(\mathrm{Yb^{3+}}\) co-doped fibers, \(\mathrm{Yb^{3+}}\) absorb the pump radiation and then resonantly transfer a portion of their energy to \(\mathrm{Er^{3+}}\) for signal amplification. We perform a sensitivity analysis showing how the absorption and the emission cross-sections of our EYDF impact the amplification process. For this purpose, we assume the cross-sections to be 70% (k = 1) and 130% (k = 1.3) of the initially estimated values (k = 1). Figure 6 shows the differences in the performance characterized using the excited ion percentage (Figure 6a), per channel gain, and noise figure (Figure 6b). We observe that, for k = 0.7, the peak of the excited Yb\({}^{3+}\) percentage becomes smaller (approximately by 5%) and moves further into the EYDF by changing its axial position. Consequently, the depletion of the population inversion is smoothed out on all energy levels, allowing the amplified signal to accumulate a certain part of the pump energy and therefore improving its ability to consume larger portions of the population inversion. This extends the length of the EYDF where the signal amplification occurs effectively, leading to a higher gain (by \(\sim\)0.7 dB) and a lower noise figure (by \(\sim\)0.1 dB). On the contrary, for larger cross-sections (k = 1.3), the Yb\({}^{3+}\) peak moves towards the signal/pump source (axial position = 0.7 m) and gets smoothed out, which brings a lower gain and a higher noise figure. Finally, the BER performance is evaluated for a 40-channel configuration of the DWDM system with and without the EYDFA (see Figure 7). The BER values are obtained for four channels: Ch1 with fc = 191.6 THz (the beginning of the C-band), Ch16 with fc = 193.1 THz (the anchor frequency of the DWDM grid), Ch26 with fc = 194.1 THz (gives the peak gain), and Ch40 with fc = 195.5 THz (the end of the C-band), but Figure 7 shows the largest  
Figure 6: Sensitivity analysis showing the impact that the fiber’s absorption and emission cross-sections have on ion excitation for coefficients k = 0.7, 1, 1.3 of the initially estimated cross-section values: (**a**) excited ion percentage vs. axial position in the doped fiber; (**b**) the EYDFA average gain and the maximum noise figure of the 40-channel DWDM system. BER at a particular value of the received power. To obtain statistically reliable results, we use a \(2^{15}\)-1 uniquely seeded pseudorandom binary sequence (PRBS) to obtain \(2^{13}\) bits used for the simulations and BER estimation that relies on the stochastic signal and noise representation. Specifically, the noise is added to the signal and the probability density function of the detected signal is approximated with the chi-square function.