# Cladding-Pumped Erbium/Ytterbium Co-Doped Fiber Amplifier for C-Band Operation in Optical Networks
## 3 Results and Discussion
Figure 3: The amplifier’s (**a**) gain, (**b**) noise figure, and (**c**) maximum gain difference between the channels as a function of the EYDF length. Lines connecting data points are only included as a guide. The characteristics are obtained considering a 40-channel dense wavelength-division multiplexed (DWDM) system with an input optical power (\(\text{p}_{\text{IN}}\)) of \(-\)20 dBm/channel and a 3 W pump source at \(\Delta\)p = 975 nm coupled in the co-propagating (blue) and counter-propagating (green) directions. Next, we explore the EYDFA characteristics (namely gain, maximum gain difference, and noise figure) under different operating conditions by varying the number of DWDM channels and their optical power levels. During the analysis, we consider a DWDM configuration with 1, 2, 4, 8, 16, 32, 40, and 64 channels, whose power levels are set between \(-25\) and \(-10\) dBm/channel, see Figure 5. The output power curves (Figure 5a) show that the higher the number of DWDM channels, the smaller the output power difference. The amplifier saturates and eventually it fails to amplify more than 40 DWDM channels even if their power is as low as \(-25\) dBm/channel. When analyzing the output power curve for \(\text{p}_{\text{IN}}=-25\) dBm/channel, we have noticed that the output power levels increase by 3.5 dB when the number of DWDM channels is increased from two to four. However, the corresponding number is 4.3 dB when the number of DWDM channels is increased from 16 to 32 channels. Such behavior indicates that low-power optical signals (i.e., with low input power and/or a small number of channels) are not able to completely exploit the population inversion generated in the gain media. Consequently, the unused portion of the population inversion eventually generates an excessive amount of amplified spontaneous emission (ASE) noise, which results in a poor noise figure, see Figure 5c. On the contrary, high input power consumes the achieved population inversion, in such a way that the obtained gain is reduced and the output power reaches its limit. For instance, the output power increases by 0.3 dB when an additional eight channels (\(\text{p}_{\text{IN}}=-25\) dBm/channel) are added to a DWDM system with 32 channels, and it remains similar even when the number of channels is increased to 64. A higher pump signal power also does not result in a higher gain or a higher output power. Even if a 4 W pump signal is used, the output power increases by not more than 0.2-0.3 dB for a 40-channel configuration with \(\text{p}_{\text{IN}}=-20\) dBm/channel. Therefore, the maximum output power of the proposed EYDFA is limited to approximately 22 dBm. The maximum gain difference curves (Figure 5b) show the following trend--the higher the input power, the smaller the gain difference in a DWDM system with 4-32 channels. The main reason is that a higher portion of population inversion is consumed to achieve similar levels of amplification for higher input power signals. Therefore, at a certain level of population inversion, optical signals with higher power get less amplified and the gain difference between the channels becomes smaller. However, the opposite situation is observed for 40 DWDM channels, where the gain difference becomes higher for higher input signal powers (e.g., compare \(-25\) dB/channel and \(-10\) dBm/channel curves). Such behavior  
Figure 5: (**a**) Output power, (**b**) maximum gain difference, and (**c**) maximum noise figures vs. the number of DWDM channels and their input power for the EYDFA configuration with a 7 m long EYDF and \(\text{p}_{\text{PUMP}}=3\) W at \(\lambda_{\text{p}}=975\) nm coupled in the co-propagation direction. Lines connecting data points are only included as a guide. occurs because higher power signals drain the ion population inversion more efficiently.