# Cladding-Pumped Erbium/Ytterbium Co-Doped Fiber Amplifier for C-Band Operation in Optical Networks
## 2 Experimental Setup and Principles  
The simulation setup used to characterize the performance of the developed cladding-pumped EYDFA is shown in Figure 1. It is realized using VPIphotonics Design Suite [20], yet the absorption and emission cross-sections of the Er\({}^{3+}\)/Yb\({}^{3+}\) co-doped phosphosilicate glass double-cladding fiber are experimentally measured in our laboratory and fed as the input data to the simulation setup (Figure 1b). Besides, the overlap factor is estimated using the proposed red, green, blue (RGB) color approach applied to the EYDF cross-section images taken by a fiber microscope (Figure 2). As we assume that EYDFA will be operated in metro-access segments of optical transport networks, the multi-wavelength operation is considered. Consequently, the setup includes three parts: (i) _n_\(\times\) 10 Gbps OOK WDM transmitters; (ii) the realistic model of our EYDFA, consisting of the EYDF itself, optical pump source (central wavelength \(\lambda_{\text{p}}\) = 975 nm at 25 \({}^{\circ}\)C and output power 3-5 W), high-power optical combiners/splitters, and the amplifier's test unit for the evaluation of its characteristics (e.g., gain spectrum and noise figure (NF)); and (iii) WDM (de-)multiplexers and receivers for signal quality estimation (not shown in Figure 1). The key component of this optical back to back (OB2B) setup is a fiber model. For our purposes, we use a stationary fiber model from VPIphotonics Design Suite [20] that can be used for Er\({}^{3+}\)/Yb\({}^{3+}\) co-doped cladding-pumped fiber amplifiers. According to its description, this model is based on the bidirectional propagation equations for signals and multilevel rate equations for ion populations. To tune this model with the respect to our EYDF, we use the measured cross-sections to specify the emission and absorption spectra, and the overlap factors to specify the WDM signal (\(\sim\)1550 nm) and the pump signal (\(\sim\)975 nm) coupling and their propagation (which depends on fiber profile and dimensions). The model is resolved in both the longitudinal and transverse directions considering the number of effects, e.g., Er\({}^{3+}\)/Yb\({}^{3+}\) energy transfer, cross-relaxation effects,  
Figure 1: (**a**) Simplified simulation setup of the cladding-pumped Er\({}^{3+}\)/Yb\({}^{3+}\) co-doped fiber amplifier (EYDFA); (**b**) absorption and emission cross-sections experimentally determined for the Yb\({}^{3+}\)/Er\({}^{3+}\) co-doped fiber using the absorption spectra measurement method. Figure 2: (**a**) Microscope image of the EYDF cross-section together with its geometrical measures, and (**b**) RGB representation of its outer cladding (red), inner cladding (green), and core (blue) used for the overlap factor estimation. excited-state absorption, Rayleigh scattering, and Kerr nonlinearity. The summary of the setup parameters is given in Table 1.  
The simulation model includes parameters that specify both the WDM system and the EYDFA under test. In this case, we operate with a 10 Gbps NRZ-COK signal whose central frequencies are arranged across the C-band (1530-1565 nm) using a 100 GHz grid. Although we consider the WDM configuration with the total number of channels up to n = 64, channels 41-64 are outside of the C-band (\(\ell_{\text{c}}>\) 195.6 THz). They are used to highlight the wavelength dependence of the amplifier's gain and noise figure characteristics, especially for high (\(\textgreater-10\) dBm/channel) and low (\(\textgreater-25\) dBm/channel) input signal powers. The category \"EYDFA pump parameters\" provides details about the optical pump source and its direction with respect to the signal propagation. Finally, the category \"doped fiber parameters\" includes the measured, estimated, and given characteristics of our EYDF used to build the EYDFA in the laboratory.