# Cladding-Pumped Erbium/Ytterbium Co-Doped Fiber Amplifier for C-Band Operation in Optical Networks
## 1 Introduction
Furthermore, the larger number of cores of MC amplifiers support larger savings. Similar conclusions are found in [5]; the authors have estimated that, by substituting all inline EDFAs in a 2230 km long loop between Paris and Marseille with cladding-pumped 12-core EYDFAs, the costs associated with signal amplification could be reduced by 33%. Moreover, a 55% reduction in the link's total power consumption would be achieved by the year 2035. Such significant cost and energy savings encourage the research in this area. A hero-experiment, demonstrating a 1 Pbps transmission over a 205 km long 32-core multicore fiber (MCF), is reported in [15] where an inline core-pumped 32-core EDFA is used for optical loss compensation. Remarkably, the authors mention that the amplifier's saturation (and not fiber nonlinearities and inter-core crosstalk (IC-XT)) was the main factor preventing the achievement of an even longer transmission. A cladding-pumped co-doped DFA addresses this issue at the expense of amplification bandwidth. A wideband operation of MC-EDFAs, covering the C and the L optical bands, is studied in [16; 17]. In the first case, a 19-core EDFA was used to achieve a 207 Tbpstransmission over a 1500 km long 12-core MCF. A transoceanic distance of more than 8000 km is demonstrated in [17] where the authors used a 31.4 km long recirculation loop, consisting of a 19-core MCF and two cladding-pumped MC-EDFAs (for the C and the L bands separately) for the inline amplification of the dual-polarization quadrature phase-shift keying (DP-QPSK) signals. When it comes to long-haul transmission, amplifier characteristics, e.g., noise figure (NF), gain profile, etc., can significantly decrease the number of amplification spans a signal can traverse before impairments degrade its quality below a certain threshold. Using the transmission link configuration with an MC-EDFA and with a standard EDFA, the authors in [5] investigate the impact that the IC-XT has on the maximum transmission distance (without regeneration) when operating with the 400G dual-polarization 16-ary quadrature amplitude modulation (DP-16 QAM). The investigation relies on the Gaussian noise (GN) model [19] to estimate the signal-to-noise ratio (SNR) after each 80 km long fiber span using the amplifier's NF and the IC-XT component. The results show that, for the considered scenario, the maximum transmission distance decreases from 11 spans to only eight spans when replacing the conventional inline EDFAs (NF = 4.5 dB) with the MC-EDFAs (NF = 4.5 dB). For NF = 6.5 dB, the corresponding numbers are 11 and six spans, respectively. Therefore, the design of MC-EYDFAs should be carefully considered to reduce the impact on the maximum transmission distance while enabling cost and power savings. Yet, the impact that the design specifications of a cladding-pumped EYDFA (e.g., the length of Er\({}^{3+}\)/Yb\({}^{3+}\) co-doped fiber, pump power and propagation directions, and Er\({}^{3+}\)/Yb\({}^{3+}\) emission cross-sections) have on the amplifier's characteristics remain unclear, especially, when it comes to the wavelength dependence. This latter aspect is of importance for the use of such amplifiers to compensate optical losses in networks/links exploiting wavelength division multiplexing (WDM). Therefore, in this article, we investigate the characteristics of an EYDFA under different operating conditions to assess the suitability for operation in a metro-access segment of optical transport networks where the dense wavelength-division multiplexing (DWDM) solutions are normally used. To perform the analysis, we have developed a simulation framework, consisting of a DWDM transmission system with up to 64 \(\times\) 10 Gbps DWDM channels allocated using the fixed 100 GHz grid and a single-core cladding-pumped EYDFA of our design. The amplifier's model is adjusted using the experimentally extracted characteristics (e.g., overlap factor, absorption, and emission cross-sections) of the commercial Er\({}^{3+}\)/Yb\({}^{3+}\) co-doped fiber.