# Cladding-Pumped Erbium/Ytterbium Co-Doped Fiber Amplifier for C-Band Operation in Optical Networks
## 1 Introduction  
An immerse variety and accessibility of multimedia services, such as Netflix, YouTube, etc., together with optical fiber penetration in rural areas and growing numbers of end users, have caused a tremendous increase in internet traffic that now must be supported by telecom infrastructure [1, 2]. This puts pressure on infrastructure providers, including optical network carriers. Therefore, they are looking for cost-efficient solutions able to supportthem in the future and even in cases of global emergency. As the COVID-19 pandemic has shown, both telecom infrastructure providers and service providers get entrusted with a role of technological pillars, bonding humanity and supporting our society and business in daily activities such as remote work and distance education. Considering these aspects of growth rates of internet traffic, the versatility of broadband and multimedia services, as well as cost and energy efficiencies, space-division multiplexing (SDM) is considered as one of the biggest breakthroughs in fiber-optic communications able to support the sustainable evolvement of telecom networks and services [3; 4; 5]. To ensure cost-efficient optical signal amplification in SDM-based fiber-optic transmission systems, cladding-pumped doped fiber amplifiers (DFAs) have attracted increased attention. They are considered as one of the most suitable amplification approaches from the perspective of both capital expenditure (CAPEX) and operational expenditure (OPEX) [6]. Such amplification schemes allow for using high-power uncooled multimode laser diodes as pumping sources, which significantly reduces the overall power consumption. Furthermore, the use of only one pumping source improves the cost-efficiency, helps to reduce the size of a multicore amplifier, and thus saves physical space in a rack when migrating to such an amplifier implementation [2]. Although multicore erbium-doped cladding-pumped fiber amplifiers (MC-EDFAs) are commonly used in SDM networks [2; 7; 8], they have an extremely low pump conversion efficiency. This is the main drawback of such amplifiers, which arises due to insufficient absorption of the pumping radiation by the gain medium [9]. Therefore, the increase in the output optical power is problematic. The typical output power values for MC-EDFAs are 14-17 dBm [10; 11]. To overcome these limits, most of the current high-power optical fiber amplifiers use the gain media with erbium (Er\({}^{3+}\)) and ytterbium (Yb\({}^{3+}\)) co-doping. In this case, the amplification happens through two stages. In the first stage, the major part of 975 nm pumping radiation is absorbed by the Yb\({}^{3+}\) that excites them to a state with higher energy. In the second stage, the excited Yb\({}^{3+}\) resonantly transfer a portion of energy to Er\({}^{3+}\) and excites them to a state with higher energy. In such a way, the absorption of the pumping radiation is significantly enhanced [12; 13; 14]. Furthermore, adding Yb\({}^{3+}\) allows an increase in the separation between erbium ions in the gain medium, therefore, Er\({}^{3+}\)/Yb\({}^{3+}\) co-doping minimizes the possibility of clustering among Er\({}^{3+}\), thus allowing an increase in Er\({}^{3+}\) concentration in the gain medium compared to the common implementation of erbium-doped fibers [9]. As the result, shorter lengths of Er\({}^{3+}\)/Yb\({}^{3+}\) co-doped fiber are required to achieve similar levels of amplification. In recent years, cladding-pumped DFA solutions have been studied intensively to reveal the pros and cons that they potentially bring once deployed in SDM networks. In the following section, we briefly cover solutions that tackle the transmission performance limits [5; 15], the wideband operation limits [16; 17], and quantify their impact on network cost and energy efficiency performance [5; 18]. For instance, a 40% power consumption reduction can be achieved by replacing the conventional core-pumped EDFAs with cladding-pumped 32-core EYDFAs [18].