Single-frequency Er3+ doped fiber amplifiers in MOPA configuration are promising candidates to fulfil the challenging requirements of laser sources at 1.5 μm for the third generation of interferometric gravitational wave detectors (GWDs) [1]. An all-fiber design ensures excellent beam quality, -stability and reliability compared to bulk concepts. However, in monolithic systems high power operation gives rise to non-linear effects such as stimulated Brillouin scattering (SBS), which eventually limit the output capacity of such fiber MOPAs [2]. Since the threshold power for SBS scales inversely with the length and cross-section of the fiber, researchers placed much attention to enhance the Er3+ doping levels and therefore decrease necessary fiber length for sufficient signal amplification. However, already at moderately low doping concentrations Er3+ ions tend to form clusters in industry-standard fused silica fibers. Consequently, the short intra-cluster distances between adjacent Er3+ ions introduce quenching effects in terms of homogenous up-conversion processes (I13/2 + I13/2 ➔ I9/2 + I15/2). Thus, high inversion levels can reduce the pump-to-signal energy conversion efficiency and amplifier’s output capacity since ions in the I9/2 state relax non-radiatively.