Single-frequency Er3+:Yb3+ co-doped fiber amplifiers in MOPA configuration are promising candidates to address the challenging requirements regarding laser sources for the next generation of interferometric gravitational wave detectors (GWDs) at 1.5µm [1]. Erbium’s low absorption properties in the spectral range of commercially available pump diodes can partially be compensated by co-doping the fibers with Yb3+, which absorbs the pump light and transfers the energy to the Er3+ ions. Bottlenecking of the transfer process can induce excessive ASE, which limits the amplifier’s performance. This effect can be significantly suppressed by pumping the amplifier off-resonantly at around 940nm [2]. However, the lower absorption cross section also requires a longer fiber hence lowering the SBS threshold. We present a numerical study for the trade-off between ASE suppression and SBS threshold at different pump wavelengths. The system was modelled as coupled 2- respectively 3-level system, linked by the energy transfer process and described by the corresponding rate-equations [3]. The power propagation along the fiber was modelled by differential equations taking into account the steady-state population densities. The simulation was stopped when either the total Yb3+-ASE or SBS was more than 1\% of the amplified signal power. In contrast to on-peak pumping, wavelengths from 940nm to 960nm enable higher output power levels while maintaining short fiber lengths. If pumped too far away from the Yb3+ absorption band, the overlong fiber length results in a decreased SBS threshold. In addition to that, we performed heterodyne measurements of the SBS spectra in commercially available Er3+:Yb3+ and passive single-mode fiber. We measured a shift of the center frequency in an EYDF with respect to its passive counterpart by 1GHz. We also investigated the influence of pump and seed power levels on the properties (bandwidth etc.) of the Lorentzian SBS gain. References [1] ET Science Team, Einstein gravitational wave Telescope Conceptual Design Study, (2011). [2] D. Creeden, H. Pretorius, J. Limongelli, and S. Setzler, Single frequency 1560nm Er:Yb fiber amplifier with 207W output power and 50.5\% slope efficiency, Proc. SPIE 9728, 97282L-97282L-6 (2016). [3] Q. Han, J. Ning, and Z. Sheng, Numerical Investigation of the ASE and Power Scaling of Cladding-Pumped Er-Yb Codoped Fiber Amplifiers, IEEE Journal of Quantum Electronics 46, 1535-1541 (2010).