656 W Er-doped, Yb-free large-core fiber laser

value excludes a fraction ∼25% of the power that emerged from the cladding, which we attribute in part to mode coupling between the 146 μm core and 700 μm inner cladding. Whereas these parameters are adequate for in-band tandem pumping of Tm-doped fiber lasers, we predict that an output power of over 1 kW is possible through pumping with state-of-the-art 0.98 μm diode lasers, even with a smaller core that allows for improved beam quality.