More than 5 kW counter tandem pumped fiber amplifier with near single-mode beam quality

Abstract

We report on what we believe is the first demonstration of a monolithic counter tandem pumped ytterbium-doped fiber amplifier based on conventional double-clad fiber. At the maximum power of 5.03 kW, the amplifier delivered a near single-mode beam quality (M2 = 1.54), without stimulated Raman scattering effect or mode instability. In addition, we theoretically and experimentally investigated the impact of the 1080 nm signal laser from the amplifier on the 1018 nm fiber lasers used as pump sources. It revealed that even a small proportion of signal light coupled into a 1018 nm fiber laser cavity could cause a considerable power decline of the pump laser. Possible solutions to reduce the influence of signal light on pump sources were extensively discussed. Moreover, we experimentally confirmed the advantage in nonlinear effects mitigation of counter-pumping compared to co-pumping. This work proves the feasibility and superiority of the all-fiberized counter tandem pumping scheme, which could provide inspiration for future power scalability.