407 W specially-designed fiber laser at 1018 nm using a gain fiber with a low core/cladding ratio of 20/400 μ m

In this paper, two monolithic 1018 nm Yb-doped fiber laser systems—one as an oscillator (OSC) and another as a specially-designed amplifier (SDA)—are investigated using 20/400 μ m Yb-doped fibers (YDF). 394 W of output power and a slope efficiency of 39.4% was obtained from the OSC setup. The SDA setup resulted in 407 W of output power with a slope efficiency of 39.9%, which is the highest reported output power via the 20/400 μ m YDF. The beam quality factor M 2 is also measured to be ~1.5 for both setups. Considerations regarding the gain fiber length were taken into account so as to suppress any detrimental effects such as ASE and self-pulsing. The output power behavior of both setups indicate that further power scaling would be possible.