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We report results from a new Yb-doped gain fiber with increased higher-order-mode (HOM) loss, compared to conventional step-index fibers. The fiber had 20 µm mode-field diameter (MFD), high absorption, and high transverse-mode instability (TMI) threshold. TMI-free operation with 5 kW output power was demonstrated from a 9 m length of gain fiber, limited by pump power. The large MFD and high absorption allowed for a 7.5 m long amplifier with greater than 80% o-o efficiency and the Raman peak more than 50 dB below the signal. These results were also enabled by a new, small-size, 7+1:1 pump-signal combiner.

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We demonstrate record high energy (1.2 mJ) nanosecond pulses with peak power of >200 kW and average power of 620 W, in a fiber amplifier using a novel 25 µm mode-field diameter Yb-doped gain fiber. The TMI threshold for this fiber was measured to be 1.8 kW. This is achieved at a diffraction limited beam quality of M2=1.08.