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.
We report new Yb-doped gain fibers with approximately 20-μm core diameter and 400-μm cladding. These fibers have mode-field diameter greater than 19 μm, and increased higher-order-mode loss compared to conventional 19-μm modefield diameter fibers. The increased higher-order mode loss allows for high transverse-mode instability thresholds in fibers that also have large MFD and high absorption. A 21-μm fiber with 6.5 m operating length, was free from transverse-modal instabilities at 3.6 kW signal power and had 81% optical-optical efficiency. In narrow-linewidth amplifier experiments, the next-generation fiber with 21 μm fiber achieved 2.74 kW output power at 6 GHz linewidth, limited by pump power. This represents almost 2x increase in the ratio of power to signal linewidth compared to existing commercially available 19.5 μm MFD fibers, achieving 510 kW/GHz.
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