Recently, ytterbium-sensitization was developed for thulium-doped fibers for operation at ~2µm. In these, pump energy at 910-980nm is absorbed by the Yb Here, we present further power-scaling of a cladding-pumped Tm:Yb codoped fiber laser. A Tm:Yb codoped preform was fabricated at the OFTC in Sydney. From this, a double-clad fiber was drawn at the ORC in Southampton. The fiber had a 18.5µm diameter, 0.22 NA aluminosilicate core doped with Tm and Yb in equal concentrations (weight fraction 2%). The pure-silica inner cladding had a 300µm diameter, D-shaped geometry. The inner cladding was coated with a low-index polymer for pump waveguiding. The absorption at the pump wavelength (975nm) was 4 dB/m. A 2.5m long TYDF was used in the experiments. The laser cavity was formed between a perpendicularly cleaved fiber facet at the pump launch end and an external, lens-coupled dichroic mirror in the other end of the cavity. A diode-stack pump source was launched into the TYDF. The fiber laser output was taken from the pump launch end via a dichroic mirror, HR around 2µm. The slope efficiency was 26% and the threshold was 7 W with respect to launched pump power. The maximum output power (75W) was limited by failure of the core at the out-coupling facet. We did not measure the beam quality or the emission spectrum, but we did determine that the laser operated in the 2µm wavelength range. Emission at ~1µm was negligible
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