Development of Tm-Doped LMA Fibres for High-Power Amplifiers/Lasers Operating in the 2 Um Spectral Range

Large mode area thulium-doped fibres are essential for high-power fibre laser systems operating in the 2 μm range. They exhibit good quantum efficiency (> 1.9) when pumped by inexpensive semiconductor laser diodes operating at a wavelength of 790 nm [1], enabling energy-efficient high-power fibre lasers with output powers up to about 1 kW. Heat load, which easily achieves values of several W/cm is the limiting factor [2]. Further power scaling is achievable through in-band pumping of thulium-doped, eventually holmium-doped fibre lasers/amplifiers. While in-band pumped lasers haven't yet reached their full potential, experiments are promising, with slope efficien-cies exceeding 88 % in 100 W, in-band pumped hybrid thulium-doped fibre laser [3], and 80 % in 35 Wall-fibre holmium-doped fibre laser [4]. For efficient in-band pumping, diode-pumped thulium-doped fibre lasers operating at short wavelengths (1900–1950 nm) are advantageous due to their higher quantum efficiency compared to Raman fiber lasers [5]. The heat load is distributed between the diode-pumped fibre lasers and a highly-efficient high-power in-band pumped laser/amplifier.