Monolithic 272W High Efficiency Tm-Doped Nested-Ring Fibre Laser

Thulium fibre lasers represent an important system in applications such as medical treatments, materials processing, remote sensing, non-linear frequency conversion, LIDAR, and defence applications [1]. High-power thulium lasers are mainly based on 790 nm cladding pumping schemes, which allow exploitation of a two-for-one cross-relaxation process with theoretical efficiency up to 80 %. However, power scaling beyond 150 W, in general, requires utilisation of LMA fibres, which suffer from lower efficiencies [2]. The nested-ring fibre was proposed to investigate the power-scaling potential of a novel non-uniform Tm doping profile employed across a small core diameter without pedestal. The confinement of Tm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>+ doping to a thin ring towards the outer edge of the fibre core reduces the spatial overlap between the propagating mode and the doped region. This allows efficient operation at the shorter wavelengths without the onset of parasitic lasing at longer wavelengths. Crucially, this design also reduces pump absorption per unit length, while still maintaining a high Tm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>+ doping concentration within the doped region of the core (4 wt. %), to allow efficient exploitation of the two-for-one cross-relaxation process. This reduces the thermal load density of the fibre, allowing for a greater power extraction before thermally-induced failure [3].