Development of power-scalable thulium-doped silica fibre lasers and investigation of their novel applications

Abstract

A range of applications can benefit from the development of high-power thulium (Tm)-doped silica fibre lasers emitting in the two-micron band. These include the materials processing,medical, and defence industries. Current efforts to further scale output power require the development of novel fibre designs and fabrication methods. The use of a non-uniform Tm-doping profile in the fibre core allows a reduced overall pump absorption density to be maintained, while achieving a high efficiency by exploiting the ‘two-for-one’ cross-relaxation process which occurs when optically pumping at 793 nm. This enables a reduced thermal loading density compared with uniform Tm-doping, by spreading the thermal load over a longer fibre length. However, this increases the impact of core propagation loss on efficiency. A novel method of quantitively determining the quantum efficiency and core propagation loss via simple measurements of laser performance has been developed. Characterisation of four different Tm-fibres was conducted,including a novel non-uniform fibre, referred to as the ‘nested-ring’. These studies found that quantum efficiency scales expectedly with Tm-doping concentration. However, the measured core propagation loss was ~0.15 dB/m for all fibres, indicating that high core propagation loss may be a significantly limiting factor in the efficient power-scaling of Tm-doped silica fibre lasers.A 100 W-level all-fibre laser was constructed, achieving an output power of 122 W at an absorbed slope efficiency of 61.6±0.2%. The heat-sinking architecture was fairly simple, with the presence of energy transfer up conversion apparent and thermal issues at splice positions encountered. A 200 W-level all-fibre laser featuring an improved heat-sinking architecture achieved an output power of 273 W at the point of splice failure. This result is the highest output power achieved by a single-mode, pedestal-free, non-LMA Tm-doped fibre. The absorbed slope efficiency was 70.7±0.6%, higher than the predicted performance. The reason for this is currently unclear, however potential explanations are discussed. A wavelength-tuneable Tm-fibre laser achieved a total tuning range of 160 nm and operation above 70 W. The absorbed slope efficiency was 45%, which was likely caused by intra cavity loss in the free-space wavelength-tuning setup.The attenuation of optical radiation by fog droplets in the atmosphere presents an issue for many applications, including the deployment of laser directed energy weapons. The use of a high power Tm-fibre laser to evaporate a low-loss channel through fog was investigated. Theoretical models of the absorption and scattering losses produced surprising results when applied to large diameter droplets. Therefore, an experimental study was completed within the constraints of the laboratory environment. Challenges involving atmospheric thermal blooming were encountered and overcome in this study. There was no measurable increase in visible transmission through fog as a result of a co-linear Tm-fibre laser beam. This may be explained by fog droplets of an unrealistically small diameter, which cause the loss to be strongly dominated by Mie scattering.

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Identifiers

ORCID for Martin Paul Buckthorpe: orcid.org/0000-0002-0846-9685

ORCID for Peter Shardlow: orcid.org/0000-0003-0459-0581

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