Novel power scaling architectures for fibre and solid-state sources

Abstract

This thesis explores approaches for scaling the output power of rare-earth ion doped fibre lasers and amplifiers, fibre amplified spontaneous emission sources, and solid state laser oscillators. Scaling output power from laser sources has been a topic of interest ever since the first laser was demonstrated. The development of new geometries and novel techniques for reducing effects that limit the maximum output power are particularly important. Three approaches for power scaling are demonstrated here. The first is an all fibre geometry for producing predominately single-ended operation. By exploiting the high available gain in rare-earth-ion doped fibres, predominately single-ended laser output can be achieved in a high loss cavity with feedback at one end considerably lower than the other. This was demonstrated with an Yb doped fibre laser using a low loss end termination scheme to produce 29W and 2W in the forward and backward directions, respectively, for launched pump power of 48W. This corresponds to a slope efficiency for the forward direction of 77%. The single-ended scheme was also applied to a Tm-doped fibre ASE system, producing a maximum output of 11W for 43W of launched pump, with an emission bandwidth of 36nm centred at 1958nm. Secondly, a Tm-doped fibre distributed feedback laser with 875mW of single frequency output at 1943nm was used in a master oscillator power amplifier configuration. Using three amplifier stages, the output was scaled to 100W of output with a final polarisation extinction ratio of >94% and a beam propagation factor of M2 <1.25. The last laser architecture was a cryogenically cooled Ho:YAG laser in-band pumped by a diode pumped Tm-doped fibre laser. After determining the absorption bandwidth as a function of temperature at the desired pump wavelength of 1932nm in Ho:YAG, the fibre laser was constructed to have an emission line-width of <0.2nm to achieve efficient overlap with the absorption peak. This fibre laser was used to pump two different Ho:YAG laser configurations. The first was a free-running laser based on a simple two-mirror cavity design, which showed a factor of 1.7 increase in the laser slope efficiency and a factor of 10 decrease in threshold pump power when the crystal temperature was reduced from 300K to 77K. The second cavity condition discussed was for low quantum defect operation, which was demonstrated at 1970nm corresponding to a quantum defect of just 2%. Lastly, further power scaling and other applications for all three approaches are discussed

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