Fabrication of novel geometry fibre lasers for high power applications

Abstract

This thesis details my work on the development of fabrication techniques for high power doped fibre lasers, using novel fibre geometries, and their demonstration. The main methods for increasing output power were using helical cores, multiple cores and large cores. A method for fabricating helical core fibres was proposed and implemented. Core and cladding pumped devices were successfully constructed, and were then tested in collaboration. A cladding pumped helical core fibre laser improved the beam from a 30µm core from an M2 of 3.3 to < 1.4, with a maximum output power of 64W and slope efficiency of 84%. A ribbon fibre with multiple laser emitting cores was fabricated. The difficulties in the fabrication of such a fibre required extensive research, and arise from the large aspect ratio of the preform and fibre, and the low size reduction during fibre drawing. Through many steps a ten core ribbon fibre was fabricated, which was used, in collaboration, to achieve an output laser power of 250W, with a slope efficiency of 65%. Using a spectral beam combination technique, 5 cores were locked together. The output from a number of cores within a circular fibre with no combination is a method for attaining stable high powers with moderate beam quality. A three core fibre was fabricated, and showed an M2 of 5 and slope efficiency of 75%. This fibre was then tapered down from 150µm in order to improve the beam quality further, and for 125µm and 100µm tapers, the M2 values measured were improved to 4.3 and 3.5, respectively. An Yb-doped rod was fabricated, with a large core of 140µm diameter, and machined flats to break the cladding symmetry. The power attained was only 13.4W with a slope efficiency of 20%, due to surface imperfections. A fibre created with altered fabrication techniques showed no surface defects and showed over 90% transmission for 800nm light. Experiments performed by a colleague showed a slope efficiency of 69% and an M2 of 11, indicating that the fabrication method could create an effective cladding pumped rod laser.

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