An investigation into thulium and ytterbium doped glass fibre lasers
The work described in this thesis is the result of three years of study in thulium and ytterbium doped glass optical fibres with a view to investigating and characterising laser action in these media. In particular an investigation has been carried out into the effect that pump excited state absorption (ESA) has on the 1064nm-pumped Thulium-doped silica 2µm fibre laser, resulting in the first measurement, in terms of cross section, of the ESA from the 3H4 level in the 1-1.5µm region for Tm:silica. Laser oscillation has been observed at ~1.9µm in Tm-doped lead germanate fibre pumped at ~790nm. This new glass fibre was specially developed to have an appropriate phonon energy to enhance the lasing properties of this thulium transition over that in silica or fluoride hosts. It is the first glass fibre laser based on a germanate glass. The lowest ever recorded threshold for this transition in any glass has been recorded at 3.6mW of launched power. In order to investigate infra-red pumped visible upconversion lasers in Tm:ZBLAN fibre, ytterbium doped silica fibre lasers were first studied as a route to producing the required pump wavelengths. By using a novel pumping scheme in the wing of the absorption band at 1064nm or 1047nm, efficient laser action achieved in the long wavelength range of the emission, between 1096nm and 1180nm. The lowest recorded threshold was 17mW and the highest measured slope efficiency was 87% (both with respect to launched power). These investigations confirm the versatility of Yb:silica fibre lasers. With the successful development of the Yb:silica fibre laser operating at the required pump wavelength, an investigation was carried out into single-wavelength-pumped, room temperature upconversion in thulium-doped ZBLAN with the aim of producing a blue laser. Blue lasing was observed in the range 476nm - 486nm (the longest and shortest laser wavelengths recorded for this transition) with pump wavelengths between 1100nm and 1141nm (the longest and shortest pump wavelengths reported). The lowest threshold was measured as 11mW (launched power)and the maximum slope efficiency was 23% (with respect to launched power), results which indicate potential for an infra-red diode pumped blue laser.