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Single-mode fibre lasers are a new class of active fibre device, which play a remarkable role in optical fibre communications and sensors. They also exhibit a number of new phenomena, one of the most important being polarisation effects. In this thesis, new unexpected fundamental polarisation effects in fibre lasers have been discovered. The phenomena are fluorescence depolarisation, the existence of orthogonal polarisation eigenmodes, and the dependence of the output polarisation on the pump polarisation state. A unified theoretical model, incorporating the electric dipole model for rare-earth ions in a glass matrix, is formulated. The concept of effective pump power is introduced and can be applied to existing laser theory; thus laser performance characteristics such as thresholds, slope efficiencies and the polarisation state of the output can be predicted accurately. The good agreement between theory and experiment enables the derivation of the polarised cross-section ratio. This has been done for Nd3+ and Er3+ ions in silica fibres. From the theoretical analysis the optimum condition for single polarisation operation is deduced. An effective technique for making single-polarisation single-mode (SPSM) fibre lasers has been developed using an integral fibre polariser. The advantages of this technique are low insertion loss, high polarisation extinction ratio and compatibility with existing single-mode fibre components. Laser-diode-pumped SPSM fibres of high performance, both in continuous-wave and Q-switched operations, have been achieved. |
