Mechanism of spontaneous switching of polarisation in an ytterbium-doped fibre laser

The regime of polarisation switching in a double-clad ytterbium fibre laser is studied experimentally and explained theoretically. It is shown that polarisation switching appears in lasers with the long active medium (no less than 20 m) when the pump slightly exceeds the threshold. The experimentally observed properties of spontaneous polarisation switching in the ytterbium fibre laser are explained by the competition of modes with orthogonal polarisations taking into account the polarisation-dependent saturation of amplification and losses.