Polarisation-maintaining fibre couplers with misaligned birefringent axes

A recently developed formulation of the coupling between polarisation maintaining fibres is applied to the study of the dependence of the properties of tunable polarisation maintaining couplers on the angular misalignment between the transversal principal axes of the two individual fibres. When the cores are close enough it is shown that while the same coupling ratio can be obtained at several transversal settings of the coupler, quite different behaviour is exhibited by the extinction ratio. In particular, for a transversely tunable coupler made of 3 mm beat-length fibres with a 5° misalignment, two 3 dB settings result in polarisation isolations of 18 dB and 40 dB