Preservation of polarisation in optical-fibre waveguides with elliptical cores

Elliptically cored fibres with large index differences have been drawn and shown to hold polarisation well, the modal beat length being less than 1 mm. An analysis of the propagation characteristics shows that there is a point below the higher-mode cutoff where the difference in group velocities between the two fundamental modes is zero but where there is a large enough difference in the phase velocities to preserve polarisation.