Prediction of the ultimate performance of side-tunnel single-polarisation fibre

As the result of recent progress in polarisation-maintaining optical waveguiding schemes, a basic question now arises: which of stress-induced type or geometrical type approach will finally offer better characteristics? The letter discusses the ultimate performance of a side-tunnel fibre which offers the strongest polarisation-maintaining capability among various geometrical-type ones. The result indicates that, if the structure is optimised, a relative bandwidth of truly single-polarisation characteristics of 0.21 and a normalised birefringence of 0.0045 can be achieved for a relative core-cladding index difference of 0.01. These values are comparable to or higher than the best data reported so far with stress-type ones.