Explanation of polarisation dependence of differential phase shift in two-mode elliptic-core fibre strain gauges

An appropriate elliptic-core fibre theory is applied to model the differential phase shift between the first- and second-order modes observed in recent experiments on two-mode elliptic-core fibres subjected to longitudinal strains. The theory provides an explanation of the polarisation dependence of the stretching delta l_{2 pi} required to produce a 2 pi differential phase shift between the two modes as a function of wavelength for both polarisations.