Polarisation-dependent coupling in a twin-elliptical-core optical fibre, whose cores possess both an arbitrary core eccentricity and orientation, is investigated. Exact numerical results are presented that determine the degree of form birefringence and permit the accuracy of standard approximation techniques to be evaluated. A comparison of the theoretical x- and y-polarisation coupling coefficients with experimental data suggests that stress-induced birefringence is dominant in practice.
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