Loss and Birefringence for Arbitrarily Bent Optical Fibres

We present a calculation to determine the radiation losses forarbitrarily bent, weakly guiding optical fibres. Our resultsare derived for the full set of vector Maxwell's equations,and show that the rate of energy loss due to radiation differsfor the various modes. These results are different from thatcalculated from a scalar theory, where the so-called linearlypolarized (LP) modes are used to approximate the electromagneticfield distributions, showing that in this case scalar theoriesgive incorrect loss rates. We also show that different rangesfor the size of the curvature and torsion produce quite differenteffects. In particular, we show that, for smaller amounts ofcurvature and torsion, the modes retain the basic structureof the modes in the straight fibre while losing energy slowlyat differing rates, while, for larger amounts of curvature andtorsion, the modes become distorted by the bending.