Curvature of a single-mode optical fibre gives rise to two principal forms of additional transmission loss, namely transition loss and pure bend loss. The transition loss and the associated ray radiation, which have been observed at the beginning of a bend, can be satisfactorily explained by a modified coupled mode theory. The radiation modes are represented by a quasi-guided mode having an average propagation constant beta(e). The introduction of a gradual change of curvature reduces the transition loss much more than the pure bend loss. Analysis of the microbending loss shows that the transition component is a maximum at a given correlation length which can be simply expressed in terms of beta(e). The contributions of both transition and bend components to the total microbend loss have been derived for the case of a randomly-curved fibre for several autocorrelation and density functions.
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