Microbending loss in optical waveguides

The problem of coupling from a bound mode to the radiation field in an open waveguide subject to slow variations of its axis is considered. The derivation does not depend on statistical averaging over an ensemble of waveguides, as do earlier treatments of the same problem. Nevertheless, it is shown that an average loss can be defined, the average being taken over the length of the waveguide. For this reason, the results are applicable to the realistic experimental situation, when experiments are performed on single waveguides. As an example, we calculate the microbending loss of a single-mode step-index optical fibre as a function of wavelength and compare with some recent experimental measurements of the same quantity. Good agreement is shown when the Fourier transform of the curvature autocorrelation function is assumed to have an inverse power dependence on the Fourier frequency, but the agreement is less convincing for some other possible choices of autocorrelation function.