Optical diffusion coefficients, computed from ray-tracing in step-index slab guides with small continuous width perturbations, show an angular dependence which does not in general correspond well with the mechanical deformation spectrum. This contrasts with assumptions of a one-to-one correspondence in earlier reports. The results indicate that, in a fibre having a steep cut-off in the perturbation spectrum at high mechanical frequencies, the corresponding diffusion coefficient will have a significantly less pronounced cut-off. It is also shown that, in a cylindrical guide with diameter perturbations, the effect of skew rays is to further reduce the cut-off rate in the diffusion coefficient.
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