Microbending loss in monomode fibres: solution of petermann´s auxiliary function

The spot sizes and auxiliary function that determine the microbending sensitivity of monomode fibres are shown to have analytic solutions for step-index profiles. Although a numerical solution is necessary in the core when the profile is inhomogeneous, the derivation is simplified by matching to the analytic solution within the cladding. Previously, analytic solutions had only been given for Gaussian fields.