Leakage loss analytical formulas for large-core low-refractive-index-contrast Bragg fibers

Abstract

We propose analytical formulas for large-core low-contrast Bragg fibers that predict the leakage loss of their low-order core modes, whatever their polarizations are and whatever the number of rings in the cladding is, for the first time to our knowledge. We propose also a generalized analytical formulas, which encompasses the case of Bragg fibers with a nonperiodic cladding, by decomposing their cladding into its constitutive periodic elements, whose contributions are taken into account through a simple multiplication factor. These formulas are developed thanks to a perturbation approach and to the use of the asymptotic formulation of the field in the cladding and provide a good accuracy except in the vicinity of couplings between core modes and cladding modes.