Abstract: A set of orthonormal Laguerre-Gauss functions was usedto approximate the field of the fundamental mode of aweakly guiding fiber. The parameters of the expansionwere evaluated by a variational method which maximizesthe mode propagation constant. It was verified that forfiber with both triangular and parabolic refractiveindex profile, expansion with few terms only allows toconstruct an accurate field approximation. The proposedapproximation does not require the knowledge of theactual field of the fiber and then avoids to look for anumerical solution of the scalar wave equation. Inaddition, due to the elementary form of the involvedfunctions, the parameters characterizing the fiberperformance can be analytically evaluated in a closedform. !11
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