Abstract: A set of orthonormal Laguerre-Gauss functions was used to approximate the field of the fundamental mode of a weakly guiding fiber. The parameters of the expansion were evaluated by a variational method which maximizes the mode propagation constant. It was verified that for fiber with both triangular and parabolic refractive index profile, expansion with few terms only allows to construct an accurate field approximation. The proposed approximation does not require the knowledge of the actual field of the fiber and then avoids to look for a numerical solution of the scalar wave equation. In addition, due to the elementary form of the involved functions, the parameters characterizing the fiber performance can be analytically evaluated in a closed form. !11
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