A comprehensive analysis of the various ray propagation mechanisms that take place between the different layers of highly multimode multistep index (MSI) fibers is presented. A suitable analytical expression that allows the evaluation of the power fraction transferred from a certain layer to another one from rays undergoing either refraction or frustrated total internal reflection is provided. This expression is valid for any MSI fiber of any number of layers with a radially decreasing refractive index profile. Frustrated total internal reflection is originated as a consequence of the discontinuous refractive index profile inherent in this type of fibers, and unlike in step-index and graded-index fibers, it can also affect bound rays. Such an expression will allow determination of the remaining ray power along the fiber in each possible propagation state created as a consequence of this phenomenon. The approximations made will be of great benefit for computational purposes since they will lead to a considerable simplification in the numerical prediction of ray propagation in these fibers.
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