Mode coupling and power diffusion in multimode optical fibers is considered. In particular, the mechanisms responsible for the coupling are investigated and their characteristic lengthscales explored. An appropriate local coordinate system is introduced in which arbitrary curvatures may be considered. Through an asymptotic analysis, the mode coupling equations are shown to reduce to a nonconstant coefficient diffusive equation for the power coupling. This power coupling is shown to be strongly driven by the degenerate mode structure of the multimode fiber.
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