The mode coupling among guided modes in a photorefractive crystal waveguide is investigated. We derive the coupled equations of guided modes in a photorefractive crystal waveguide. Analytical solutions of the coupled equations are obtained for a two-mode photorefractive crystal waveguide. We also numerically analyze the coupled equations of guided modes in a multimode photorefractive crystal fiber. As a result of the coupling, power may flow to the fundamental mode or the highest-order mode,

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... ending on the direction of the crystal optical axis. In addition, we report an analytical solution of the modal power in core and cladding for all guided HE m and EH m modes for cylindrical fibers made of uniaxial crystals.