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Optical fibre photorefractive intermodal couplers written with visible light can be phase-matched in the infra-red, using the dispersion of the intermodal beat length. Phase matching at 720 nm of an intermodal coupler written at 488 nm is demonstrated. The observation of mode coupling at many closely spaced wavelengths is explained by the nondegeneracy of the higher-order modes.
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François Ouellette (1989) studied this question.
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