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An effectively nonreciprocal directional coupler has been demonstrated using evanescent coupling between a single-mode and a multimode optical fibre. At least 17 dB difference in coupling efficiency between transmit and receive branches has been measured. The near-field mode patterns show coupling between the single-mode fibre and a few high-order modes of the multimode fibre.
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Whalen et al. (1985) studied this question.
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