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Dispersion in graded-index optical fibres can be calculated by ray-optical or wave-optical methods. This letter reports a seeming discrepancy between the results of both methods: in a fibre with a refractive profile that has been optimised by the WKB wave-optical method, helical rays may occur with a much larger transit-time spread than the modes computed with the WKB method. The question is what transit-time spread is relevant for the bandwidth of the fibre.
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Siegfried Geckeler (1975) studied this question.
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