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For normal incidence of a Gaussian beam on a multimode optical fibre, theory predicts that only HE1m, modes are excited, and this is confirmed experimentally for a liquid-core fibre of normalised frequency v = 125. Discrete-mode propagation is observed, indicating that the amount of mode conversion due to fibre imperfections is thus very small.
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Gambling et al. (1973) studied this question.
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