Abstract
Two- and multiple-beam interference fringes are used to measure the refractive index of graded index fibres. A general expression considering the refraction of the beam by the graded index fibre has been derived, from which expressions for homogeneous and skin-core fibres can be obtained. Comparison between the results with the method used when neglecting refraction is dealt with using Philips, LDF (TM) optical fibres and drawn polypropylene. Considering the derived expressions in the refractive index determination gives a correction even when a matching liquid is used, this suggests that the refraction method should be used especially for graded index fibre.
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