A method is described for determining unambiguously, from the far-field pattern of single-mode fibres, the core diameter and the refractive-index difference between core and cladding. It involves measurements only of the half-power width of the main lobe and the width of the first minimum. Universal curves are presented that can be used with single-mode fibres or any fibre operating in the single-mode regime. Independent determinations of core diameter over the range 4-8µm by an etching technique are in excellent agreement with those obtained from the far-field pattern.
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