Determination of core diameter and refractive-index difference of single-mode fibres by observation of the far-field pattern

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¿8¿m by an etching technique are in excellent agreement with those obtained from the far-field pattern.