The theoretical characteristics of the scattered-light pattern from a clad glass fiber illuminated by a laser beam perpendicular to its axis agree closely with results of experimental measurements of the scattered light. A simplified geometric ray-tracing technique shows that for fibers with medium and small core sizes, specific angle ranges of the scattering pattern provide determinations of fiber diameter independent of core parameters. Measurements of the fringe modulation give relatively sensitive determinations of core diameter. Light scattering is a useful technique for determining the diameters of both clad and unclad fiber. Total-diameter determinations have accuracies of ±0.2 µm and core diameters, ±0.5 µm for 0.02 refractive-index difference between core and cladding.
L. S. Watkins, "Scattering from side-illuminated clad glass fibers for determination of fiber parameters," J. Opt. Soc. Am. 64, 767-772 (1974)