Fiber-diameter measurement by occlusion of a gaussian beam

The amount of light occluded by a fiber as it passes through alaser beam can be used as the basis for fiber-diametermeasurement. This technique is analyzed with a two-dimensionalrigorous model. The occlusion seen for dielectric fibers as afunction of their diameter is highly oscillatory owing to interferencebetween the light transmitted by the fiber and the rest of thediffracted field. Scalar diffraction theory is shown to be adequatein modeling this effect. The oscillation sets a limit to theaccuracy of simple diameter measurement systems and is confirmedexperimentally for glass fibers. However, wool fibers are found tobe better treated as an absorbing material. The effect of beampolarization is investigated and found to be negligible for dielectricfibers but significant for metal fibers of small diameter.