Positioning a focused Gaussian beam inside a refractive cylinder

We present in this work a study of the diffraction of a focused Gaussian beam by a microrefractive cylinder, and the dependence of the far field diffracted pattern with the location of the beam focal plane, relative to the center of the cylinder. A numerical study of the problem is carried out and validated with experimental verification. We center our attention on two parameters: the divergence and the number of peaks of the diffracted field. Both present a minimum for a particular position of the focal plane inside the refractive cylinder. This analysis can be used as a novel control technique for ablation of optical fibers and it can also be used as a criterion for the proper location of a focused laser beam inside an optical fiber with high accuracy. This criterion, based on the distribution of the far-field diffracted pattern, can be automated or used as a visual clue for a human operator.