A new method is presented to determine the refractive index and the diameter of unclad optical fibers. The technique is based on an analysis of the back-scattered light when a beam from a cw laser impinges upon the fiber. A geometrical-optics analysis shows that the position of a sharp cutoff in the radiation pattern determines the refractive index, whereas the distance between certain successive minima gives the diameter. The theory is compared to experimental observations, with excellent agreement.
H. M. Presby, "Refractive index and diameter measurements of unclad optical fibers," J. Opt. Soc. Am. 64, 280-284 (1974)