Diameter determination of graded-index optical fibers from backward-scattered pattern

The backscattered pattern of an optical fiber with arbitrary refractive-index profile is used to determine the fiber diameter. Two methods are presented: the fiber diameter can be determined either by noting the position of a particular fringe maximum or by counting the total number of fringes in the pattern. This method is fast in that it does not require any mechanical moving part, and the measurement error is 1%. This method is based on the prior knowledge of the refractive-index profile of the fiber, but it is shown that the error incurred by assuming the wrong profile is less than 1%. It is also shown in this paper that the method can be used to monitor the diameter variation while the fiber is being pulled. This is done by tracking the displacement of a particular fringe. It is shown that this method is also applicable to fibers with elliptical cross section.