Rapid Automatic Index Profiling of Whole-Fiber Samples: Part 1

Multimode optical fiber systems can achieve high bandwidth if the refractive index profile of the fiber core approximates very closely the theoretically predicted almost-parabolic shape. This poses the problem for the fiber fabricator of being able to produce preselected index profiles with high accuracy and also of being able to check whether the desired profile has actually been obtained. It is thus necessary to have measuring equipment capable of displaying the index profiles of fibers after manufacture quickly, accurately, and, if possible, nondestructively. Means for measuring refractive index profiles by accurate interferometric techniques have previously been described.1"3 However, the 867 most successful earlier technique, the slab method, requires elaborate sample preparation because a thin slice has to be cut out of the fiber and polished to a high degree of flatness and parallelism. If the interferogram could be made by shining light not longitudinally but transversely through the fiber core, fiber preparation would become unnecessary, except for the need for preventing the fiber cladding from contributing a very large amount of fringe shift. This can be accomplished by submerging the fiber in index-matching oil. This general idea has been described in Ref. 4. A similar method has been used in Ref. 5, but the analysis used to extract the refractive index profile from the interferogram was restricted to power law profiles that can be characterized by two constants, the refractive index difference (between the maximum value at the core center and the cladding value) and the power law coefficient.