Novel Technique for Measuring the Index Profile of Optical Fibers

The accurate measurement of index profiles at various wavelengths may help design multimode fibers whose transmission capacity would go well beyond what has been presently achieved. Indeed, numerical calculations and theoretical analyses1-2 show that there exist index profiles (usually not power-law profiles), which, for quasi-monochromatic sources, provide transmission capacities of about 1.6/A2 Mb/s X km, where A = A n/n. Measured transmission capacities are about 10 times smaller. To determine the optimum profiles, it is indispensable to know the variation of dn/d0 (where X0 is the operating wavelength) as a function of n for the class of materials considered with an accuracy of about 1 percent. The required variation of dn/dD as a function of n can be obtained, in principle, from measurements on bulk samples (e.g., Ref. 3). We question, however, whether measurements on bulk samples are applicable to the fiber material with sufficient accuracy. For that reason and also because the fabrication and measurement of bulk samples is time-consuming, the direct measurement of index profiles at various wavelengths is highly desirable. Once the optimum profile applicable to the class of materials considered has been determined, we measure the departures of the profile n(r) of the fabricated 1489