A new method of obtaining the refractive-index profile of an optical fiber preform is described. The technique is based on the refraction of light emerging from an endface cut normal to the preform axis. Two forms have been developed. In one, quantitative index data are obtained by a spatial-filtering method. The result is displayed as a 3-D graphic plot. The other uses a new type of spatial modulation which permits a direct display of the 3-D index profile in the form of a visual image
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