Method of determining an index of refraction profile of an optical fiber

The method of determinig an index of refraction profile of the core of an optical fiber having a substantially cylindrical core encased within a surrounding cladding comprising the steps of:

• (a) forming an interferogram with a beam of radiant energy passing transversely through the optical fiber with the interferogram having a pattern of mutually spaced interference fringe lines shifted from a reference line distances determined by differences between indices of refraction radially through the fiber core and that of the fiber cladding;
• (b) measuring the interferogram fringe line shift at a plurality of points along a fringe line corresponding to a series of mutually parallel ray chord paths extending through the core at mutually diverse minimum radial distances from the core axis;
• and
• (c) sequentially calculating from core peripheral region inwardly towards the core center that portion of the measured fringe line shift attributable to discrete differences in indices of refraction of cylindrical core ring portions of thicknesses defined by successive minimum radial distances in the series of ray paths and that of the cladding.