Optical fiber index profiling by video analysis of interference fringes

An electronic technique to make fringe displacement measurements on the output field of an interference microscope to 1/1000 of a fringe, not including systematic errors, is described. The method, based upon line-selected video signal analysis, is applied to the evaluation of the refractive index profile of a graded index optical fiber. The index difference at a given point between the core and cladding is determined to better than 1 part in 10(5), implying a precision in the profile parameter alpha of +/-0.005, an order of magnitude better than existing techniques.