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Phase-velocity measurements using prism output coupling for single- and few-mode optical fibers.

A measurement technique based on evanescent coupling between guided fiber modes and radiation modes in a bulk medium is described. Modal phase velocities have been measured to an accuracy of better than 0.01%. Direct measurement of modal beat lengths and cutoff wavelengths are also possible. For a step-index fiber, refractive indices for the core and cladding were determined to an accuracy of 10−4; the core diameter was determined to an accuracy of 1%.