Interferometric measurement of group and phase refractive index

An experimental apparatus has been designed to measure group refractive index (n(g)) by observing the shift of the fringe visibility envelope upon insertion of a sample into one arm of a Twyman-Green interferometer. A criterion is developed for the limiting bandwidth and thickness for which good visibility may be expected and for predicting the bandwidth for the narrowest visibility curve. It is demonstrated that the measured group index data can be converted to phase index data with a previously described technique [J. R. Rogers and M. D. Hopler, J. Opt. Soc. Am. A 5, 1595-1600 (1988)] to an accuracy of approximately 0.0006 across the visible spectrum.