The phase ambiguity in dispersion measurements by white light spectral interferometry

•We present a self-consistent method to determine refractive index dispersion.•Refractive indices are fitted to a polynomial function and errors are estimated.•High precision refractive indices (errors less than 0.01%) are obtained. In this work, we address the phase ambiguity in white light spectral interferometry. This ambiguity prevents one from obtaining the refractive index over a broad spectral range with high accuracy. We first determine the error when the refractive index is fitted to a linear combination of power functions. We demonstrate that the error is proportional to wavelength and independent of sample thickness. We show how to reduce the error over the entire spectral band by measuring the spectral phase at the output of the interferometer for some suitable wavelengths as a function of sample orientation.