Methods for refractive-index homogeneity calculation using Fourier-transform phase-shifting interferometry

Fourier-transform phase-shifting interferometry brings the possibility of a very precise measurement of refractive-index variation in a material. In a four surface cavity six first order interferograms are observed. In the standard way an interference between waves reflected from a front and a rear surface of a sample is used to obtain refractive-index homogeneity. But another three interferograms contain information about inner inhomogeneity and can be used for its calculation as well. In this paper all four calculation methods are presented and compared. The measurements and simulations show, the methods give us globally similar outputs. However due to calculation from different interferograms every result is affected by different undesirable effects. This difference can be used for suppression of the effects and thus to refine the results.