Evaluation of group index in spectrally resolved white light interferometry

Different methods to determine the group index by measuring the material dispersion in spectrally resolved white-light interferometry were analysed. Four methods were investigated: computing the phase derivative directly, using the group index definition to calculate it from the refractive index derivative, locating the equalisation wavelength, and locally evaluating the fringe periodicity. For each method, different paths or variants were investigated to evaluate their efficacy and determine their advantages and drawbacks. We considered the measurement range of a typical CCD camera, from 400 to 1000 nm, and tested every method with two different samples: a 1-mm thick BK7 plate and an optical cell filled with deionised water; obtaining similar results in both cases. Only methods requiring the acquisition of a few frames were considered.