Estimation of the effective wavenumber for a collimated beam in an interferometer, case study for FG5/X absolute gravimeters

The paper describes an improved method for the estimation of the effective wavenumber in optical interferometers previously published by the authors, which allows reaching accuracy better than one part per billion. The method is based on analyses of spatial intensity profiles of a laser beam and its propagation at different distances, when images are captured by a digital camera. The method has been used for the evaluation of the so-called diffraction correction for FG5 and FG5X absolute gravimeters, which represents one of the most important contributions in the uncertainty budget of these gravimeters. We show that the commonly used estimates tend to systematically underestimate the correction.