  The uniformity of the intensity and phase of laser beams is crucial to
high-performance atom interferometers. Inhomogeneities in the laser intensity
profile cause contrast reductions and systematic effects in interferometers
operated with atom sources at micro-Kelvin temperatures, and detrimental
diffraction phase shifts in interferometers using large momentum transfer beam
splitters. We report on the implementation of a so-called top-hat laser beam in
a long-interrogation-time cold-atom interferometer to overcome the issue of the
inhomogeneous laser intensity encountered when using Gaussian laser beams. We
characterize the intensity and relative phase profiles of the top-hat beam and
demonstrate its gain in atom-optics efficiency over a Gaussian beam, in
agreement with numerical simulations. We discuss the application of top-hat
beams to improve the performance of different architectures of atom
interferometers.
