  Diffraction of atoms by laser is a very important tool for matter wave
optics. Although this process is well understood, the phase shifts induced by
this diffraction process are not well known. In this paper, we make analytic
calculations of these phase shifts in some simple cases and we use these
results to model the contrast interferometer recently built by the group of D.
Pritchard at MIT. We thus show that the values of the diffraction phases are
large and that they probably contribute to the phase noise observed in this
experiment.
