Composite techniques for phase retrieval in the Fresnel region

In free-space propagation of optical beams, the linear contribution of the object-plane phase to the intensity distribution in the Fresnel region is described by the Transport of Intensity equation (TIE). In the near-Fresnel region, i.e. where the Fresnel number is larger than unity, the linear term can often be dominant. Therefore in this region the TIE can provide a good initial approximation which can then be refined by other techniques, e.g. by Gerchberg-Saxton-Fienup (GSF) type algorithms. Multigrid methods with the TIE applied on coarser grids and GSF algorithms applied on finer grids are shown to be particularly effective in this context. (C) 2003 Published by Elsevier Science B.V.