We present a detailed coupled-mode analysis of superimposed phase grating guided-wave structures for multibeam splitting (MBS) and wavelength demultiplexing (WDM) applications, taking into account the intergrating coupling effects. The case of small angular separation between the output waves and the case of small wavelength
separation between the spectral components of the input wave, which are of great importance for high fanout devices and have so far not been adequately treated, have been examined. For the MBS case (single-wavelength diffraction), critical parameters affecting the diffraction characteristics are found to be the relative index modulation between the gratings, the relative and absolute angular positions of the diffracted waves, the number of gratings, and the wavelength and polarization of the incident wave. For the WDM case (multiwavelength diffraction), wavelength separation between the spectral components of the incoming beam is shown to be an additional critical parameter, due to parasitic waves in undesired output channels. To obtain high-performance devices (high fanout density, high channel efficiencies, high angular and wavelength selectivities, and low cross talks between channels), the analysis suggests the use of long grating structures with large diffraction angles and gratings with close index modulations.