The diffraction of electromagnetic waves by periodic structures continues to be of great interest and practical importance owing to numerous applications in a variety of fields such as acousto-optics, electro­optics, integrated optics, spectroscopy, optical computing, optical interconnects, and quantum electronics. Optical gratings may be planar (slab, volume gratings) or surface-relief (corrugated gratings). The periodic modulation may be in the permittivity (resulting in index of refraction gratings), or in conductivity (absorption) or in a combination of these. Diffraction gratings can be fabricated in dielectrics, semiconductors, metals, or even plasmas. They can be characterized as isotropic, uniaxial-, biaxial-anisotropic or gyrotropic depending on the properties of their materials. They can be of constant or of varying modulation and can be cascaded or multiplexed. The grating diffraction methods of analysis can be divided into two major categories, the integral methods,1 and the differential methods.1–7 The most common and accurate differential methods are the coupled-wave approaches2–5 and the modal approaches.6,7