Fresnel zones for modes

The notion of Fresnel zones for modes is introduced which is analogous to the usual Fresnel zones introduced for rays. It is shown that by using Fresnel zones for modes one can simplify the analysis of mode scattering at large-scale inhomogeneities in multimode waveguides. The mode amplitude variations due to scattering by an opaque screen and by refractive index inhomogeneities are considered. In the latter case the formulas obtained in the scope of this approach are statistical averaging suited and similar to those of geometrical optics and the Rytov method widely used to calculate fluctuations of ray complex amplitudes. The results of a tank experiment carried out to test the predictions concerning the mode scattering by an opaque screen are presented.