Analysis of leaky modes supported by a slab waveguide

Abstract

AbstractThe electromagnetic waves propagating along an open waveguide can be classified in several ways. For instance: (1) guided modes and leaky modes; (2) real modes and complex modes; and (3) proper modes and improper modes. However, their mutual relationships are not clear. In addition, the physical meaning of the leaky mode is not well understood in comparison with the guided mode.In this paper, the relationship of the modes propagating in a dielectric slab waveguide is investigated. First, from the dispersion equation, the guided mode cutoff frequency and the leaky mode cutoff frequency are obtained so that the regions of the guided mode and the leaky mode are found. It is shown also that there exists an improper mode region with real propagation constants between the guide and leaky mode regions.This improper mode does not decay in the direction of propagation and the amplitude increases away from the waveguide along a straight line with an arbitrary angle from the propagation direction. In this region, the group velocity may exceed the speed of light in free space or become infinite. Thus, in this paper, this region is called the anomalous group velocity region. The width of this region is inversely proportional to the mode number and the waveguide width.