The theory that studies the general law of coupling between two or more modes of electromagnetic waves is also called coupled wave theory. Broadly speaking, it is a general theory to study the coupling between two or more waves.
Coupling will occur between electromagnetic waves of different modes in the same waveguide (or cavity) or between electromagnetic waves of different modes in different waveguides (or cavities). Usually, coupling occurs between the same wave, but it may also occur between different types of waves, such as the coupling between two modes of electromagnetic waves and two modes of space charge in a traveling wave tube.
Extended data:
Different forms of coupled-mode equation In order to derive the coupled-mode equation, it is necessary to expand the field in Maxwell's equation according to the orthogonal function set, and different coupled-mode equations can be obtained by using different orthogonal function sets.
For example, the orthogonal function set in a waveguide corresponds to all modes of its electromagnetic wave (for an open waveguide, radiation modes should also be included). Any independent transmission along the waveguide without coupling is called normal mode, and the coupled mode is non-normal mode. The electromagnetic wave in non-uniform waveguide can be expanded according to the normal mode set in the reference waveguide, and different reference waveguides are selected to correspond to different normal mode sets to obtain different coupled mode equations.
Taking the waveguide with variable cross-section as an example (Figure 2), three groups of normal modes corresponding to three reference waveguides at different cross-section positions are represented by dashed lines: ideal mode, intrinsic mode and super-intrinsic mode.
The reference waveguide corresponding to the ideal mode is a uniform waveguide, and its cross-sectional shape and size are consistent with the actual waveguide input; The reference waveguide corresponding to the local mode is a uniform waveguide, and its cross-sectional shape and size are consistent with the actual waveguide at the observation point.
The reference waveguide corresponding to the ultra-local mode is a trumpet waveguide, whose shape is consistent with the actual waveguide of the observation point, and its longitudinal section boundary line is tangent. The latter two modes change with the location of observation points, and their mode characteristics are mainly determined by "local" characteristics.
References:
Baidu Encyclopedia-Coupling Mode Theory