Waveguide
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In physics, optics, and telecommunication, a waveguide is an inhomogeneous (structured) material medium that confines and guides a propagating electromagnetic wave.
In the microwave region of the electromagnetic spectrum, a waveguide normally consists of a hollow metallic conductor, usually rectangular, elliptical, or circular in cross section. This type of waveguide may, under certain conditions, contain a solid or gaseous dielectric material.
In the optical region, a waveguide used as a long transmission line consists of a solid dielectric filament (optical fiber), usually circular in cross section. In integrated optical circuits an optical waveguide may consist of a thin dielectric film.
In the radio frequency region, ionized layers of the stratosphere and refractive surfaces of the troposphere may also act as an atmospheric waveguide.
In digital computing, the term waveguide can also be used for data buffers used as delay lines that simulate physical waveguide behavior, such as in digital waveguide synthesis.
Waveguide propagation modes depend on the operating wavelength and polarization and the shape and size of the guide. In hollow metallic waveguides, the fundamental modes are the transverse electric TE1,0 mode for rectangular and TE1,1 for circular waveguides, seen here in cross-section:
A dielectric waveguide is a waveguide that consists of a dielectric material surrounded by another dielectric material, such as air, glass, or plastic, with a lower refractive index. An example of a dielectric waveguide is an optical fiber. Paradoxically, a metallic waveguide filled with a dielectric material is not a dielectric waveguide.
A closed waveguide is an electromagnetic waveguide (a) that is tubular, usually with a circular or rectangular cross section, (b) that has electrically conducting walls, (c) that may be hollow or filled with a dielectric material, (d) that can support a large number of discrete propagating modes, though only a few may be practical, (e) in which each discrete mode defines the propagation constant for that mode, (f) in which the field at any point is describable in terms of the supported modes, (g) in which there is no radiation field, and (h) in which discontinuities and bends cause mode conversion but not radiation.
A slotted waveguide is generally used for radar and other similar applications.
Sources: in part from Federal Standard 1037C and from MIL-STD-188, and ATIS