waveguide

For more information on waveguide, visit Britannica.com.

A device that constrains or guides the propagation of electromagnetic radiation along a path defined by the physical construction of the guide. Electromagnetic waves may propagate in space, as radio waves, but for many purposes waves need to be guided with minimum loss from the generating point to a point of application. Several guiding systems are of importance, including two-conductor transmission lines, various forms of striplines used in microwave integrated circuits, hollow-pipe waveguides, and dielectric wave­guides. Hollow-pipe guides are used primarily in the microwave region of the spectrum, dielectric guides primarily in the optical region. See also Coaxial cable; Electromagnetic wave transmission; Microwave; Transmission lines.

Hollow-pipe waveguides consist of a dielectric region, usually air, surrounded by a closed good conductor such as silver, copper, aluminum, or brass. The cross-sectional shape is usually rectangular, but may be circular or of a variety of other shapes. Voltage and current concepts, so useful for transmission lines, are not so useful for waveguides. Distributions of electric and magnetic fields, obtained from Maxwell's equations, are needed. See also Maxwell's equations.

Solution of the field equations shows that there is an infinite number of modes for these guides, where a mode is a solution that maintains its transverse pattern but attenuates and shifts in phase as it propagates along the guide. These modal patterns may be likened to the resonant modes of a drumhead, with low-order modes having only a few transverse variations and high-order modes having many.

All modes in these guides have cutoff properties. Below a certain critical frequency, the mode will not propagate but only attenuates if excited; above the cutoff frequency, though, it may propagate with a finite phase velocity and small attenuation (zero attenuation for perfect dielectric and conductor). Higher-order modes usually have higher cutoff frequencies.

If the guiding pipe is perfectly conducting, the modes may be divided into transverse magnetic (TM) and transverse electric (TE) types. For the former, the magnetic field is confined to the transverse plane, while the electric field is so confined for the latter. These classifications remain useful for the practical metallic conductors used for such guides.

A variety of circuit elements is required for exciting desired modes, filtering, coupling to passive and active elements, and other necessary networking functions. Excitation of a particular mode may be by probes, along the direction of the mode's electric field, by loops normal to magnetic field lines, or by the charge streams of an active vacuum-tube or semiconductor device placed within the guide.

Other important waveguide elements are the directional coupler and isolator. In the directional coupler, there is coupling to an auxiliary guide in such a way that the output of one of its ports is proportional to the wave traveling in the forward direction, and the output of the other is proportional to the reverse wave. The isolator makes use of the nonreciprocal properties of ferrites with an applied magnetic field to pass the forward-traveling wave of the guide but to eliminate the reflected wave. See also Directional coupler.

A dielectric waveguide consists of one dielectric material, called the core, surrounded by a different dielectric, called the cladding. The permittivity (dielectric constant), or refractive index, of the core is larger than that of the cladding, and under proper conditions electromagnetic energy is confined largely to the core through the phenomenon of total reflection at the boundary between the two dielectrics. See also Permittivity; Reflection of electromagnetic radiation; Refraction of waves.

Early dielectric guides were so lossy that they could be used only over short distances. Dielectric light pipes found surgical and laboratory use. In 1969 silica fibers were developed with attenuations of 32 dB/mi (20 dB/km), low enough to be of use for optical communication applications. Since then, further improvements have reduced losses to as low as 0.3 dB/mi (0.2 dB/km). Fiber guides are now the basis for a worldwide optical communication network. See also Fiber-optics imaging.

Planar, rectangular, and thin-film forms of dielectric guides are also important in guiding optical energy from one device to another in optoelectronic and integrated optic devices, for example, from a semiconductor laser to an electrooptic modulator on a gallium arsenide substrate. Because of the simpler geometry, the planar forms will be used to explain the principle. See also Integrated optics; Laser; Optical modulators.

By far the most important dielectric guide at present is the optical fiber used for optical communications. Here the round core is surrounded by a cladding of slightly lower refractive index. The combination is surrounded by a protective jacket to prevent corrosion and give added strength, but this jacket plays no role in the optical guiding. See also Optical fibers.

A rectangular, circular or elliptical tube through which electromagnetic waves are transmitted. An optical fiber is an optical waveguide.

Download Computer Desktop Encyclopedia to your iPhone/iTouch

Rectangular or circular pipe used to guide electromagnetic waves at microfrequencies.

Look up waveguide in Wiktionary, the free dictionary.

A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguide for each type of wave. Waveguides differ in their geometry which can confine energy in one dimension such as in slab waveguides or two dimensions as in fiber or channel waveguides.

Contents 1 Electromagnetic waveguides 2 Optical waveguides 3 Acoustic waveguides 4 Sound synthesis 5 External links

Electromagnetic waveguides

Main articles: Waveguide (electromagnetism) and transmission line

Waveguides can be constructed to carry waves over a wide portion of the electromagnetic spectrum, but are especially useful in the microwave and optical frequency ranges. Depending on the frequency, they can be constructed from either conductive or dielectric materials. Waveguides are used for transferring both power and communication signals.

Optical waveguides

Main article: Waveguide (optics)

Waveguides used at optical frequencies are typically dielectric waveguides, structures in which a dielectric material with high permittivity, and thus high index of refraction, is surrounded by a material with lower permittivity. The structure guides optical waves by total internal reflection. The most common optical waveguide is optical fiber.

Other types of optical waveguide are also used, including photonic-crystal fiber, which guides waves by any of several distinct mechanisms. Guides in the form of a hollow tube with a highly reflective inner surface have also been used as light pipes for illumination applications. The inner surfaces may be polished metal, or may be covered with a multilayer film that guides light by Bragg reflection (this is a special case of a photonic-crystal fiber). One can also use small prisms around the pipe which reflect light via total internal reflection [1]—such confinement is necessarily imperfect, however, since total internal reflection can never truly guide light within a lower-index core (in the prism case, some light leaks out at the prism corners).

Acoustic waveguides

Main article: Waveguide (acoustics)

An acoustic waveguide is a physical structure for guiding sound waves. A duct for sound propagation also behaves like a transmission line. The duct contains some medium, such as air, that supports sound propagation.

Sound synthesis

Main article: Digital waveguide synthesis

Uses digital delay lines as computational elements to simulate wave propagation in tubes of wind instruments and the vibrating strings of string instruments.

External links

• Francesco Errante, The behaviour of RF balanced transmission lines in a progressive wave regime

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - bølgeleder

Français (French)
n. - (Élec) guide d'ondes

Deutsch (German)
n. - Hohlleiter

Ελληνική (Greek)
n. - (τεχνολ.) κυματοδηγός

Italiano (Italian)
guida d'onda

Português (Portuguese)
n. - guia de ondas (f)

Русский (Russian)
волновод

Español (Spanish)
n. - guía de ondas

Svenska (Swedish)
n. - vågledare

中文（简体）(Chinese (Simplified))
波导, 波导管, 波导器

中文（繁體）(Chinese (Traditional))
n. - 波導, 波導管, 波導器

한국어 (Korean)
n. - 극초단파를 전달하거나 구속하는 금속관

日本語 (Japanese)
n. - 導波管

עברית (Hebrew)
n. - ‮שפופרת מתכת המעבירה גלים קצרים‬

If you are unable to view some languages clearly, click here.

To select your translation preferences click here.