dispersion

dispersion: Any phenomenon in which the velocity of propagation of an electromagnetic wave is wavelength dependent. Note 1: In communication technology, "dispersion" is used to describe any process by which an electromagnetic signal propagating in a physical medium is degraded because the various wave components (i.e., frequencies) of the signal have different propagation velocities within the physical medium. Note 2: In an optical fiber, there are several significant dispersion effects, such as material dispersion, profile dispersion, and waveguide dispersion, that degrade the signal. Note 3: In optical fiber communications, the incorrect terms "multimode dispersion" and "intermodal dispersion" should not be used as synonyms for the correct term "multimode distortion." Note 4: In classical optics, "dispersion" is used to denote the wavelength dependence of refractive index in matter, (dn/d

, where n is the refractive index and

is the wavelength) caused by interaction between the matter and light. "Dispersion," as used in fiber optic communications, should not be confused with "dispersion" as used by optical lens designers. Note 5: Three types of dispersion, relating to optical fibers, are defined as follows:

material dispersion: In optical fiber communication, the wavelength dependence of the velocity of propagation (of the optical signal) on the bulk material of which the fiber is made. Note 1: Because every optical signal has a finite spectral width, material dispersion results in spreading of the signal. Note 2: Use of the redundant term "chromatic dispersion" is discouraged. Note 3: In pure silica, the basic material from which the most common telecommunication-grade fibers are made, material dispersion is minimum at wavelengths in the vicinity of 1.27 m (slightly longer in practical fibers).
profile dispersion: In an optical fiber, that dispersion attributable to the variation of refractive index contrast with wavelength. Profile dispersion is a function of the profile dispersion parameter.
waveguide dispersion: Dispersion, of importance only in single-mode fibers, caused by the dependence of the phase and group velocities on core radius, numerical aperture, and wavelength. (188) Note 1: For circular waveguides, the dependence is on the ratio, a /, where a is the core radius and is the wavelength. Note 2: Practical single-mode fibers are designed so that material dispersion and waveguide dispersion cancel one another at the wavelength of interest.

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