Dispersion

The process of separating radiation into various wavelengths.

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/dλ, where n is the refractive index and λ 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. Note 1: For circular waveguides, the dependence is on the ratio, a/λ, where a is the core radius and λ 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.