Multimode distortion is a distortion mechanism occurring in multimode fibers and other waveguides, in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes. Other names for this phenomenon include modal dispersion, multimode dispersion, modal distortion, intermodal distortion, intermodal dispersion, and intermodal delay distortion. Look up waveguide in Wiktionary, the free dictionary. ... Wave propagation refers to the ways waves travel through a medium (waveguide). ... The velocity of an object is its total speed in a particular direction. ... Various normal modes in a 1D-lattice. ...

In the ray optics analogy, multimode distortion in a step-index optical fiber may be compared to multipath propagation of a radio signal. Rays of light enter the fiber with different angles to the fiber axis, up to the fiber's acceptance angle. Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than rays that enter at a steeper angle (which reflect many more times off the boundaries of the core as they travel the length of the fiber). The arrival of different components of the signal at different times distorts the shape. In optics, a ray is an idealized narrow beam of light. ... In telecommunication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. ... For other uses, see Radio (disambiguation). ... In optical fibers the acceptance angle θ is the half-angle of the cone within which incident light is totally internally reflected by the fiber core. ... Spheres reflecting the floor and each other. ...

Multimode distortion limits the bandwidth of multimode fibers. For example, a typical step-index fiber with a 50 µm core would be limited to approximately 20 MHz for a one kilometer length, in other words, a bandwidth of 20 MHz·km. Multimode distortion may be considerably reduced, but never completely eliminated, by the use of a core having a graded refractive index. The bandwidth of a typical off-the-shelf graded-index multimode fiber, having a 50 µm core, may approach 1 GHz·km or more. Multimode graded-index fibers having bandwidths approaching 3 GHz·km have been produced. This article or section does not cite its references or sources. ... A micrometre (American spelling: micrometer, symbol µm) is an SI unit of length equal to one millionth of a metre, or about a tenth of the size of a droplet of mist or fog. ...

Purists insist on calling this effect an optical distortion, since dispersion is a wavelength-dependant phenomenon and multimode distortion may occur at a single wavelength. The term dispersion has become common for describing this effect, however. Dispersion of a light beam in a prism. ... The wavelength is the distance between repeating units of a wave pattern. ...

A special case of multimode distortion (or modal dispersion) is polarization mode dispersion (PMD), which comes from a superposition of two modes that normally travel at the same speed due to symmetry (for example, two orthogonal polarizations in a waveguide of circular or square cross-section), but which travel at different speeds due to random imperfections that break the symmetry. Polarization mode dispersion (PMD) is a form of modal dispersion where two different polarizations of light in a waveguide, which normally travel at the same speed, travel at different speeds due to random imperfections and asymmetries, causing random spreading of optical pulses. ...

References

• Federal Standard 1037C