Intermodal Dispersion | previous | next | feedback |
Definition: the phenomenon that the group velocity of light propagating in a waveguide structure depends on the waveguide mode
Intermodal dispersion (also called modal dispersion) is the phenomenon that the group velocity of light propagating in a multimode fiber depends not only on the optical frequency (→ chromatic dispersion) but also on the propagation mode involved. The strength of this effect can be quantified as the differential mode delay (DMD) and depends strongly on the refractive index profile of the fiber in and around the fiber core. For example, the effect would vanish for a parabolic index profile, whereas e.g. for a step-index profile the higher-order modes have lower group velocities, and this can lead to differential group delays of several nanoseconds per kilometer.
In systems for optical fiber communications based on multimode fibers, intermodal dispersion can severely limit the achievable data transmission rate (bit rate): a short input pulse launched into a multitude of modes will result in a sequence of output pulses, having a relative spacing which is determined by intermodal dispersion. In order to avoid strong signal distortion, it is usually necessary to keep the pulses long enough to maintain a reasonable temporal overlap of components from different modes, and this unavoidably sets a limit on the data rate.
The natural way of eliminating intermodal dispersion is to use fiber links based on single-mode fibers, but intermodal dispersion can also be minimized by using multimode fibers with a parabolic refractive index profile.
See also: dispersion, chromatic dispersion, fibers, multimode fibers, higher-order modes
Category: fibers and other waveguides
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