A type of intersymbol interference (ISI) created when optical pulses overrun each other in a fiber optic transmission system (FOTS). Pulse dispersion is the result of modal dispersion, which is an issue in systems employing multimode fiber (MMF). Such systems permit optical signals and signal components to propagate along multiple modes, or physical paths, within both the core and the cladding of a fiber. Some light rays take a relatively direct path through the center of the fiber core. Other rays veer towards the edge of the core, where they reflect off the interface between the core and cladding on one side of the fiber, and then the other side of the fiber, and so on as they propagate across the link. Because some paths are more direct than others, and because the time of arrival is directly related to the distance traveled, some portions of the signal arrive before others. As the distance of the circuit increases, the differences in distances traveled by the various portions of each light pulse become greater as the effects of modal dispersion become more pronounced. As the speed of transmission increases, the bit time decreases, and the separation between bits is lost.The overall impact is that the pulses of light tend to smear together as they lose their shape and overrun each other, as illustrated in Figure P-8. See also bit time, chromatic dispersion, dispersion, MMF, material dispersion, modal dispersion, mode, polarization mode dispersion, and waveguide dispersion.
In optical fibers, a general term for the broadening of a pulse by the time it reaches the receiving end. It can be the net effect of any or all of modal, chromatic and polarization dispersion. See dispersion.