Graded index multimode fiber is a type of optical fiber where the refractive index is higher at the axis of the core and then it decreases gradually towards the core-cladding interface. That is to say, the refractive index of a graded-index fiber gradually decreases from its center, and eventually decreases to the same value as the cladding at the core edge. The change in refractive index causes refraction rather than total internal reflection. When light passes through a layer with a lower refractive index, the light will fold back to the fiber axis. Total internal reflection does not occur because refraction folds the light back into the fiber axis before it reaches the cladding boundary.
Modal dispersion 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