step-index fiber is a fiber where a uniform refractive index exists within the core and a sharply decreased refractive index exists in the core-cladding interface because of the lower refractive index in cladding. For step-index multimode fiber, the light propagates in the shape of a zigzag along the fiber/core axis according to the principle of total reflection. Light entering the fiber at different angles of incidence will go through different paths. Although the incident lights propagate at the same speed simultaneously at the input, the time to reach the output of the fiber is different, resulting in a temporal dispersion called modal dispersion.
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