A distortion mechanism, occurring in multimode fibers, in which the signal is spread in time because the velocity of propagation of the optical signal is not the same for all modes. Note 1: In the ray-optics analogy, multimode distortion in a step-index optical fibre may be compared to multipath propagation of a radio signal. The direct signal is distorted by the arrival of the reflected signal a short time later. In a step-index optical fiber, rays taking more direct paths through the fibre core, i.e., those which undergo the fewest reflections at the core-cladding boundary, will traverse the length of the fibre sooner than those rays which undergo more reflections. This results in distortion of the signal. Note 2: Multimode distortion limits the bandwidth of multimode fibers. For example, a typical step-index fibre with a 50-m core would be limited to approximately 20 MHz for a one-kilometer length, i.e., a bandwidth of 20 MHz•km. Note 3: 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-m core, may approach 1 GHz•km or more. Multimode graded-index fibres having bandwidths approaching 3 GHz•km have been produced. Note 4: Because of its similarity to dispersion in its effect on the optical signal, multimode distortion is sometimes incorrectly referred to as "intermodal dispersion," "modal dispersion,"or "multimode dispersion. " Such usage is incorrect because multimode distortion is not a truly dispersive effect. Dispersion is a wavelength-dependant phenomenon, whereas multimode distortion may occur at a single wavelength. Synonyms intermodal delay distortion, intermodal distortion, modal distortion.