MULTIMODE DISTORTION

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. ( 188 ) Note 1: In the ray -optics analogy, multimode distortion in a step-index optical fiber 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 fiber core , i.e. , those which undergo the fewest reflections at the core- cladding boundary, will traverse the length of the fiber 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 fiber 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 fibers 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. [After FAA ] Synonyms intermodal delay distortion , intermodal distortion, modal distortion.