Multimode distortion
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Multimode distortion 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. Other names for this phenomenon include modal dispersion, multimode dispersion, modal distortion, intermodal distortion, intermodal dispersion, and intermodal delay distortion.
In the ray optics analogy, multimode distortion in a step-index optical fiber may be compared to multipath propagation of a radio signal. Rays of light enter the fiber with different angles to the fiber axis, up to the fiber's acceptance angle. Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than rays that enter at a steeper angle (which reflect many more times off the boundaries of the core as they travel the length of the fiber). The arrival of different components of the signal at different times distorts the shape.
Multimode distortion limits the bandwidth of multimode fibers. For example, a typical step-index fiber with a 50 µm core would be limited to approximately 20 MHz for a one kilometer length, in other words, a bandwidth of 20 MHz·km. 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 µm core, may approach 1 GHz·km or more. Multimode graded-index fibers having bandwidths approaching 3 GHz·km have been produced.
Purists insist on calling this effect an optical distortion, since dispersion is a wavelength-dependent phenomenon and multimode distortion may occur at a single wavelength. The term dispersion has become common for describing this effect, however.
A special case of multimode distortion (or modal dispersion) is polarization mode dispersion (PMD), which comes from a superposition of two modes that normally travel at the same speed due to symmetry (for example, two orthogonal polarizations in a waveguide of circular or square cross-section), but which travel at different speeds due to random imperfections that break the symmetry.
