l fiber, the power between adjacent modes is coupled back and forth, which leads to a partial mode delay equalization. The second is related to the physics of propagation. Modes that are weakly confined in the core spread out in the cladding, and if they meet higher loss than in the core, then the number of modes propagating is limited to those strongly confined in the core. additional loss might be present in the cladding because the material used has a higher intrinsic loss or some of the modes might be radiated into the secondary cladding. These two effects limit the spread of the delay per mode, reducing the intermodal dispersion. In addition to pure fiber effects, there is another source of unpredictability in the mode struc- ture of the light source and its statistics, leading to modal noise (Epworth, 1978). To appreciate the quantities involved, consider a multimode step index fiber of 10 km length with a core refractive index of 1.5 and A = 2%. Then from equation 5.26 the r.m.s. pulse broadening is 288ns The maximum transmission bitrate in terms of the pulse r.m.s. width is given by Multimode Fibers (5.27) Therefore, for Os = o = 288 ns, the maximum bitrate is 868 Kbit/s, which is not a useful value for most modern applications. The key question now is whether the differential delay for a multimode fiber can be improved. The reason the differential delay between the axial mode and the extreme meridi- onal mode is high is that the meridional mode has to reach the boundary between core and cladding before it is reflected back into the core. If the flight time of a meridional mode is reduced, then the differential delay will also be reduced. This can be achieved with the use of graded index fiber. The basic concept here is to vary the refractive index from a maximum at the center of the core to a minimum at the core-cladding interface. The general equation for the variation of refractive index with radial distance is (5.28) rza cladding where A is the relative refractive index difference, r is the axial dist