(DSF) The material dispersion parameter, D (a) is usually defined as in The dispersion of a multimode fibre consists of the material and Equation 53.17, which gives Equation 53.18. waveguide dispersion of a single mode fibre and of the intermodal dispersion. The nature of the intermodal dispersion can be under- (53.17) stood by considering a step index multimode fibre. ps/nm.km The light launched into this fibre can propagate in the extremes along two paths as in Figure 53.7(a). The first is along the axis of the fibre and along this path the pulse will be subjected to the (53.18) minimum delay, ATmin' given by Equation 53.21. The physical origin of waveguide dispersion is the non-linear (53.21) dependence of the propagation constant of the waveguide on the optical frequency. This is the irreducible minimum dispersion and it is present in the waveguide even for zero material dispersion. The The second is the extreme meridional ray which will experience waveguide dispersion can be approximated by Equation 53.19. the maximum delay, given by Equation 53.22. 53/8 Fibre optic communications exhibit higher losses because they penetrate deep in the cladding. The second and not so obvious is mode mixing. Because of the irregularities at the core-cladding interface of an actual fibre energy is transferred from slower to faster modes and vice versa, with the Axial ray result that in a long fibre the intermodal dispersion is reduced. With mode coupling between all guided modes the dispersion is given by Equation 53.26, where Lm is the characteristic length for mode mixing. (53.26) Extreme meridional ray In practice L is established by measurements. In spite of the effect of mode mixing in reducing intermodal dispersion the most successful approach in reducing the dispersion Axial ray of multimode fibres is by grading the refractive index of the core. The most popular grading profile is the parabolic which yield a dispersion given by Equation 53.27. (53.27) Extreme meridional ray The pulse broadening given by Equation 53.26 