and material dispersion are of opposite sign and as a result the persion is the dispersion whose origin lies in the interaction of the wavelenght of zero dispersion moves to longer wavelengths. In material of the waveguide and of the waveguide itself with the general waveguide dispersion is small and by itself a second order optical signal. It consists of two main components, material or effect, but by a suitable choice of fibre parameters it can be made to chromatic dispersion and waveguide dispersion. The origin of the cancel the material dispersion yielding a fibre with zero dispersion intramodal dispersion, known also as chromatic dispersion, is the at longer wavelength than the natural zero dispersion of the silica. finite spectral width of the optical source and its interaction with the It is particularly important that the dispersion zero can be moved to waveguide material properties (material dispersion) and the struc- 1550nm to coincide with the global loss minimum of the silica fibre. ture (waveguide dispersion). The total dispersion of a single mode fibre is shown in Figure 53.6. Intermodal dispersion, also referred to as modal dispersion, is the result of the propagation delay between modes in a multimode fibre. Both types of dispersion are present in a multimode fibre because each mode is subject to intramodal dispersion but as should be clear by now only intramodal dispersion is present in a single mode fibre. Since intramodal dispersion is more fundamental we examine it first. Consider a pulse of near monochromatic radiation of average wavelength propagating in a single mode fibre of length L. Then combining Equations 53.11 and 53.12 the pulse delay due to the material dispersion is given by Equation 53.15. (53.15) Assuming that the r.m.s. spectral width of the radiation is o then the pulse broadening due to the material dispersion is given by Equation 53.16. Figure 53.6 The dispersion for single mode fibre: (a) pure (53.16) silica; non-dispersion shifted fibre (NDSF); (c) dispersion shifted fibre 