 the light sources used in is the minimum refractive index at the cladding boundary. The the equipment, i.e. lasers and LEDs. larger the NA, the more modes the fibre will support but the A typical value for graded index fibres is 120ps/km per nm at a baseband response may be limited. Fibres for telecommunication centre wavelength of 850nm, that is a pulse emitted from a 10nm purposes have an NA of typically 0.2. width source would be subjected to 1.2ns widening in a 10km The simplest form of multimode fibre is a step-index design in length. With increasing wavelength, material dispersion in silica which there is a well defined step in the refractive index between fibres falls and becomes negative above about 1270nm. As attenu- the core glass and the cladding. In such a fibre the light rays (highest ation also falls with wavelength, 1300nm is an attractive operating order modes) experiencing most reflections travel a longer path window. The value of material dispersion at 1300nm is usually down the fibre than rays experiencing fewer reflections (lower order taken as 6ps/km nm. modes). Consequently the arrival time of the higher order modes at the end of the fibre is later. A transmitted pulse at the output end of Material dispersion in the next transmission window of 1550nm a fibre will be longer than that inputted. To an analogue signal this where attenuation is still lower, is around 17ps/km nm. By increas- effect shows as a bandwidth limitation. ing doping concentrations, the material dispersion zero can be The most commonly encountered form of multimode fibre is the shifted to 1550nm, but at the expense of some slight increase in graded index type in which the refractive index decreases with attenuation at 1550nm (See paragraph 15.4.2. on single mode offset from the centre to an approximately parabolic law. The fibres). difference between the refractive index at the axis and the boundary In systems planning calculations, the effects of both material and of the core is about 1%. This has two effects. Firstly, t