coating curing have a sufficiently low pneumatic resistance. oven, and then finally the fibre is wound on the take up drum. It will 15/10 Telecommunication cables Figure 15.3 Illustration of numerical aperture be noted that the primary protective coating is applied virtually as Then, 1ns of pulse distortion measured at half amplitude, corre- the fibre is drawn. sponds to a drop of 6dB measured in the amplitude/frequency response at 440MHz. 15.5.1 Features of multimode fibres In an installed cable of jointed fibre lengths, mode mixing takes place at the joints and at small irregularities within the fibre lengths Standard multimode fibre for telecommunication application has a and this has the effect of reducing the effects of modal dispersion. core diameter of 50um and a cladding surface diameter of 125um. The summation of modal dispersion in a real route is given by dL ° The numerical aperture (N.A.) of a fibre is a measure of the angle where d is the dispersion per kilometre, L is the length in kilometres of acceptance of the light rays presented to it as shown in Figure and c is the concatenation index. The concatenation index for most 15.3 which illustrates numerical aperture. Rays at too steep an angle fibres lies between 0.5 and 1.0 and is usually taken as 0.7. will not be totally reflected and not be guided. Commercially the modal dispersion of multimode fibres is usually If 0 is the critical angle for total reflection, and 0 is the expressed in terms of bandwidth per kilometre. A typical grading maximum incident angle, then the numeric aperture is given by would be from 400MHz to 1600MHz in 400MHz steps. Equation 15.25. The refractive index of glass is a function of the light wavelength. As the propagation rate is a function of the refractive index it (15.26) follows that different wave lengths have different propagation speeds in the fibre. Where is the maximum refractive index of the core and nce Thus pulse dispersion arises from variation of the refractive index across the band of wavelengths emitted by