d dissipated through non-radi- ative processes. Fibre used in optical communications is made from From the performance and manufacturing view point a very glasses with high silica content doped with various oxides. For these attractive profile is the one with a=2 known appropriately as the glasses there are two absorption resonances. One is the ultraviolet parabolic profile, shown in Figure 53.4(a). The change of refractive and one in the infrared. At the ultraviolet part of the spectrum the index affects the number of modes and for a parabolic profile the loss is 1dB/km at 0.62pm but falls rapidly at longer wavelengths and number of propagating modes is given by Equation 53.8. at 1240nm it reaches 0.02dB/km. The infrared loss is around 0.03dB/km at 1500nm and 0.5dB/km at 1700nm. (53.8) If one assumes that 0.5dB/km loss is a reasonable one then there is a 1.5um window between the two absorption edges. However, at this point scattering enters the picture. The scattering, known as The number of modes therefore is halved. Single mode fibre are Rayleigh scattering, is the result of microscopic material inhomo- usually step index fibre because graded index single mode fibres do geneities. The inhomogeneities arise both from the glass and the not offer any significant advantages. various dopants which are used to produce the index difference, An, The single and multimode fibres just outlined are made of silica between core and cladding. Rayleigh scattering is sensitive to wave- or multicomponent glass compounds and dominate the communica- length because it varies as 2-4. Because of that scattering is insigni- tion market where performance is the prime consideration. For low ficant at long wavelenths. Figure 53.5 shows the total loss of single bandwidth applications multimode plastic clad fibre or all plastic mode fibre and it is interesting to notice that the global loss mini- fibre can be used with significant cost reduction. The plastic clad mum of silica fibre is at 1550nm where the loss attains the minimum fibre ha