pec- < 2.045 tion of the fibre. One other restriction imposed by single mode propagation is that since only one mode propagates the optical source which excites the fibre must also be single mode. The parameter which must change, other things being equal, to It was assumed up to now that the refractive index of the fibre make a multimode fibre single mode is the fibre radius. For the changes in a step-like way at the core-cladding boundary, as in multimode fibre discussed above and for single mode propagation Figure 53.4(a). This fibre is known as step index fibre and it is easy at 0.85um the fibre radius should be reduced to 4.85um. However, to make and is widely used. However, as it will be seen later, the this only shows the size of dimensions involved. information bandwidth of the step index multimode fibre can be Light acceptance Cladding Ray lost through radiation and absorption in cladding Extreme ray for acceptance Figure 53.3 The light acceptance cone geometry Principles of light transmission 53/5 Refractive index Figure 53.4 Fibre types: (a) step index profile; (b) graded index parabolic profile improved if the refractive index of the core is graded with the teristics. From the transmission point of view these can be reduced maximum value at the centre of the core, as in Figure 53.4(b). to essentially two: attenuation (loss) and dispersion (bandwidth). Using axial symmetry in general the index variation can be written The total loss in an optical fibre consists of two elements: intrinsic as in Equation 53.7, where a is the profile parameter. losses due to the fundamental properties of the materials used to make the fibre and extrinsic losses which are attributed to the actual n(r)= n,(1-2An(r/a)")" r < a (core) fibre and the details of the usage. The intrinsic losses are essentially two: absorption and scattering. Absorption is a property of the material and in optical wavelength ( cladding ) (53.7) it takes place when the energy of the incident optical radiation is absorbed by electronic transitions an