engths, that is (53.12) around 1550nm, will possibly lead to higher bend losses. The physical cause of bend losses is the loss by radiation of the energy, since a tight bend forces the radiation to propagate into the For normal dispersion the higher frequencies are delayed more cladding. The bending losses introduced and discussed up to now than the lower ones, with the result that the pulse width increases. were due to gross fibre bends introduced by the user. However, there Assuming linear operation and that the output pulse emerging from are bending losses introduced randomly along the waveguide, with the waveguide is Gaussian of standard deviation (r.m.s. value) o each of them introducing a very small loss but the aggregate loss can then the 3dB optical bandwidth is given by Equation 53.13. be quite high. This is known as microbending loss and it can be caused by pressure exerted by other fibre within a fibre cable. (53.13) One additional form of loss is the result of the small variation in the core diameter of a fibre arising during the manufacturing phase. Principles of light transmission 53/7 The 3dB electrical bandwidth is defined as the frequency at which (53.19) the optical power is down to 0.707 of that at zero frequency, as in Equation 53.14. (53.14) For V in the range 1.5<V<2.4 the parameter b can be approxi- B Wek - 0.1325 mated by Equation 53.20. (53.20) After this brief introduction to the concept and impact of disper- sion we proceed to examine the effect of dispersion in single and multimode fibres. Since for single mode transmission 2.0 < V < 2.4 and d2(Vb)/dV2 is of the order 0.1 to 0.2. At short wavelengths, that is around 1um, both waveguide and chromatic dispersion have the same sign and 53.1.2.1 Dispersion they are added. As it was stated above the material dispersion for silica decreases The dispersion of a fibre of arbitrary parameters can be separated to zero around 1.28um. At wavelengths longer than that the wave- into two elements, intramodal and intermodal. The intramodal dis- guide 