o about 600Mbit/s. At higher bit rates additional allowances may have to be made for very short term 15.5.2 Single mode fibre laser wavelength changes during the 'on' period which are con- verted by the small but finite dispersion of the fibre to an additional noise source. The bandwidth limitations of multimode fibres arise from the vari- ation in multipath propagation times. The best graded index fibre offers a bandwidth of about 1.5GHz/km and is expensive and difficult to produce. (On a 30km route this would reduce to some- 15.6 Optical fibre cables thing like 140MHz which is inadequate for high bit rate systems.) There are two primary considerations determining the design of This major limitation is overcome in single mode fibre designs. an optical fibre cable: The number of modes propagated by a fibre is given approximately by Equation 15.26. To minimise optical attenuation increments associated with the manufacture and use of the cable. (15.26) 2. To maintain the physical integrity of the fibre during the cabling process and its subsequent installation and service environ- ment. where a is the core radius Macrobending loss arises due to a partial loss of guidance in the a is the wavelength fibre when it is bent into a curve having a radius less than a few tens is the refractive index of cladding of millimetres. When the fibre is SO bent, the evanescent wave in the nco is the refractive index of the core. cladding needs to travel above the velocity of light in the medium, which it cannot do, and SO guiding is lost and energy is radiated. At As the fibre diameter is reduced, the number of modes which can wavelengths approaching the mode cut-off wavelength, more en- be propagated falls, and in the extreme only a single mode is ergy is carried in the evanescent field in the cladding and bending transmitted. In order to obtain a usable core size the core cladding loses more easily occur. In multimode fibres, there are always some index difference is also reduced from the order of 1% to 0.1% modes close to cut-of