he fibre modal dispersion can be considered to be of Gaussian shapes and becomes self focusing and secondly, transit speed in the outer, lower added by taking the square root of the sum of the squares. refractive index regions of the fibre is faster than in the higher Low attenuation coupled with wide bandwidth, is probably the refractive index region at the core. That is, there is an equalising most remarkable property of optical fibres and accounts for their effect on the transit times of the higher order modes and the pulse outstanding success. Values of around 0.5dB/km at 1300nm are spreading effect is reduced by a factor of 100. commonly achieved on cables for general telecommunication appli- For many fibres the pulse spreading which results from these cations, and even lower values at 1550nm for submarine telecom- multipath differential propagation times is of a Gaussian shape. munication systems. Optical fibre cables 15/11 There are two basic loss mechanisms in optical fibres. These are width between the points across the fibre where the optical field absorption and scattering: amplitude measured is of the maximum value. Absorption loss. Pure silica is transparent in the wavelength The overall diameter over the cladding of the fibre as drawn is 120 range from approximately 200nm to 10000nm, with a maxi- micron, the same as for standard multimode fibre. The cladding is mum transparency at a wavelength of 1550nm. Small traces of thus very much larger than the core and is of optically low loss. One metallic impurities in the silica can increase the loss of the fibre consequence of this is that care has to be taken to exclude cladding considerably and metallic ion (fe, Cu, Ni etc.) concentrations light when making measurements, for example by taking the fibre must be limited to less than 0.01ppm. A common impurity is through a bath of liquid of higher refractive index than the cladding. water in the form of hydroxyl ions (OH) which can produce Some fibre protective coatings are also designed to act as cladding