This book presents new and important research on optical fibres. An optical fibre is a glass or plastic fibre designed to guide light along its length by confining as much light as possible in a propagating form. In fibres with large core diameter, the confinement is based on total internal reflection. In smaller diameter core fibres, (widely used for most communication links longer than 200 meters) the confinement relies on establishing a waveguide. Fibre optics is the overlap of applied science and engineering concerned with such optical fibres. Optical fibres are widely used in fibre-optic communication, which permits transmission over longer distances and at higher data rates than other forms of wired and wireless communications. They are also used to form sensors, and in a variety of other applications. The term optical fibre covers a range of different designs including graded-index optical fibres, step-index optical fibres, birefringent polarisation-maintaining fibres and more recently photonic crystal fibres, with the design and the wavelength of the light propagating in the fiber dictating whether or not it will be multi-mode optical fibre or single-mode optical fibre. Because of the mechanical properties of the more common glass optical fibres, special methods of splicing fibres and of connecting them to other equipment are needed. Manufacture of optical fibres is based on partially melting a chemically doped preform and pulling the flowing material on a draw tower. Fibers are built into different kinds of cables depending on how they will be used.