In the last few years the subject of optical cornmunications has moved rapidly from being a promising research area to a practical reality already being installed and carrying traffic in trunk networks in many countries. At the same time new applications for fibre technology are emerging and are placing new demands on the system components. In telecommunications there is a steady increase of interest in the use of fibres for undersea cables, in local area networks and wideband links, and a little further ahead the possibility of coherent communications systems. With an optical carrier...