n (1390km). This system is the first to use the lasers is to use DSF. However, the use of external modulators such 1550nm window with DFB lasers, p-i-n detectors and integrated as LiNbO or electro-optic absorption devices offers possible alter- silicon bipolar receivers. The section length is 120km using NDSF. natives if the use of DSF is not possible. This is particularly important in upgrading installed NDSF fibre. The design of a single 53.6.2 Terrestrial fibre optic systems mode system is similar to that of a multimode system outlined earlier. The first application of fibre optic communications were in terre- strial systems. The first systems were operating at 850nm using multimode transmission but the advances made in the performance Applications of fibre optic of the fibre at the 1300nm window and of the electro-optics forced communications the migration of the applications to this wavelength. Bit rates of 140MBit/s and above are used in the trunk network The basic characteristics of fibre optic communications are the low and those below in the junction network. Trunk systems use lasers loss and large bandwidth of the channel (the fibre), the high perfor- and high sensitivity p-i-n/FET or APD receivers and because the mance, compactness and reliability of the components (sources and aim is to maximise section length, single mode fibres are used. detectors) and the high performance subsystems possible (optical Junction systems usually use LEDs and multimode fibre because the receivers and transmitters). These features, combined with the rapid dispersion penalty is negligible. progress made in integrated electronics, has ensured fibre optic The length of trunk system varies and depends on the distribution penetration into virtually all the communication applications rang- of population. System length can be as short as 50km but as long as ing from submarine systems to data buses for avionics. Considering 1000km. In Europe and some areas of the USA system lengths are that it was only twenty years ago when it b