al dispersion is 130ps/km. Assuming that 1dB dispersion of maximising the section length of a 2.4Gbit/s system. Again the penalty can be tolerated the maximum dispersion limited section optimum wavelength window is around 1550nm where top quality length for a quarter of a bit duration dispersion is 13.7km. This single mode fibre has a loss of 0.2dB/km. The dispersion at 1550nm length corresponds to a fibre loss of 4.1dB and the system is depends on the class of the fibre. Fibre designed for zero dispersion dispersion limited. Because of that one can design a receiver with at 1300nm has 16 to 18ps/km.nm dispersion at 1550nm. On the reduced sensitivity using a p-i-n detector. For the example discussed other hand fibre designed for zero dispersion at 1550nm is usually here let us assume that the theoretical receiver sensitivity is - specified as having a window within which the dispersion is less 53/24 Fibre optic communications than a given number. A usual number for this tolerance is to fibre, but also the transmission format, the system configuration 3ps/km.nm. and the long term prospects of submarine communications. The key In the design of high capacity single mode systems using non-dis- to this change is the high reliability of the electro-optics compo- persion shifted fibre one of the problems is laser chirping. Chirping nents. Submarine systems are planned and designed for 25 year takes place because in directly modulated lasers the injection of the service with a maximum of three repairs, because repairs of these modulation current forces a small but nevertheless crucial wave- systems are time consuming and expensive operations. length shift in the single longitudinal mode of the device. For well The first submarine system was the Transatlantic telephone cable designed DFB lasers the chirping is of the order of 0.1 mm to 0.3nm. (TAT-8) connecting the USA with the UK and France. The distance is about 6000km and the information capacity of the system was In spite this small value chirping imposes a significan