ield, is modulated. For analogue modulation the optical source is biased at a given Modulation point and the modulation current is superimposed on it. Figure 53.27 shows a driver which can be used for analogue modulation. The modulation waveforms vary depending on whether the opti- cal device is a LED or a laser, as shown in Figures 53.28 and 53.29. Figure 53.29 Analogue laser modulation Clearly there are issues of linearity as with any analogue system 53/22 Fibre optic communications which has to be resolved. The usual techniques of feedback and typical arrangement to maintain the output power constant is shown pre-distortion can be used but improved device performance is the in Figure 53.31. critical issue in analogue modulation. The photodiode monitors the power at the back facet of the laser For digital modulation the device is based either around zero and produces a signal proportional to the average optical power. (LEDs) or at threshold (lasers). If lasers are biased at zero current This signal is then compared with a reference and the controller then there is significant turn on delay, which impairs the perfor- adjusts the laser bias current accordingly. This control scheme mance at high bit rates. The typical high speed digital modulator is cannot control the level of 'ones' and 'zeros' individually and for current mode logic otherwise known as the emitter coupled switch, some advanced applications, such as long haul systems, more ela- shown in Figure 53.30. The advantage of this configuration is the borate schemes are used which control the level of 'ones' and large bandwidth available and the ability to control the modulation 'zeros'. 53.5 Optical system design The design of optical systems follows the same approach as any LED or other communication system. The approach can be summarised in laser a number of steps as follows; 1. The required performance against an agreed performance index is established. 2. The basic system parameters are selected. 3. The performance of suitable subsystems is establishe