ws: of devices suitable for fibre communications are summarised in LEDs: light output vs current, spectral density, information bandwidth. Lasers: light current characteristics, spectral density, turn on Table 53.1 Typical LED performance parameters with multimode fibre tail (50/120um) delay, relaxation oscillation, spectral broadening under modu- lation, partition noise and information bandwidth. The light current characteristics of a LED is shown in Figure (MHz) 53.14 for three device structures. Notice that for the surface and edge emitting LEDs the device exhibits a linear characteristic for low currents with the edge emitting device being linear for a substantial part of the characteristic. The spectral density of a LED Table 53.1. As one can see from this table the output power into a standard multimode fibre tail is of the order of a few tens of microwatts only in spite of the fact that the power from the facet of the device is of the order of 5mW to 15mW. The reason for this difference is the broad far field radiation pattern of the LEDs and the relatively small NA of the standard multimode telecommunica- tion fibre. The light current characteristic of a laser is shown in Figure 53.15. In contrast to the same curve for LEDs there is now a sharp 'knee'. The current corresponding to the knee is the threshold current, and it is an important parameter of the device. At currents below the laser behaves like an LED, that is, the radiation is spontaneous and the power low. Above I the power rises very rapidly and now the device operates in the regime of stimulated emission. The spectral density of the laser is now very different from that of an LED. It is narrow and consists of a number of spectral lines under an envelope that is nearly Gaussian. The spectral density depends on the driving conditions of the device especially with large digital or analogue signals. One special feature of the laser is the turn on delay. With the device biased at an arbitrary current level, I below threshold, the Current (mA) time 