the relaxation oscillation frequency. It was mentioned above that multimode lasers are subject to spectral problems. The solution to these problems lie in operating the device in a single longitudinal mode. Fabry Perot devices cannot Input pulse generally operate in a stable single mode. As was mentioned earlier a successful single mode device is the Distributed Feedback laser (DFB). These devices are used in all long haul systems operating at 1550nm with or without dispersion shifted fibre. However, even Figure 53.16 The concept of turn on delay in these devices are not free of imperfections. They suffer from chirp- semiconductor lasers: tout time ing, that is the small change of wavelength during the duration of the current pulse corresponding to a digital '1' symbol. 53/14 Fibre optic communications Table 53.2 Typical laser performance parameters with single and multimode fibre tails for digital applications (1) corresponds to DFB laser peak (nm) th (mA) P out (mW) Al (nm) t (ns) t (ns) Device class Fibre tail Class see(1) see(1) The power from the facet of lasers designed for fibre communica- (pu(1)=Re(1)2-RP(1) (53.41) tion is around 5mW to 10m W but the power available from the fibre tail is of the order at 1mW to 2mW. This again is due to the rather broad and asymmetric far field of the lasers. The most successful detector structure has been the p-i-n photode- The performance of lasers varies depending on the application but tector, shown in Figure 53.18. The device consists of three layers. for fibre communications Table 53.2 summarises the characteristics Two heavily doped layers, p+ and and a lightly doped n layer of some typical state of the art lasers. sandwiched between the two. The n layer is so lightly doped that it The performance of state of the art lasers for analogue modulation is virtually an intrinsic semiconductor. Hence the name p-i-n. is summarised in Table 53.3. In order to minimise the capacitance, the device operates reversed These tables contain but a fraction of what is available on s