detection. Mathematically it is stated as in Equation range of interest. Using Equation 53.41 the photocurrent can now 53.41, where R is a constant to be discussed later. Table 53.3 Typical laser performance parameters with single and multimode fibre tails for analogue applications (1) is in dBc units a peak (nm) Poul(mW) AN (nm) BW (GHz) 2nd H. Dist. 3rd Or. Int. Fibre tail (dbm) (dbm) 1300 1550 -40 1 -50() 1310 1550 -60 ¹ -65(1) Optical detectors 53/15 Contact AR coating Depletion layer Figure 53.18 The p-i-n photodetector: (a) the schematic of a p-i-n; (b) silicon p-i-n photodetector be written as in Equation 53.44, where e is the electronic charge and The principle of the operation of the p-i-n photodetector is that in R the device responsivity in [A/W]. the limit as n 1 one photon produces one electron-hole pair. However, for reasons which will become apparent in the section on P (1) (53.44) receivers, if the device could yield for each photon many electron- iph(t) = n q e h f = = RP(1) holes pairs then the impact of the noise of the electronic amplifiers WAVELENGTH (um) Figure 53.19 The optical absorption coefficient for some materials used in fibre optic communications 53/16 Fibre optic communications following the detector on the total (S/N) ratio will be minimised. ciency down to 850nm in spite the fact that the Si absorption starts Such devices, known as Avalanche Photodetectors (APDs), make tailing off. The InGaAsP device is specially designed for long possible the design of high sensitivity optical receivers. wavelengths in the sense that the absorption and multiplication do In an APD, electron-hole pairs released by the primary photoelec- not take place in the same layer. Such devices, known as separate tron effect are accelerated to velocities capable of initiating impact absorption and multiplication detectors (SAMs for short), aim at ionisation. The secondary pairs, who owe their presence to ionisa- minimising the multiplication noise. tion, in their turn generate other pairs and the final current