nt reduction Equation 53.55, where T is the average transit time. Gain - (53.55) (53.49) The noise of the p-i-n detector is due to the flow of current, shot The C of APDs is usually larger than that of p-i-n detectors noise, and due to thermal effects primary in R_ The shot noise because of the complexity of the devices. The state of the art of consists of two contributions; shot noise due to the dark current, available p-i-n detectors is summarised in Table 53.4 and that of the given as usual by Equation 53.50, and shot noise due to the signal photocurrent itself, given by Equation 53.51. Table 53.4 Parameters for p-i-n detectors <id>>=2eldth (53.50) Diameter Materio (A/W) (MHz) (53.51) The total p-i-n noise is given by Equation 53.52. 200.0 (53.52) InGaAs InGaAs It should be clear now that even if the dark current is zero there will always be a noise component due to the signal. This noise-in- signal component establishes the performance limit of an optical receiver. APD in Table 53.5. Because of the superior performance of In- Because of the random nature of the impact ionisation process GaAsP devices these tables do not include data on Ge detectors. there is a fundamental difference between the noise of a p-i-n and These detectors were selected as being suitable for communica- that of an APD. The APD noise is given by Equation 53.53 where tions. There are available even faster detectors but there are used 1dcm is the part of the dark current subject to multiplication, the mainly in optical measurements. un-multiplied dark current and F the excess noise factor. Table 53.5 Parameters for avalanche photodetectors apd>e(phsc = (53.53) Materia Diameter Had the ionisation process been deterministic then F = 1.1 In terms (A/W) (MHz) of gain, the excess noise factor can be expressed as in Equation 53.54. 0.025 InGaAs 80 F = M* (53.54) InGaAs 50 The exponent X depends on the material and the design of the APD and for a well designed and used detector 0<x<1. 53/18 Fibre optic communications Output Optical receiver Post