rons in the conductors that constitute the electronic circuitry in the receiver. It varies inversely with the load resistance on the electronic amplifier and SO can be reduced (but not eliminated) with appropriate receiver circuit design. Both shot noise and thermal noise are proportional to the bandwidth, Ber of the electrical filter in the receiver, but there is limited scope to reduce either by use of a very narrow passband, for fear of eliminating important spectral components from the signal. The RIN results from the superposition in the semiconductor laser of the random field of the amplified spontaneous emission with the coherent field of the stimulated emission. We saw in the subsection on optical transmitters that it is manifest as high-frequency inten- sity fluctuations on the signal, even when the injected electrical current is highly stabilized and constant in time. An earlier section on amplifier noise explained that o 2 s-ase results from the beating (i.e., the coherent superposition) of the signal with all of the spectral components of the ASE from the amplifiers in the system, while ase-ase is due to the beating of each spectral component of the ASE with every other one. The ASE-ASE beat noise can be greatly reduced with the Bit Error Rate aid of an optical filter, and the wavelength de-multiplexer in a WDM system commonly satisfies this need for each channel. However, signal-ASE beat noise is always present in amplified systems. The best that can be done to minimize it is to use optical amplifiers with low noise figures and to follow the system design guidelines that were presented in an earlier section and that will be further discussed in the final section of the chapter. Another complication is the type of detector chosen: APD or p-i-n. The avalanche process in APDs provides a gain, Mapd, which enhances the receiver's sensitivity but unfor- tunately imposes additional noise that is normally accounted for by including an APD noise figure, Fapd, within the expression for the variance of the shot