y spaced amplifiers: How the total noise figure varies with the number of amplifiers used. See Figure 6.10. Note that nonlinear crosstalk and dispersion are ignored. Number of Amplifiers There are two ways of interpreting the situation: mathematical and physical. From a mathematical viewpoint, the use of many low-gain amplifiers allows us to place our first amplifier earlier in the system and SO suffer less from the adverse effect of a large loss immediately after the transmitter, which the Friis formula predicts as having a particularly detrimental effect. In terms of physics, when there are many low-gain amplifiers, the signal power cannot fall to levels where the shot noise is as influential. In other words, if the signal power is always maintained at a high value, the noise component associated with the random photon arrivals makes a less important contribution to the overall SNR (Bristiel, 2006). Figure 6.11 allows us to conclude that if noise is the dominant concern, we must use the (expensive) strategy of many low-gain amplifiers. However, the fiber nonlinear effects also influence our decision. (See the later section Nonlinear Crosstalk in Amplifier Chains.) Rayleigh Backscattering The amplifier noise that we have considered SO far is due to ASE, but there is another con- tribution that can be important in amplified systems: Rayleigh Backscattering (RBS) (Jiang et al., 2007; Bromage, Winzer, & Essiambre, 2004). Even in fiber that has been fabricated to a high quality, all waves experience a weak distributed reflection without any shift in fre- quency that is due to microscopic perturbations in the refractive index on the scale of one- quarter wavelength. RBS acts on the pump, the signals, and the bidirectional ASE. Moreover, the retro-propagating waves also experience RBS, resulting in what is known as double RBS. Normally the powers, especially for double backscattering, are very low, but the presence of amplifier gain can allow them to grow. When there is a chain of discrete amplifiers (e.g., EDFAs), th