e signals experience multi- ple RBS as they pass through the transmission fiber and the amplifiers provide periodically spaced gain elements. When the gain is high or there are many amplifiers, the amplified backscattered power can become sufficient to contribute to the system's total noise. The scattered waves arrive at the detector, having made multiple transits through the chain. Single and double backscattering of the bidirectional ASE from the amplifiers enhances the noise power, while double backscattering of the signal causes time-delayed replicas to be superposed at the detector. In both cases, the BER is increased. Chapter 6 Optical Fiber Transmission RBS can also be substantial within the fabric of fiber-based optical amplifiers. It grows most when the amplifier medium is long, the effective area is small, and the germania content of the glass is high. These circumstances can sometimes apply in L-band EDFAs, where the doped fibers can be ~200 m, but they are most marked in FRAs. In Raman amplifiers RBS is nearly always a significant contributor to the overall noise figure because the Raman pumps provide sufficient gain to enable the backscattered waves to grow and contribute additional noise. When RBS is included, equation 6.3 must be modi- fied to give (6.9) in which Fint is the "intrinsic" noise figure and PDRB is the forward-propagating RBS power as measured within an electrical filter of bandwidth Be (Hz). The "net" (or "input-output") gain, Gnet of an FRA of length L is calculated by numerically solving complicated propaga- tion equations, but a simple approximation for single-channel operation can be obtained by assuming that the signal power is low: (6.10) Ppump is the launched pump power, which can be either co- or contra-directional with respect to the signal, is the fiber's gain coefficient, and as is the loss at the signal wavelength. Leff is the fiber's effective length, given by Leff = [1 - exp(-apL)], Ap, in which Ap = a (Apump). Equation 6.10 identifies an alternative definition of gain, 