ent within the receiver unit. The preamplifier provides gain, G, SO that the 1 bits arrive at the detector with power GPinc-1 and are then converted into a mean photocurrent, Ip1 = RGPinc-1. However, a preamplifier adds signal-ASE beat noise and (small levels of) ASE-ASE beat noise to the 1s and ASE-ASE beat noise to the 0s. In most situations the two beat noise terms dominate over both the thermal and shot noise components. If we can also neglect RIN to a first approximation and say that the transmitter has a high extinction ratio, equations 6.21 and 6.24 give (6.25) Equation 6.25 can be used to demonstrate that the receiver sensitivity improves sub- stantially when a preamplifier is used (Agrawal, 2002; Steele, Walker, & Walker, 1991) and Chapter 6 Optical Fiber Transmission this result applies despite the addition of amplified spontaneous emission noise on both the 1 and 0 bits. The optical power incident on the detector is greater by a factor of G, and it normally more than compensates for the additional noise terms. The best results for an optically preamplified system depend on the preamplifier having a low noise figure. For this reason, when EDFAs are incorporated into the receiver node, they tend to be pumped at 980 nm (Becker, Olsson, & Simpson, 1999). In an earlier section it was argued that distributing the gain with the aid of Raman amplification provides a lower effective noise figure than can be achieved by an equivalent discrete preamplifier. Even though Raman amplifiers are subject to an additional noise component due to Rayleigh backscatter, this argument remains valid when calculating BER, and it has been shown experimentally that distributed amplifiers can provide impressive overall system perfor- mance (Morita et al., 2000; Masuda, Kawai, & Suzuki, 1999). It would appear from the arguments presented in this section that all we need to do to achieve good BER performance is to ensure that a sufficiently high signal power is incident on the detector (with or without the aid of an optical preampli