sion. There are semiconductor laser amplifiers, Raman amplifiers, Brillouin amplifiers, and erbium-doped fiber amplifiers (EDFAs). Certainly the EDFAs show the widest acceptance. One reason is that they operate near the 1.55-um wavelength region, where fiber loss is at a minimum. Reference 11 states that it is possible to achieve high amplifier gains in the range of 30 dB to 40 dB with only a few milliwatts of pump power when EDFAs are pumped by using 0.980-um or 1.480-um semiconductor lasers. Figure 9.31 is a block diagram of a low-noise EDFA. In Figure 9.31, optical pumping is provided by fiber pigtailed 19 semiconductor lasers with typically 100 mW of power. Low-loss wavelength division multiplexers efficiently combine pump and signal powers and can also be used to provide a pump bypass around the internal isolator. The EDFA has an input stage that is codirectionally pumped and an output stage that is counterdirectionally pumped. Such multistage EDFA designs have simultaneously achieved a low-noise figure of 3.1 dB and a high gain of 54 dB (Refs. 10, 11). Two-Stage Design Signal Er3+ -doped fiber input Output 980-nm pump laser 980-nm or 1480-nm pump Low-noise section High gain 20-50 dB Figure 9.31 An EDFA block diagram. (From Ref. 9. Courtesy of Hewlett-Packard.) 19 A pigtail is a length of fiber factory connected to an active component. CONCEPTS IN TRANSMISSION TRANSPORT In-line amplifiers Fiber Fiber Figure 9.32 Applications of EDFAs. The loops of fiber should be noted in Figure 9.32. These are lengths of fiber with dopant. The length of erbium-doped fiber required for a particular amplifier application depends on the available pump power, the doping concentration, the design topology, and gain and noise requirements. EDFAs are often installed directly after a semiconductor laser source (transmitter) and/or directly before the PIN or APD receiver at the distant end. Figure 9.32 illustrates this concept. With the implementation of EDFAs, the length of a fiber-optic link can be extended without repeaters an ad