wavelength. Optical amplifiers offer a more attractive solution for implementation of long-haul DWDM systems and they can be used to boost the DWDM output signal, to amplify all wavelengths on the line, or to amplify the received signal before the optical demultiplexer as shown in Figure 4.34. There are many different optical amplifiers, but erbium-doped fiber amplifiers (EDFAs) in particular have become popular in long-distance trans- missions of high-capacity DWDM signals. They operate in a low attenuation wavelength range from 1,520 to 1,565 nm (ITU grid) and gain signals at all wavelengths by typically 30 dB or more. Figure 4.35 shows a simplified dia- gram of an EDFA. A weak optical input signal containing many wavelengths in the 1.54-um range enters the erbium-doped fiber, into which light at 980 or 1,480 nm is injected using pump lasers. This injected light stimulates the erbium atoms to release their stored energy as additional 1,540-nm light as the input signal is inserted [5]. As this process continues down the doped fiber, all optical signals in the 1,540-nm range grow stronger. With pump power from a few milliwatts to 100 mW, EDFAs achieve gains from 20 to 50 dB extending fiber sections between amplifiers to 100 to 200 km. The spontaneous emission in the EDFA also adds noise to the sig- nal, which limits the number of concatenated optical amplifiers. At distances longer than 600 to 1,000 km, the signal must be regenerated, which requires Optical input Coupler Coupler Isolator Optical output signal, 1.54 um signal, 1.54 um Erbium-doped 980 or fiber 980 or 1,480 um 10-50m 1,480 um laser laser Figure 4.35 Optical EDFA amplifier for DWDM signals. Introduction to Telecommunications Network Engineering optical demultiplexing, optical-to-electrical conversion, electrical regenera- tion, electrical-to-optical conversion, and optical multiplexing. 4.8.9 Microwave Relay Systems Microwave relay systems are radio systems that may be used for point-to- point transmission instead of copper or optical cable systems.