ing the light signal, and a coupler connects the pump laser directly to the amplifier. In simple EDFAs, amplification takes place in 50 m or SO of erbium fiber coiled inside. Erbium in the doped fiber Optics Primer 1X2X3 Figure 2-43 Power Power Functions of a general optical amplifier. Wavelength Wavelength Amplified wavelengths Amplified wavelengths Power/Booster In-line In-line Amplifier Amplifier Amplifier amplifier Pre: Amplifies aggregated wavelengths prior to being demultiplexed (In)Line: Amplifies aggregated wavelengths within the span Booster: Amplifies individual wavelengths prior to being multiplexed Advantages over Electronic Systems No OE, EO conversion Low noise, high gain Greater bandwidth than electronic repeaters Transparent to bit rates Transparent to modulation formats Simultaneous generation of multiple WDM wavelengths provides maximum amplification at 1,550 nm. Figure 2-46 is a typical amplification profile. A gain-flattening filter can extend an EDFA amplifier's range of opera- tion, as shown in Figure 2-47. Introducing optical amplification into a long-haul link (see Figure 2-48) obviates the need for retiming, reshaping, and regenerating (otherwise known as 3R). However, EDFAs have a tendency to create noise in the Chapter 2 Optical Amplifier Figure 2-44 Optical amplifier. Pump Lasers (980 or 1480 nm) Erbium Amplified Fiber Signal In Signal Out Coupler Isolator Isolator Erbium-Doped Amplifier (EDFA) Structure Figure 2-45 EDFA structure. Simultaneously amplifies wavelengths in the 1530-1565 nm region Inherent spontaneous emissions are also amplified and become noise Erbium Fiber Optical Signals (1550 nm window) Optical Isolator Input Output Residual Pump Laser (980 or 1480 nm) Wavelength-Selective coupler fiber-optic network, and electrical regeneration is still needed at various points in the network to suppress that noise. Regeneration, we know, is expensive and limits the bandwidth that can be supported. To regenerate an optical signal, a wavelength must be converted from an optical signa