A type of amplifier used in fiber optic transmission systems (FOTS) and comprising a short length of fiber that has been doped with erbium and spliced into the operating single-mode fiber (SMF) in a configuration known as discrete amplification, or lumped amplification. A three-port wavelength division multiplexer (WDM) is used, with one incoming port connected to the operating fiber carrying the primary signal in the 1550 nm window, one incoming port attached to a pump laser operating at 980 nm or 1480 nm, and the one outgoing port connected to the operating fiber (see Figure E-2).The pump laser excites the erbium atoms.Weak incoming light from the operating system causes the erbium atoms to drop from their excited state. As they do so, they release the extra energy, which transfers to the primary signal and amplifies it. An EDFA can simultaneously amplify a number of wavelengths in an operating range around 1550 nm, which is in the optical C-Band.A single-pump EDFA involves a pump laser on the upstream side of the erbium-doped fiber section and provides a gain varying from +10 dB (1,000%, or 10:1), to as much as approximately +17 dB (approximately 8,000 percent, or 80:1). A double-pump EDFA involves one pump laser on the upstream side and another on the downstream side of the erbium-doped fiber section, and provides a gain of close to 30 dB (100,000 percent, or 1,000:1). Note: The pump lasers can operate in either direction. Optical isolators, placed on both sides of the EDFA, act like diodes, serving to prevent optical signals from traveling in more that one direction. EDFAs are highly effective and less costly than optical repeaters, but generally are limited to no more than 10 spans over a total distance of 800 km or so. At that point a repeater must be applied to retime and reshape the signal, thereby filtering out the accumulated noise caused by various forms of dispersion. EDFAs are further limited by their inability to amplify wavelengths shorter than 1525 nm. An EDFA performs a type of amplification known as lumped amplification, as it is concentrated in a single point. See also amplifier, C-Band, diode, dispersion, dopant, erbium, FOTS, lumped amplification, noise, repeater, SMF, WDM, and window.