https://www.fiberlabs.com/glossary/erbium-doped-fiber-amplifier/
Erbium-Doped Fiber Amplifier (EDFA) is an optical amplifier used in the C-band and L-band, where loss of telecom optical fibers becomes lowest in the entire optical communication bands.
EDFA is now most commonly used to compensate the loss of an optical fiber in long-distance optical communication.
Another important characteristic is that EDFA can amplify multiple optical signals simultaneously, and thus can be easily combined with WDM technology.
