Optical fibers are sometimes doped with the rare earth element, erbium, which can amplify Light in the 1550 NM region when pumped by an external light source.

Fiber optic amplifiers, such as erbium doped fibers, re-amplify an attenuated signal without converting the signal into electrical form. Fiber amplifiers are used to support dense Wavelength division Multiplexing (DWDM) and to expand to the other wavelength bands supported by fiber.