An erbium amplifier, also called optical amplifier or an erbium-doped fiber amplifier or EDFA, is an optical or IR repeater that amplifies a modulated laser beam directly, without opto-electronic and electro-optical conversion. The device uses a short length of optical fiber doped with the rare-earth element erbium. When the signal-carrying laser beams pass through this fiber, external energy is applied, usually at IR wavelengths. This so-called pumping excites the atoms in the erbium-doped section of optical fiber, increasing the intensity of the laser beams passing through. The beams emerging from the EDFA retain all of their original modulation characteristics, but are brighter than the input beams.

In fiber optic communications systems, problems arise from the fact that no fiber material is perfectly transparent. The visible-light or infrared (IR) beams carried by a fiber are attenuated as they travel through the material. This necessitates the use of repeaters in spans of optical fiber longer than about 100 kilometers.

A conventional repeater puts a modulated optical signal through three stages: (1) optical-to-electronic conversion, (2) electronic signal amplification, and (3) electronic-to-optical conversion. (The term optical encompasses IR as well as visible-light energy in this context.) Repeaters of this type limit the bandwidth of the signals that can be transmitted in long spans of fiber optic cable. This is because, even if a laser beam can transmit several gigabits per second (Gbps) of data, the electronic circuits of a conventional repeater cannot.

Besides eliminating complex and inefficient conversion and electronic amplification stages, the EDFA allows the transmission of signals that employ wavelength-division multiplexing (WDM). This increases the realizable bandwidth relative to conventional repeaters still further.