An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. It is used to amplify weak signals in optical fibers, allowing them to travel longer distances without needing to be regenerated.

There are three main types of optical amplifiers: erbium-doped fiber amplifiers (EDFAs), Raman Amplifiers and semiconductor optical amplifiers (SOAs). EDFAs are the most commonly used type of optical amplifier, while SOAs are used in applications where a higher gain is required.

EDFA (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. Invented in 1987 [1], 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.

EDFAs are used as a booster amplifier, line amplifier, and pre-amplifier in an optical transmission line, as schematically shown in Figure 1.

.   The booster amplifier is placed just after the transmitter to increase the optical power launched to the transmission line.

.   The inline amplifiers are placed in the transmission line, compensating the attenuation induced by the optical fiber.

.   The pre-amplifier is placed just before the receiver, such that sufficient optical power is launched to the receiver.

A typical distance between each of the EDFAs is several tens of kilometers.