The development of the optical amplifier (OA) in the late 1980s revolutionized communication systems. Optical amplifiers had an important impact similar to the invention of the laser in early 1960s. Both devices contributed to the development of communication systems and other applications, such as lower pump optical power, single pixel multicolour displays, and light emitting devices. An optical amplifier is a device that amplifies the optical signal directly, without converting it to an electrical signal and then to an optical signal again. OAs are used for amplifying a weak optical signal in order to increase the distance the signal can be transmitted down the transmission lines. In comparison, repeaters and generators convert the signal to electrical form, regenerate or amplify the signal, and then convert it to optical form again. The conversion of the signal from one form to another is a complex process, subject to high losses, slow speed, and more costly than simpler optical amplifiers.