Optical Fiber Amplifiers
Optical fiber Amplifier are optical amplifiers based on optical fibers as gain media. In most cases, the gain medium is a glass fiber doped with rare earth ions such as erbium (EDFA = erbium-doped fiber amplifier), neodymium, ytterbium (YDFA), praseodymium, or thulium. This active dopant is pumped (provided with energy) with light from a laser, such as a fiber-coupled diode laser; in almost all cases, the pump light propagates through the fiber core together with the signal to be amplified. A special type of fiber amplifiers are Raman amplifiers.
The originally dominating application of fiber optical amplifier was in optical fiber communications over large distances, where signals need to be periodically amplified. Typically, one uses erbium-doped fiber amplifiers with signals of moderate optical power in the 1.5um regions. Often, many different wavelength channels are simultaneously amplified in a single fiber optical amplifier (WDM).
Other important application areas of fiber optical amplifiers have been developed later. In particular, high-power fiber amplifiers can produce output powers of hundreds of watts or even many kilowatts. Such optical amplifier are now widely used in laser material processing, replacing solid-state bulk lasers and CO2 lasers, for example. Typically, they are based on ytterbium-doped double-clad fibers for signals in the spectral region of 1.03–1.1um.
It is often very convenient (e.g. in optical fiber communications) that light from a fiber can easily be sent into a fiber amplifier, and the amplified light can be sent into further transmission fiber. Besides the active (amplifying) fiber, some additional components may be used, e.g. dichroic fiber couplers and Faraday isolators