Fiber amplifiers 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 (see below).