“Erbium Doped Fiber Amplifier (EDFA) EDFAs differ from the normal method of regenerative or electro-optic repeaters in that light does not have to be converted to an electrical signal, amplified, and then converted back to light. Optical amplifiers contain a length of fiber that is doped with erbium (a rare earth substance) that provides the gain medium, and a LASER source that “pumps” the doped fiber with light at the correct frequency to thereby amplify an input optical signal. One of the most important features, after the fact that EDFAs are not frequency-dependent, is that they allow bandwidth upgrades (within limits) without replacing the entire transmission system.”