UD-1550D Series C-Band UD EDFA (Erbium Doped Fiber Amplifer) is a representative one in the optical amplifier. As the EDFA’s wavelength is C-Band, it is in line with the low-loss band of fiber and its technology has been relatively mature, so widely used. Erbium-doped fiber is the core components of the EDFA, it makes quartz optical fiber as matrix material, and incorporate a certain proportion of rare earth element erbium ions (Er3 +) in the core of a fiber. When certain amount of pump light is injected into the erbium-doped fiber, Er3 + have been excited from the low-energy level to the high energy level, due to Er3 + has a very short lifespan on the high energy level, and soon transit to a higher level by the form of a non-radiative, and form the population inversion distribution between this energy level and low-energy-level. Because the energy between these two energy levels is exactly equal to the photon energy of C-Band, stimulated emission of C-Band light can only occur, we can only enlarge 1550nm optical signal.