EDFAs consist of erbium-doped fiber having a silica glass host core doped with active Er ions as the gain medium. Erbium-doped fiber is usually pumped by semiconductor lasers at 980nm or 1480nm.
Basic elements of an EDFA are shown in the figure above. The gain medium in the amplifier is a specially fabricated optical fiber with its core doped with erbium(Er). The erbium-doped fiber (EDF) is pumped by a semiconductor laser, which is coupled by using a wavelength selective coupler, also known as a WDM coupler, that combines the pump laser light with the signal light. The pump light propagates either in the same direction as the signal (co-propagation) or in the opposite direction (counter-propagation). Optical isolators are used to prevent oscillations and excess noise due to unwanted reflection in the assembly. More advanced architecture of an amplifier consists of multiple stages designed to optimize the output power and noise characteristics while incorporating additional loss elements in the mid-stage.
The energy level scheme of the erbium ion and the associated spontaneous lifetime in the glass host are are shown above. The atomic levels of Er-ions are broadened by local field variations at the microscopic level in the glass host. The light emission due to optical transitions from the first excited state to the ground level are perfectly matched with the transmission window of silica transmission fiber (1525-1610nm).
The erbium ions can be excited to the upper energy levels by 980nm or 1480nm pumps. In both cases, it is the first excited state, that is responsible for the amplification of optical signals. The amplification is achieved by the signal photons causing stimulated emission from the first excited state.
A three-level model can be used to describe the population of energy levels in the case of 980nm pumping, while a two-level model usually suffices for the 1480nm pumping case. Nearly complete inversion of Er ions can be achieved with 980nm pumping, whereas due to the stimulated emission at teh pump wavelength the inversion level is usually lower in the case of 1480nm pumping.
A higher degree of inversion leads to lower noise level generated from the spontaneous emission process and is therefore highly desirable for the pre-amplifier stage. The quantum efficiency of the amplifier is higher for 1480 pumping due to better closer match between the signal and pump energies.
The spontaneous lifetime of the metastable energy level is about 10ms, which is much slower than the signal bit rates of practical interest. These slow dynamics are responsible for the key advantage of EDFAs because of their negligible inter-symbol distortion and inter-channel crosstalk.