Erbium Doped Fiber Amplifiers Erbium Doped Fiber Amplifiers are considered the most important invention of the 1990’s in the telecommunication industry. EDFAs have made WDM possible which is in turn the backbone of all wide band networks, first and foremost the Internet. EDFAs are simple and their properties almost ideal. For example: High gain broad gain bandwidth and large saturation powers Polarization independence Low noise easy integration with fibers

EDFA Basic Structure

Most important pump bands

Three level Model of an EDFA The simplest EDFA model considers Erbium in a glass host as a three level system Ground state 2 Meta stable state with a long life time 3 Pumping state The important transition is12 Its energy difference corresponds to ~ 1550 nm

Pumping rate from to is 1 3 Stimulated emission rate from to is not important 3 1 There are two transitions from level : Radiative and Non radiative, where The transition is mainly non radiative 3 The rate of stimulated absorption and emission from to is and 2 1 The rates of spontaneous emission from are2

Three level Stark Split System The three level model described so far is too simple for proper modeling of Erbium atoms in a glass host. A more appropriate description is obtained using a three level stark split laser model Charge distribution in the glass induces electric fields (called Ligand fields) which initiate the Stark effect which splits each of the energy bands. Each band is split in this model into g sub levels with Consequently, there are no discrete levels but rather energy manifolds centered around some level. For example, the manifold at level is the reason for the ability to directly pump this level from the ground state (at 1480 nm) 2

1 j g1g1 P 1j 1 k g2g2 P 2k 1 l g3g3 P 3l Pump R jl A kj Spont. Emission W kj gain NR relaxation

Bolzman factor

Gain Coefficient