Fundamentals

Absorption and emission
The pump and emission process is shown in the figure on the right. The pump radiation is absorbed at the transition: 4I15/2 - 4I11/2
The so generated population of the 4I11/2 state is radiation less transferred very rapidly to the 4I13/2 state. In principle an electronic transition between these levels, creating a photon, is not allowed. But due to the fact that the Erbium ions are located in a glass host, they are subject to electrical fields caused by the surrounding atoms in the host lattice. The degenerated atomic levels of the Erbium ions are now split into a manifold of different multiplets, also known as Stark splitting, allowing electric dipole transitions between the manifold of the 4I13/2 and that of the 4I15/2 state. The involved long lifetime (14 msec) of the 4I13/2 state is many orders larger than that of the nonradiative lifetime of the upper 4I11/2 state. This is the reason why a population inversion between the 4I13/2 and 4I15/2 level can be generated leading to the desired amplification of photons around 1530 nm. Once an inversion is achieved, the depopulation of the 4I13/2 takes place as spontaneous and induced emission. The induced emission finally is responsible for the desired amplification whereas the unavoidable spontaneous emission (SE) generates just noise which unfortunately is also amplified leading to the effect of amplified spontaneous emission (ASE).

Gain of the EDFA
Measurement of the gain is performed by monitoring the output of the fibre by means of the InGaAs PIN photodiode with the oscilloscope. The signal of the photodiode is displayed as trace A and the injection current of the pump diodelaser as trace B. Since the gain depends on different parameters as wavelength of the pump diodelaser, coupling efficiency of the signal as well as pump radiation, its influence can be measured and observed in real time.