Erbium Doped Fibre
Amplifier EDFA

Within only three years optical fibre amplifiers have revolutionised optical fibre networks. The main reason for the strong impact that they have and will have on data communication is that they are unique in simultaneous amplification of optical signals with different rates or data formats over a broad spectral region. When using optical amplifiers instead of semiconductor amplifiers, one can remove the barrier of narrow and fixed bandwidth of electrical repeater. Among different types of optical fibre amplifier nowadays the EDFA is the most commonly used. The basic principle of this rapidly growing technique is the optical pumping of a material with which optical fibres can be doped. Lasers and optical pumping of doped glasses reach far back to the roots of the early laser techniques, for example the Nd doped glass laser and amplifier. However, it turned out that Erbium doped fibres pumped with laser diodes with wavelengths of 980 or 1480 nm amplifying 1530 nm optical signals showed the optimum characteristics for use in data communication. Although this technique is quite young and one can certainly expect further developments in this area, the EDFA pumped with diode lasers at 980 nm has been proven to be the amplifier of the future. This and the fact that a great variety of physical aspects, combined with practical training with lasers and optical fibres, makes this experiment a valuable tool for the education of students as well as for teachers, since this technique is very young, but nevertheless it belongs to one of the most important ones of this century. A 17 metre long Erbium doped fibre (EDF) is pumped by a 980 nm diode laser.
The signal of 1550 nm is produced also by a laser diode. Both radiation are combined by means of a dichroic mirror and coupled into the EDF. Each laser diode is mounted on a thermo electrical cooler and connected to a separate controller which allows the variation of the temperature and the injection current. The output of the EDF is monitored by a PIN InGAs photo detector which is connected to one of the pre-amplifiers of the controller.

Fundamentals

The figure on the right shows the absorption spectra of an Erbium doped fibre. A variety of absorption bands can be seen. Optical pumping with radiation in the visible range of the spectrum as well as pumping at 800, 980 and 1480 nm have been performed. For several reasons it turned out that pumping at 980 nm is the most advantageous method for constructing low noise optical amplifiers. The peak at 1530 nm corresponds to the 4I13/2 and 4I15/2 transition when no optical pumping process takes place.


Examples of investigation and measurement

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 radiationless transferred very rapidly to the 4I13/2 state. In principle an electronical 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).

Output power of the laserdiode
The characteristic data of the laserdiode can be measured in relative units. If a laser power meter is available, this can also be done in absolute units. Output power as a function of injection current, slope efficiency and threshold current are determined. By means of the polarisation analyser the degree of polarisation of the laserdiode radiation is measured. Since the emission is a mixture of LED and diodelaser radiation, the polarisation of the laser radiation becomes dominant for higher injection currents.

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 laserdiode as trace B. Since the gain depends on different parameters as wavelength of the pump laserdiode, coupling efficiency of the signal as well as pump radiation, its influence can be measured and observed in real time.