# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 3 Optical Amplifiers in Telecommunications Networks
### Erbium-Doped Fiber Amplifiers
Figure 6 shows a more detailed description of the energy levels with splitting due to both spin-orbit coupling and fine splitting owing to the structure of the host silica glass. From this figure, we can deduce the mechanism of amplification in different spectral areas; in this case, for the erbium amplifier, the spectral area is from 1530 nm to 1565 nm. Different pumping schemes are possible, and the most efficient pumping wavelengths are 980 nm and 1480 nm. Two different configurations can be realized. The first configuration has a pump and signal propagating in the opposite direction; this configuration is referred to as backward pumping. The forward pumping configuration is the second configuration, in which the pump and signals propagate in the same direction. Both schemes are frequently used; a combination of backward and forward pumping is employed when more uniform gain is required. As with any real device, optical amplifiers have some limiting factors in practical deployment. The most important factor is amplifier noise, which is usually expressed as a noise figure (NF). The cause of this behavior is ASE. ASE is the random return from an excited state to a normal energy state. ASE can be used to produce a broadband light source, which is undesired in optical amplifiers. The ideal theoretical NF for EDFAs is 3 dB; typical NFs vary from 4 dB to 8 dB. A total of 980 nm pumps can provide a better NF than 1480 nm pumps. Amplifier noise is a very limiting factor in long-haul transmission because not only data signals are amplified [52]. The described EDFAs are referred to as lumped amplifiers, in contrast to the distributed Raman amplification techniques that are described in this section. However, even EDFAs can be used as distributed-gain amplifiers when the fiber is doped with erbium. These distributed EDFAs were investigated but were never massively deployed in reality. Another rare earth element used for amplification is praseodymium. Praseodymium-doped fluoride fiber amplifiers (PDFFAs), which have sometimes been referred to as PDFAs to make the name more visually similar to EDFAs, can be used to amplify signals in the original O-band, which covers  
Figure 6: (**a**) Schematic diagram of erbium energy levels and (**b**) detailed energy levels for erbium. 1260-1360 nm. Compared with EDFAs, these amplifiers for the O-band are different in one important aspect. Pr (and Nd) operate on the four-level principle, which implies slightly worse parameters, such as output powers or noise figures. In contrast to a three-level system, the population inversion in four-level systems is permanently positive. However, this issue is beyond the scope of this paper. When pumping does not occur, for example, after pump failure, transmitted signals do not suffer gain or achieve attenuation. This effect is in contrast to the three-level Er system, which becomes a strong absorber, and in reality, no signal is transmitted [52]. People may ask why signals should be amplified in the original lossy area of 1310 nm when all long-haul systems use C- and L-bands. The answer is chromatic dispersion (CD) and higher speeds. Even for 10 gigabit Ethernet (GE), 1310 nm transceivers were available and substantially less expensive than their 1550 nm counterparts. Czech Education and Scientific Network (CESNET) performed few experiments with PDFFAs in the 2000s [67], especially when extending the all-optical reach of 10 GE server adapters or network interface controllers (NICs). Note that NICs with 1550 nm transceivers were not available on the market partly due to the high prices of 1550 nm transceivers. From the original distances of approximately 10 km, we were able to reach more than 100 km with PDFFAs and almost 200 km with PDFFAs augmented with Raman amplification.