# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 3 Optical Amplifiers in Telecommunications Networks
### Erbium-Doped Fiber Amplifiers
The only drawback of PDFFA is fluoride fibers, which are difficult to manufacture (fluorine is very hazardous, the fluoride composite glasses are hygroscopic and the mechanical properties are not as relaxed as those of the silica glasses used for EDFAs); therefore, few vendors can manufacture them. PDFFAs are noisier than EDFAs as well. However, problems with chromatic dispersion and higher speeds occur with pluggable transceivers for 100 Gb/s and 200 Gb/s. The price difference between the shorter reach of 1310 nm and the longer reach of 1550 nm transceivers is significant, and PDFFAs can offer economically profitable solutions. Thulium fiber amplifiers are used for PONs for signals in the 1490 nm spectral window. Ytterbium is frequently used as a codopant in EDFAs to achieve higher optical output powers [52]. Fiber optic amplifiers operate based on the principle of stimulated emission. The principle is similar to that of lasers. An EDFA amplifier consists of a laser pump diode (laser source of optical radiation) and special erbium (Er)-doped fiber. Due to the radiation added from the pump to the Er fiber, the gain is achieved in the range of C-band wavelengths. A simple schematic is shown in Figure 7. The principle of working is referred to as \"3-layer\" [64]. * Optical radiation from the laser pump is coupled to an Er\({}^{+}\) fiber with a length of a few meters (10-100 m). * Due to this process, the atoms of erbium (Er\({}^{3+}\) ions) are excited. * Absorbed energy allows migration to higher energetic layer E3. * Ions in this so-called metastable state remain only for a short time (a few milliseconds). * Then, the atoms migrate to the conductivity layer--E2 (nonradiative transition). * After the state of \"population inversion\" is achieved, the highest proportion of Er ions is in an excited state, and the energy is released via the transmitted signal. * The excited ions return to the basic energy layer E1 in the valence band. This is accompanied by the stimulated emission of radiation with the same wavelength and phase as the transmitted signal. * This is how to temporally store the energy achieved by the laser pump. The transmitted signal is amplified in the C-band in the area of 1550 nm. Note that a useful signal and noise are amplified in the amplified band. While the use of 980 nm and 1480 nm is possible, only 980 nm pumps are currently used due to a higher degree of population inversion. With the exception of the C-band (1530-1565 nm), we can use EDFAs for amplification in the L-band (1570-1625 nm). A difference is primarily observed in the Er fiber length--for the C-band, the Er fiber must be longer. The gain of EDFA amplifiers is approximately 30-50 dB depending on the Er fiber length and the power of the pump laser. The higher the quantity of ions is, the higher the energy level and the more frequent the stimulated emission. These phenomena increase the gain of the optical amplifier Amplification is the result of the population inversion state of doped ions due to the pump laser. If the power of the optical signal increases or the power of the pump decreases, the inversion state is reduced and the power is decreased. This phenomenon is known as \"saturation\". EDFA amplifiers are used below the saturation threshold. Spontaneous emission and ASE are reduced, which is referred to as \"gain compression\" [64]. EDFAs are the most usable, and their advantages are described as follows:  
* full optical system,
* high gain, 30-50 dB,
* low noise Figure (4-6 dB),
* polarization independent,
* the same phase and frequency as an input signal,
* high power transfer efficiency from pump to signal power (50%),
* can act as a shutter--when the EDFA is unpumped (e.g., if the electricity fails), it acts as shutter,
* large dynamic range,
* directly and simultaneously amplify a wide wavelength band (80 nm) in the 1550 nm region, with a relatively flat gain,
* flatness can be improved by gain-flattening optical filters,
* suitable for long-haul applications.