# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 3 Optical Amplifiers in Telecommunications Networks
### Raman Amplifiers  
Another principle used for optical amplification is based on stimulated Raman inelastic scattering (SRS). This process differs from stimulated emission, as exhibited by EDFAs and SOAs, where incident photons stimulate emission of another photon with the same energy (i.e., frequency). In SRS, incident photons create another photon with lower energy (i.e., with lower frequency), and the remaining energy is absorbed in the fiberglass as molecular vibrations (optical phonons). Materials absorb energy, which is subsequently emitted. If the energy of emitted photons is lower than the energy of absorbed photons, the effect is referred to as Stokes Raman scattering. If the energy of emitted photons is higher than the energy of absorbed photons, materials lose energy, and the effect is referred to as anti-Stokes Raman scattering. This scattering process is spontaneous, i.e., in random time intervals, and occurs when signal photons (sometimes referred to as Stokes photons) are injected into materials with pump photons, as known to occur in EDFAs. While Raman amplification in optical communications was demonstrated in the early 1970s, the Raman effect was predicted in the 1920s and published in 1928 [70]. The first demonstration of Raman amplification in optical fibers was performed in the early 1970s, and many research papers indicated the potential of the Raman effect and amplifiers in fiber optic networks. However, as with coherent systems, Raman amplifiers were overtaken by EDFAs. In the 2000s, Raman amplification started to emerge in real transmission systems, especially with long-haul and ultralong-haul systems, but with improved devices. The Raman effect in silica fiber is weak, and much higher pump powers are required than those with EDFAs. Polarization dependency is also a problem, but it can be solved with the use of two orthogonally polarized pump sources, and the gain profile is not spectrally constant (refer to Figure 10). The problem with gain is true for every amplifier, and solutions to mitigate this effect are known [71]. The principle of the Raman amplifier is based on the interaction between photons that spread in the optical environment and this environment (material). The result of the interaction is a frequency shift. Raman amplifiers produce stimulated Raman scattering (SRS) in the material of the optical fiber. Due to optical pumping at specific wavelengths, interaction between photons and phonons of the material is possible, where the energy of molecules is added to the energy of photons (refer to Figure 11). Due to this change, a new mode with a 100 nm wavelength shift is created. The wavelength is shifted to longer wavelengths. Therefore, if we need to amplify optical signals in the 1550 nm band, 1450 nm pumping sources must be used. Raman scattering is an elastic scattering mechanism that does not require a population inversion. The maximal gain is approximately 30 dB [64]. As previously mentioned, the amplified band is given by the wavelength of the pumping diode. Due to this capability, Raman amplifiers can function in an extensive range of wavelengths. Figure 9: Gain dependence on the output power. The power of the Raman amplifier depends on the power and wavelength of the pumping diode, spectral efficiency, fiber length and mode field diameter (MFD). SRS can occur in forward and backward directions. Therefore, Raman amplifiers can operate in two modes:  
* Distributed Raman amplifier (DRA),
* Lumped Raman amplifier (LRA). Amplification in Raman amplifiers is very different from that in EDFAs, PDFFAs and SOAs: The transmission fiber is used as the media for amplification, and therefore, Raman amplifiers are distributed. Other optical amplifiers may be considered to be \"lumped\". DRA uses backward pumping. When the pump is situated at the end of an optical link, the gain contributes to all-optical links, and the power loss is continually compensated. DRA amplifiers have a low noise figure, high gain and low nonlinear distortion [71].