# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 3 Optical Amplifiers in Telecommunications Networks
### Raman Amplifiers
It should be noted that lumped Raman amplifiers were also introduced in telecommunication transmission systems but in a slightly different manner. Raman pumping was combined with a dispersion compensating fiber (DCF). The diameters of DCFs are smaller than those of standard single-mode fibers, so the interaction in DCFs is stronger, and Raman amplification is more  
Figure 11: Schematic diagram of energy levels for the stimulated Raman inelastic scattering (SRS) process (silica fiber with very short high-energy-level lifetimes). Figure 10: Raman gain for copolarized and orthogonally polarized pump and signal. efficient. DCFs are \"lumped\" because they are periodically inserted into the transmission line. Thus, adding Raman pumps to previously lumped DCFs can create lumped Raman amplifiers. With the deployment of coherent transmission systems, DCFs are removed because compensation of chromatic dispersion is not necessary [59]. The Raman effect is broadband, but the drawbacks are the polarization dependency (where a common solution is to use pump depolarization) and the low gain coefficient in silica glass (see Figure 12). For this reason, high optical powers must be applied. In CESNET, experiments were performed in which the launched powers often exceeded 500 mW. These powerful lasers introduce serious safety eye hazards, even when automatic laser shutdown (ALS) is implemented (in some cases of fiber cuts or angled physical contact (APC) connectors, ALS may encounter difficulties in detecting fiber failure). For this reason, Raman amplifiers are rarely deployed in common optical systems. They are used in specific cases when very long fiber spans must be lightweight--for example, submarine links between the mainland and islands or similar specific conditions. The present DWDM system transporting coherent signals over long distances needs to deploy Raman amplifiers together with EDFAs to cope with lossy segments with high attenuation. This hybrid solution helps to keep the overall optical signal-to-noise ratio (OSNR) acceptable. It is interesting to note that in some of the literature [60], because of the rather weak interaction in silica glass, the maximum required pump power to achieve a gain of 30 dB is calculated to be 5 W. Experiments performed in CESNET showed that even pump powers less than 1 W caused very strong distributed Rayleigh scattering (DRS) and could not be used. Some vendors of transmission equipment do use pump powers below 500 mW for Raman amplification, which is another peculiarity on the other end of the power scales; both SRS and stimulated Brillouin scattering (SBS) are nonlinear effects, so some threshold optical powers are required to \"kick-start\" the mechanism. We tested and verified the Raman amplification for pump powers of 10, 50, 100, 150, 200, 250 and 300 mW of optical power. The central wavelength of the pump was \(\approx\)1455 nm, and the signal wavelength was 1552.064 nm, which is adequate for a 97 nm wavelength shift (refer to Table 1). The continuous wave signal from the laser diode was coupled to a fiber spool with a length of 50 km. If we consider a fiber attenuation of approximately 4% per kilometer (corresponding to 0.18 dB/km, which is the normal attenuation coefficient for 1550 nm in a standard single-mode optical fiber), then after 50 km, the signal is attenuated by approximately 9 dB (\(\approx\)87% loss of power!). In the scheme depicted in Figure 13 we can see two important things. First, the amplification is more effective if the pump signal propagates in the opposite direction from that of the data signal. Second, the Raman gain  
Figure 12: Schematic diagram of backward Raman amplification (counterdirectional pumping). coefficient is highly polarization dependent; therefore, two pump diodes (Pump 1 and Pump 2) are used to generate depolarized light. It is also very important that the fiber length must be long enough to generate Raman scattering [72]. As shown in Figure 14, the saturation power of the amplified signal is quasilinear.