ceiver 19.5 dB Transmission Systems Architectures and Performances Attenuation is driven by a constant coefficient that does not depend on the pumps and on the signal, while the gain depends on the pump. This means that regulating the DCF length while the pump powers are maintained constant the Raman gain can be maximized. This fact is also illustrated in Figure 6.32, where the Raman gain in the DCF is shown versus the DCF length. The Raman gain is built in the first part of the DCF, up to a maximum where the pumps have produced all the possible gain, from the maximum on, the fiber attenuation overcome the gain and the gain goes down up to a final zone in which the curve is almost linear (in dBm), demonstrating that the pumps have almost completely exhausted their gain capabil- ity and the signal evolution is governed by the attenuation only. Once determined the optimum DCF length, the dispersion parameter of the two DCF elements can be derived from the equation D1L1 +D2L2-RR+DL: - (6.89) Single channel power (dBm) Link length (km) Link length (km) FIGURE 6.31 Single-channel optical power profile in the Raman amplified single span system depicted in Figure 6.30. Raman gain (dB) FIGURE 6.32 Raman gain in the DCF versus the DCF length: the pumps are maintained, fixed, and constant. The amplifier data are those relative to the system DCF length (km) depicted in Figure 6.30. Telecommunication Networks where D and D2 are the dispersion parameters of the DCF at the transmitter and at the receiver RR is the wanted residual dispersion D is the SSMF dispersion coefficient Since we have only one equation we can assume D1 = D2=DDootaining (6.90) The obtained DCF parameters are summarized in Table 6.12. At this point the SNo can be evaluated by assuming a standard noise factor for the Raman amplifiers (see previous section) obtaining SNo = 18.4 dB. This figure can seem very small but it has to be taken into account that the optical bilateral bandwidth is 2B = 75 GHz, five times greater than the bit rate; thus, when the signa