This example demonstrates the bidirectional WDM signal transmission and Raman pumping to compensate attenuation in the transmission fiber. Figure 11 shows the design.

Figure 11. Bidirectional DWDM System Exploiting C and L Bands
Using Hybrid EDF/Raman Amplification

In this system configuration, the C band is employed for signal transmission in one direction and the L band to propagate signals in the opposite direction. The fiber attenuation is partly compensated by the distributed Raman amplifier. Accordingly, applying the backward pumping scheme for each band requires the L-band pump to be placed at the same fiber end as the C-band transmitter and vice versa. Obviously, this configuration implies the bidirectional pumping.

When modeling such systems, it is crucial that the power exchange between the co- and counter-propagating signals and pumps (pump-to-pump, pump-to-signal, and signal-to-signal) is accurately considered. Therefore, simplified approaches neglecting, for example, pump depletion are not suitable. Accurate modeling is only possible if all bidirectional interactions are modeled.

Typical signal and pump spectra are shown in Figure 12. The nonflatness of the signal spectrum is due to the Raman gain shape. It can be overcome with gain flattening filters placed right after the EDFAs.

Figure 12. Spectrum at Output of Transmission Fiber (Both Directions)

It is quite interesting to look at Figure 13, which shows the signal and pump propagation in both directions. The L-band signal launched in the backward direction at the far fiber end (z = 100 km) experiences a significant Raman amplification of the backward-propagating C-band pump wave, which is also launched at the far end. On the other hand, the C-band signal, which is launched in the forward direction at the near fiber end (z = 0 km) experiences Raman amplification of the forward-propagating L-band pump. Such signal amplification by foreign pump waves is possible because of the large bandwidth of Raman gain. Thus, the signal is amplified two times, one time by the foreign pump in the vicinity of the launch point and the second time by its own pump at the fiber output.

This example also shows that careful modeling of pump-to-pump interactions is of importance. At the far fiber end, the C-band pump significantly amplifies the L-band pump. Pump depletion of the C-band pump occurs at the near fiber end.

Figure 13. Propagation of C-Band and L-Band Signals and Pumps