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Figure 14. DWDM System for Investigation of Optimized Span Input Power Using Different Types of Hybrid EDF/Raman Amplification
Advantages of hybrid amplification were investigated for a 40-channel DWDM system. Channels transmit at 10 Gbps and are placed equidistantly 50 GHz apart. The dispersion map consists of a span of 100 km dispersion shifted fiber (DSF) or SSMF. The accumulated fiber attenuation is 20 dB. Ideal precompensation of chromatic dispersion is assumed for both cases.
Three different amplification scenarios are compared—first, backward Raman amplification, second, bidirectional Raman amplification, and third, pure EDF amplification (with noise figure of 4 dB). The span power profile for the three scenarios was shown in Figure 6. To investigate impact of fiber nonlinearities on one hand and amplifier noise on the other hand, channel launch powers are varied between -5 dBm and 20 dBm. The eye-closure of the central channel (1550 nm) is measured after a receiver unit consisting of optical drop filter, photodiode, and post-detection filter before and after fiber propagation.
Figure 15 shows the eye-closure penalty versus channel power for the three investigated amplification schemes after propagation over the DSF.
Figure 15. Eye-Closure Penalty versus Channel Power for Different Amplification Schemes after Propagation over DSF
Figure 15 clearly indicates optimum values for the channel powers with respect to eye-closure penalty. At low channel powers, performance is limited by amplifier noise, while for high channel powers, it is limited by fiber nonlinearities, namely cross-phase modulation (XPM) and four wave mixing (FWM). Regardless of the applied amplification scheme, all three penalty curves rise with almost equal gradient.
The systems using Raman amplification outperform the one using an EDFA by the optimum achievable eye-closure penalty and the tolerance to power fluctuations. For the given set of parameters, widest tolerance with respect to the launch power is found for the case of bidirectional Raman amplification.
For comparison, Figure 16 shows the eye-closure penalty versus channel power for the three investigated amplification schemes after propagation over standard SMF.
Figure 16. Eye-Closure Penalty versus Channel Power for Different Amplification Schemes after Propagation over Standard SMF
Again, Figure 16 indicates optimum channel powers with respect to eye-closure penalty. However, there is now a clear difference visible with respect to tolerance of fiber nonlinearities.
For both considered propagation fibers, the optimum launch powers differ by up to 7 dBm, depending on the applied amplification scenario. This has an impact on WDM systems using a high number of channels, as more channels can be amplified with the same amount of pump power. Also, the minimum values of eye-closure penalty differ, which indicates that different total transmission distances are possible.
The results of this example case study show the importance of including nonlinear propagation effects in the system design process when deciding on optimum signal and pump powers.
