# Unrepeated 240-km 64-QAM Transmission Using Distributed Raman Amplification over SMF Fiber +
## 3 Transmission Results and Discussions  
Due to the constrains caused by available hardware limitations, the error-free back-to-back performance of the 64-QAM transmitter was not achievable. The optimal performance versus OSNR was measured experimentally and confirmed with theoretical and numerical simulations. Data presented in Figure 2 show great agreement with all approaches. The limits imposed by the launch signal in the transmitter reduce the overall potential achievable distance that could be reached with better back-to-back performance and give very little room for any extra noise; hence, proper optimization of the Raman amplification scheme in this case is very important. The bit error rate (BER) results using first-order backward Raman amplification with different input powers (I/P) over 160-km and 200-km fiber are shown in Figure 3. As expected, in both experiments, the best BER improvement using digital back propagation (DBP) was obtained for the  
Figure 2: Experimental and theoretical back-to-back performance of a 64-QAM transmitter versus optical signal-to-noise ratio (OSNR). highest launch powers due to higher nonlinearities experienced primarily in the beginning of the span. We may notice that there is no significant difference in the DBP improvement using higher backward Raman pump powers, which indicates that the BER improvement was limited by amplified spontaneous emission (ASE) noise. The maximum span length in the unrepeatered transmission experiment with average BER (based on 10 consecutive measurements) below the soft FEC limit (\(1.9\times 10^{-2}\)) was 200 km, as shown in Figure 3.  
Although higher-order amplification was proven [3; 4; 5] to be superior in long-haul and unrepeatered transmissions, we could not achieve BER performance that was below the FEC limit in a 240-km experiment using any of the proposed higher-order, bidirectional, and hybrid Raman pumping schemes. Based on previous experimental knowledge in unrepeatered experiments with lower-order modulation formats (QPSK and 16-QAM) using the proposed amplification scheme [5; 15; 16], we believe that, with improved back-to-back performance, a successful 240-km SMF-28 transmission with BER below FEC is achievable. The achievable distance would also be improved considerably by the implementation of low-loss fibers. Here, out of the two amplification schemes (second-order Raman with the additional first-order backward pump at 1455 nm and second-order pump at 1366 nm and URFL), we only present the best results, which were achieved with the URFL configuration. Since it is difficult to predict optimal forward and backward pump powers, we took the measurements for all the possible combinations, varying pump powers by 100-mW steps for input signal powers ranging from 0 to 8 dBm (for clarity, we show the signal power \(\rm P_{launch}\) with a 2-dB step), and then processed the data offline. The results for the 240-km transmission approach with and without DBP for different launch and total pump powers are shown in Figure 4. We can notice that, similarly to previous transmission experiments, the highest BER improvement employing DBP was achieved for the signal with the highest launch power. A further increase of pump powers introduces noise and effectively deteriorates achievable transmission results. To visually illustrate the transmission improvement using DBP in Figure 5, we show the best scenario constellation diagrams for all distances before (top) and after (bottom) applying a DBP algorithm. Even with such a low BER in the 240-km case, there is a visible improvement using DBP (Figure 6e,f). Figure 3: Bit error rate (BER) vs. back propagation (BP) powers for different launch powers in 160-km (left) and 20- km (right) experiments with (dashed) and without (solid) DBP. Figure 4: BER vs. total pump powers for different launch powers in a 240-km experiment with (dashed) and without (solid) DBP.