# Real-Time Spatial-Division Multiplexing Transmission with Commercial 400 Gb/s Transponders
## 3 Real-Time MDM Transmission Using Non-Degenerate LP Modes of the WC-FMF
Each transmission link consists of an FMF and a pair of mode MUX/DEMUX for LP\({}_{01}\) and LP\({}_{02}\) modes. The insertion loss and modal crosstalk matrices of the three FMF links are measured. The results are shown in Tables 3-5. Compared to the single-roll 60 km FMF link, the extra insertion loss of the 20-roll FMF link is 0.3 dB and 0.47 dB for LP\({}_{01}\) and LP\({}_{02}\) modes, respectively. It can be seen that the insertion loss and crosstalk of the 20-roll 3 km FMF link only degrade slightly due to the 19 splice points. The modal crosstalk of the 150 km FMF link is larger than the other two cases, which could be balanced by adjusting the input power of the two modes. Figure 4: (**a**) Refractive index profile of the FMF and \(n_{\text{eff}}\) of the four LP modes. (**b**) Structure of the side-polished MSC. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Input Modes of the 60 km FMF**} & \multicolumn{2}{c}{**Output Modes**} \\ \cline{2-3}  & **LP\({}_{01}\)** & **LP\({}_{02}\)** \\ \hline \(\text{LP}_{01}\) & \(-14.42\) & \(-34.21\) \\ \hline \(\text{LP}_{02}\) & \(-34.12\) & \(-19.63\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Insertion loss (diagonal data) and inter-mode crosstalk (nondiagonal data) of the 60 km FMF with mode MUX (Unit: dB). Figure 5 shows the experimental setup of real-time 2-LP-mode MDM-WDM transmission with commercial 400 G transponders. Twelve 400 G transponders are utilized in this experiment and each mode carries six wavelengths. The six central frequencies of the transponders for each mode are set to 191.6 THz, 191.7 THz, 193.3 THz, 193.4 THz, 196 TH and 196.1 THz which are at the beginning, middle and end of the C band. Other parameters of the transponders are the same as the MCF transmission. The two WDM signals are amplified by two EDFAs and then mode-multiplexed by the all-fiber mode MUX. After the FMF transmission, the MDM signals are demultiplexed by the mode DEMUX into two SMFs. Finally, the single-mode WDM signals are demultiplexed by another two WSSs and fed into the receivers for detection. In the experiment of a single-roll 60 km FMF link, the OSNR and BER performances versus input power of LP\({}_{01}\) and LP\({}_{02}\) modes in single-mode and MDM transmission are both measured. The results of the four cases (LP\({}_{01}\) only, LP\({}_{02}\) only, LP\({}_{01}\) MDM, LP\({}_{02}\) MDM) are shown in Figure 6a,b. Because the performance of different WDM channels is similar, the results at 193.4 THz is demonstrated only. The modal crosstalk is regarded as noise when calculating OSNR. It can be seen that as the input power increases, the OSNR increases while the BER decreases in each of the four cases. The performance of single-mode transmission is better than the MDM transmission due to the modal crosstalk. Figure 6c shows the BERs of the MDM transmission at six frequencies, in which the input power of LP\({}_{01}\) and LP\({}_{02}\) modes are 7.66 dBm and 10.07 dBm, respectively. It can be seen the BERs are all below the FEC threshold. The BER and OSNR performance of the 20-roll \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Input Modes of the 20 \(\times\) 3 km FMF**} & \multicolumn{3}{c}{**Output Modes**} \\ \cline{2-3}  & **LP\({}_{01}\)** & **LP\({}_{02}\)** \\ \hline LP\({}_{01}\) & \(-\)14.72 & \(-\)33.23 \\ \hline LP\({}_{02}\) & \(-\)32.70 & \(-\)20.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Insertion loss (diagonal data) and inter-mode crosstalk (nondiagonal data) of the 20\(\times\)3 km FMF with mode MUX (Unit: dB). Figure 5: Experimental setup of real-time non-degenerate 2-LP-mode MDM-WDM transmission with commercial 400 G transponders. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Input Modes of the 150 km FMF**} & \multicolumn{3}{c}{**Output Modes**} \\ \cline{2-3}  & **LP\({}_{01}\)** & **LP\({}_{02}\)** \\ \hline LP\({}_{01}\) & \(-\)33.87 & \(-\)52.89 \\ \hline LP\({}_{02}\) & \(-\)51.00 & \(-\)41.24 \\ \hline \hline \end{tabular}
\end{table}