## 5 Experimental Results of the Ultra-High-Speed ICS CDM Transmission System  
In this section, we present the experimental results of our ultra-high-speed ICS CDM transmission system utilizing 130 Gbaud 1.2 Tb/s OTUs and 200 Gb/s ICS OTUs over the field-deployed MCF cable. We first investigate the BER performance of each fiber core channel using 1.2 Tb/s OTU transceivers. Three wavelength channels with central wavelengths of 1529.55 nm, 1549.72 nm, and 1566.72 nm are selected to evaluate the system performance over the C-band. The three tested wavelength channels are successively denoted as \(\lambda_{1}\), \(\lambda_{2}\), and \(\lambda_{3}\), respectively. And the three wavelength channels are realized by adjusting the central wavelengths of the two available 1.2 Tb/s OTUs. The single-wavelength input power to each fiber core channel of the two CDM links is uniformly set to 8 dBm. And ASE signals with the same single-wavelength input power are filled into all the remained wavelength channels, as previously mentioned. The seven fiber core channels are tested one by one using the same approach, and the results of the three tested wavelength channels after 70.76 km and 141.52 km CDM transmissions in each fiber core are shown in Figure 4. We can find that the BER values of all channels are lower than \(2.1\times 10^{-2}\) after the 70.76 km transmission and remain below the SD-FEC limit after the 141.52 km transmission. This demonstrates that an ultra-high-speed 1.2 Tb/s optical signal with a PCS-64 QAM modulation format can be transmitted error-free over 140 km in the field-deployed seven-core fiber cable. And some differences can be observed in the BER performance across different fiber core channels, which is mainly caused by the differences in span losses, inter-core crosstalk, and the performance of EDFAs among different fiber core channels. The BER performance of the ICS transmission channel using 200 Gb/s ICS OTUs is also experimentally investigated. The ICS signal uses a central wavelength of 1540.16 nm and is transmitted over the central core of the MCF. The results show that the ICS signal maintains a BER below \(1\times 10^{-4}\) after the 141.52 km transmission, which can easily achieve error-free transmission after FEC. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Core \#1** & **Core \#2** & **Core \#3** & **Core \#4** & **Core \#5** & **Core \#6** & **Core \#7** \\ \hline Span loss & 25.3 & 24.2 & 20.1 & 23.9 & 24.1 & 23.8 & 25.7 \\ Total crosstalk & \(-47.1\) & \(-44.3\) & \(-42.7\) & \(-48.2\) & \(-46.2\) & \(-44.3\) & \(-47.5\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Span loss and the total inter-core crosstalk of the first 70.76 km CDM link (Unit: dB). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Core \#1** & **Core \#2** & **Core \#3** & **Core \#4** & **Core \#5** & **Core \#6** & **Core \#7** \\ \hline Span loss & 25.1 & 25.3 & 19.8 & 23.8 & 25.2 & 23.2 & 24.6 \\ Total crosstalk & \(-45.5\) & \(-45.3\) & \(-43.1\) & \(-46.8\) & \(-42.5\) & \(-46.6\) & \(-45.4\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Span loss and the total inter-core crosstalk of the second 70.76 km CDM link (Unit: dB). We then adjust the central wavelength of the 1.2 Tb/s OTUs across the C-band to evaluate the transmission capacity of the 1.2 Tb/s ICS CDM transmission system. The central wavelength of the ICS OTUs remains at 1540.16 nm, which is denoted as \(\lambda_{4}\). The single-wavelength input power is still 8 dBm, and the ICS signal is also transmitted over the central core. The BER values of the 32 wavelength channels after the 141.52 km transmission are shown in Figure 5. And the normalized optical spectrum at the receiver is depicted in Figure 6. It can be obtained that the BER values of all 32 wavelength channels are below the SD-FEC limit for each fiber core. And the result of the ICS channel is not shown in Figure 5, which is because its BER is too low to fall within the display window of the figure.