## 2 The Field-Deployed 4-Core MCF Cable  
The MCF cable, which is deployed in Jinan, China, with the routing shown in Figure 1a, connects the Jinxiu Chuan and Xiying data centers and has a length of 17.69 km. The entire MCF cable was laid through a combination of direct burial, overhead installation, and pipeline laying, and is formed by splicing 11 segments of short cables. The length of each segment of short cable is depicted in Figure 1c. It should be emphasized that the length of the MCF cable in our field trial was slightly longer than when it was first deployed, and the number of splicing points has also increased. This is mainly because it was damaged by construction work before this field trial and repaired by splicing a spare optical cable. The MCF cable contains eight 4-core fibers, eight 7-core fibers, four G.652.D SMFs, and four G.654.E SMFs, which is equivalent to a total of 96 fiber cores. In our work, 4-core fibers, whose cross-section is shown in Figure 1b, were utilized to conduct the field trial. The diameters of the cladding and the coating of the 4-core fiber are 125 \(\upmu\)m and 245 \(\upmu\)m,respectively, consistent with those of standard SMFs, which enables the use of standard fiber fabrication and cabling processes. The core-to-core distance is 42.5 \(\upmu\)m, which is enough to effectively suppress the crosstalk between the different fiber cores. Each fiber core in the 4-core fiber complies with the ITU-T Recommendation G.652, and low-refractive-index fluorine-doped trenches are applied around each fiber core to further reduce the inter-core crosstalk and bending loss. To ensure the precise alignment of the fiber cores at the two cross-sections during the fusion splicing and to avoid misconnections between different fiber cores, a tiny marker core is also processed in the MCF fiber as depicted in Figure 1b. In addition to the parameters of the MCF already mentioned above, the other main characteristics of the 4-core fiber after cabling at 1550 nm are presented in Table 1. The characteristics of the 4-core fiber are similar to those of SMFs. The inter-core crosstalk is rather low and has little impact on the transmission performance. In our field trial, to achieve unrepeated long-span transmission, we cascaded six of the eight 4-core fibers in the MCF cable as depicted in Figure 1c. This cascading approach enabled the establishment of a single-span 106 km MCF link and allowed the OTN transceivers to be installed in the same data center. The entire 106 km 4-core fiber link contains 65 multi-core fusion splicing connections, which is equivalent to 260 single-core fusion splicing connections. We measured the splicing loss of each single-core fusion splicing at 1550 nm using a high-resolution optical time-domain reflectometer (OTDR) and \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Mode Field** & **Dispersion** & **Attenuation** & **Polarization** & **Cut-Off** & **Inter-Core** & **Bending Loss** \\
**Diameter** & **Coefficient** & **Coefficient** & **Dispersion** & **Wavelength** & **Crossstalk** & **with R = 30 mm** \\
**(\(\upmu\)m)** & **(ps/nm/km)** & **(dB/km)** & **(ps/km1/2)** & **(nm)** & **(dB/10 km)** & **(dB/100 Turns)** \\ \hline
9.8 & \(\leq\)22 & \(\leq\)0.21 & \(\leq\)0.08 & \(\leq\)1300 & \(\leq\)\(-\)50 & \(<\)0.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main characteristics of the 4-core fiber after cabling. Figure 1: **(a)** Route of the field-deployed MCF cable; **(b)** Cross-section of the 4-core fiber in the MCF cable; **(c)** Structure of the 17.69 km MCF cable composed of 11 segments of short MCF cable and the schematic diagram of six cascading MCFs for long-span transmission. a FIFO device. In this process, we utilized the OTDR to launch a pulsed optical signal into each fiber core of the 4-core fiber one by one through the FIFO device, and we obtained the splicing loss of each single-core fusion splicing from the reflected backscattering curve.