## 4 The Field-Deployed Seven-Core MCF Cable  
The performance of our proposed ultra-high-speed MCF-based ICS OTN prototype system is investigated over a field-deployed 7-core fiber cable, which is deployed in Jinan, China, with the route shown in Figure 3a. Based on available cable-laying resources such as utility poles and pipelines, the MCF cable is deployed using a combination of three methods including direct burial, overhead installation, and pipeline laying. Figure 3b presents a photograph of part of the overhead-installed MCF cable. The MCF cable has a total length of 17.69 km and is formed by fusion-splicing 11 segments of short cables. The length of each segment of short MCF cable is depicted in Figure 3c. The number of short MCF cables and the length of each short cable are also designed according to available cable-laying resources. The MCF cable contains eight identical 7-core fibers, and the cross-section of the 7-core fiber is illustrated in Figure 3a. The numbering of the fiber cores in Figure 3a matches that used in our subsequent field trial. And the third fiber core is the central core that will be employed with 200 Gb/s ICS OTUs to enable the real-time sensing of environmental disturbances. The fiber cladding and coating diameters are 150 \(\upmu\)m and 245 \(\upmu\)m, respectively. The fiber cladding diameter is designed to be larger than that of conventional SMFs, primarily to ensure a sufficient distance between different fiber cores and thereby suppress inter-core crosstalk. However, the overall coating diameter of the 7-core fiber remains compliant with existing standard SMFs, which ensures compatibility with legacy cabling processes while achieving a sevenfold increase in core density within the optical cable. Each fiber core in the 7-core fiber is non-polarization-maintaining (non-PM) and complies with the ITU-T Recommendation G.657. And low-refractive index fluorine-doped trenches are assigned to each fiber core, which can suppress light leakage from the fiber core and thus reducing inter-core crosstalk as well as bending loss. To further prevent misconnections between different fiber cores during fusion-splicing processing, a tiny marker core is integrated into the MCF. Besides the parameters of the 7-core fiber already introduced, its remaining key characteristics after cabling are listed in Table 1. These characteristics are measured at 1550 nm. We find that the measured characteristics are close to those of the SMF cable, and the inter-core crosstalk is rather low and will have a negligible impact on transmission performance. In our field trial, to fully explore the transmission capability of our system, we cascade every four of the eight 7-core fibers in the MCF cable to form two segments of 70.76 km 7-core fiber links. The cascading approach and the schematic diagram of the 70.76 km 7-core fiber link are depicted in Figure 3c. This cascading method also ensures that the transmitting and receiving stations are located in the same data center, which will facilitate the conducting of our experiment. The two segments of 70.76 km 7-core fiber links contain a total of 86 fusion splicings, which is similar to the widely deployed SMF cables. A photograph of a fusion-splice point within the fiber cable is shown in Figure 3d. Using an optical time-domain reflectometer (OTDR) and a FIFO device, we measure the loss of each fusion splicing at 1550 nm. The results show that the splicing losses are all less than 0.5 dB, and the average fusion-splicing loss is no more than 0.18 dB. We further connect each 70.76 km 7-core fiber with a pair of FIFO devices to construct two 70.76 km CDM links for optical link performance testing and the subsequent field trial of the ultra-high-speed