## 2 Fabrication of the Ultra-Bend-Resistant 4-Core SXC  
The XT between adjacent cores and macro-bending loss in multicore fiber resulting in the degradation of signal quality can be suppressed by adjusting the RIP design [1; 16]. In our work, by introducing the trapezoid index and deep assisted trench around the central core, the 4-core fiber was designed to keep a low XT and low macro-bending loss for transmission in the O-band [17]. Figure 1 shows the measured RIP of core preform by using PK2600 (Photon Kinetics, Beaverton, OR, USA). The whole fabrication process flow of the ultra-bend-resistant 4-core SXC can be derly divided into three parts: preform deposition, fiber drawing and cabling, which are, respectively exhibited in Figure 2. For the preform deposition, in order to avoid the OH-absorption during the manufacture process of the preform as much as possible [18], we used the in-tube chemical vapor deposition with a deep fluorine-doped process for the wave-guided core and the OVD process for the high-purity silica rod, respectively. The original diameter and length of the core preform are 24.89 mm and more than 950 mm, respectively. By using the fine extending and corrosion processes, the outer diameter of the core preform was changed around 15.90 mm to match with the target inner diameter of the drilling hole. Then, based on a high-precision drilling process, the core preforms were slit and inserted into a silica rod to assemble the multicore fiber preform. The final outer diameter and length of the 4-core fiber preform were 80.02 mm and 300 mm, respectively. Compared with the Stack-and-Draw manufacture process of MCF [19], our Drilling-and-Draw manufacture process aims to further improve the geometric accuracy of the core layer in MCF, which may reduce the mismatch with fan-in/fan-out (FIFO) devices. In the experiment, by using the in-line fiber drawing process, the 4-core fiber preform was drawn into fiber by optimizing a lower heating temperature around 1650 \({}^{\circ}\)C and higher drawing tension (100 \(\pm\) 3 g), so as to keep a better geometrical uniformity. The silica fiber was coated by using UV radiation-curable acrylate material as a flexible protective layer, and the effective drawn length of the 4-core fiber was more than 92 km. Figure 1: Measured refractive index profile of core preform. The cross-section of the 4-core fiber is shown in Figure 3a, where the core pitch, outer diameters of cladding and coating are 41.2 \(\pm\) 0.2 \(\upmu\)m, 124.5 \(\pm\) 0.6 \(\upmu\)m and 245.0 \(\pm\) 1.0 \(\upmu\)m, respectively. Figure 3b shows the measured RIP of the 4-core fiber by using IFA-100 (Interferometric Fiber Analyser, Sharon, MA, USA). Limited by the measured position resolution, the measured RIPs of the 4-core fiber along the \(x\) and \(y\) directions are different from those of the core preforms. It can be found that the index depth of the assisted trench is closed to \(-\)12.0 \(\times\) 10\({}^{-3}\) after fiber drawing, which can confine the guided optical wave within the core as much as possible. Two improvements of enhancing the mechanical robustness of 4-core SXC were carried out during the cabling process. One improvement is that the tensile strength of the 4-core fiber was increased to 107 kpsi which is 7% higher than that of SC-SMF. Another improvement is using aramid yarn mixed with polyester yarn and modified PVC as the reinforcement and sheath of the 4-core SXC, ensuring that the elongation at break and the tensile strength of the 4-core SXC can reach up to 180% and 15 MPa, respectively. Figure 3c,d show the schematic and external views of the structure of the fabricated 4-core SXC, where the outer diameters of the tight buffer and sheath are 0.85 \(\pm\) 0.05 mm and 2.0 \(\pm\) 0.1 mm, respectively. Figure 3: Fabricated 4-core fiber and 4-core SXC. (**a**) Cross-section of the 4-core fiber, (**b**) measured RIP of the 4-core fiber, (**c**) schematic view of the structure of the 4-core SXC, and (**d**) external view of the 4-core SXC.