## 5 Design of Flattened Dispersion TA-MCFs with Low Bending Loss
Figure 9: Cross-section of the seven-core TA-MCF and its equivalent refractive index profile. Due to their high degrees of freedom in structure design, MSFs are undoubtedly currently one of the most promising schemes in achieving wideband nearly zero flattened dispersion fibers. Many flattened dispersion MSFs were proposed. MSFs usually refer to fibers containing microstructures in their cross-section. From this perspective, the TA-MCFs proposed in this paper can also be regarded as MSFs. Below, we summarize and list some results of the reported flattened dispersion MSFs to make a comparison with our proposed fiber [8; 9; 10; 11; 12; 13; 14; 15; 16]. From Table 1, it can be seen that the two fibers proposed in this paper have the simplest structures of all, with only 18 or 19 inclusions to form the microstructure. Meanwhile, all solid schemes instead of commonly used air-silica structures were employed in our design. By doing this, the disadvantages due to the introduction of air holes, such as to keep tens or even hundreds of cladding air holes from deformation for dozens of kilometers during fiber drawing, can be avoided. Furthermore, all the materials used in our design are pure, germanium-doped, and fluorine-doped silica glass with index differences less than 0.016. These glasses are commonly used in SIF fabrication and have perfectly matched physical characteristics of transition temperature, coefficient of thermal expansion, coefficient of viscosity, etc. Compared with all solid MSFs formed by two or more glasses of high index difference [12; 13], our design greatly reduces the difficulties in glasses pairing and fiber fabrication. Figure 11: Bending losses of the seven-core TA-MCF, seven-core MCF without trench, and six-core TA-MCF by removing the central rod at 1550 nm with different bending radii. Figure 10: The nearly zero flattened dispersion of the seven-core TA-MCF as a function of wavelength. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Reference**} & \multirow{2}{*}{\(A_{\text{eff}}\) **@1550 nm (\(\mu\)m2)**} & \multicolumn{2}{c}{**Fiber Structure**} & \multicolumn{2}{c}{**Dispersion**} \\ \cline{3-6}  & & \multicolumn{2}{c}{**Number of Air**} & \multirow{2}{*}{**Material**} & \multicolumn{2}{c}{**Interval**} & \multicolumn{2}{c}{**Wavelength**} \\  & & **Holes or Inclusions** & & & **ps/(nm-km)** & **Window/Range (nm)** \\ \hline
[8] & 8.55 & 126 & Air-silica & 0 \(\pm\) 0.5 & 1190-1690/500 \\  & 8.95 & 126 & 0 \(\pm\) 0.4 & 1230-1720/490 \\ \hline
[9] & 13.2 & 60 & Air-silica & 0 \(\pm\) 0.25 & 1295-1725/430 \\ \hline
[10] & 10.86 & 126 & Air-silica & 0 \(\pm\) 0.25 & 1410-1640/230 \\ \hline
[11] & 7.64 & 37 & Air-silica & 0 \(\pm\) 0.5 & 1100-1700/600 \\ \hline
[12] & 5 & 168 & Two soft glasses: & \(-\)29.25 \(\pm\) 4.25 & 1370-2620/1250 \\  & 7 & 168 & F2 (n\({}_{\text{PE}}\) = 1.594874 at \(\lambda\) = 1.55 \(\upmu\)m) & \(-\)95.5 \(\pm\) 8.5 & 1120-2280/1160 \\ \hline
[13] & 7 & 168 & Two soft glasses: & \(-\)50 to \(-\)30 & 1100-2700/1600 \\  & & & NC21A (n\({}_{\text{NC21A}}\) = 1.5113 at \(\lambda\) = 1.55 \(\upmu\)m) & & \\ \hline
[14] & - & 276 & Air-silica & 0 \(\pm\) 3.4 & 1373-1627/254 \\ \hline
[15] & - & 126 & Air-silica & 0 \(\pm\) 1.00 & 1262-1722/460 \\ \hline
[16] & - & 109 & Air-silica & 0.82 \(\pm\) 0.30 & 1120-1510/390 \\ \hline Proposed 1 & 48.41 & 18 & Germanium-doped and fluorine-doped & \(-\)1.03 to 0.75 & 1520-2010/490 \\ Proposed 2 & 42.80 & 19 & silica glass & \(-\)0.46 to 1.35 & 1500-2040/540 \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison between the proposed flattened dispersion trench-assisted multicore fibers (TA-MCFs) in this paper with other studies. Another obvious advantage of our design over other flattened dispersion MSFs is that the TA-MCFs proposed herein have relatively larger \(A_{\text{eff}}\). This is because conventional single-core MSFs use the microstructured cladding to manage their dispersion.