## 1 Introduction  
In recent years, transmission capacity achieved several terabit/s using dense wavelength-division multiplexing (DWDM) systems [1]. Group velocity dispersion (GVD), which deteriorates the performances of DWDM systems due to its temporal broadening effects on optical pulse, is one of the most important parameters of optical fibers. Fibers with wideband nearly zero flattened dispersion are more preferable in DWDM systems, because they can reduce the GVD-induced temporal broadening, as well as keep a uniform response among different channels [2; 3; 4]. To achieve wideband nearly zero dispersion, the waveguide dispersion \(D_{\text{w}}\) needs to be regulated to counterbalance the influences of the material dispersion \(D_{\text{m}}\) in the entire interested band, which is a difficult task for step index fibers (SIFs). With artificially periodic arranged air holes in the cladding, microstructure fibers (MSFs) possess more design flexibility compared with SIFs. By employing air holes with different sizes [5], or using elliptical instead of circular air holes [6], lots of flattened dispersion MSFs were proposed. Currently, MSFs are undoubtedly the most promising schemes in achieving wideband flattened dispersion fibers [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. However, the design flexibility of MSFs also indicates the complexity of the fiber structure. It is difficult to keep tens or even hundreds of cladding air holes from deformation for dozens of kilometers during fiber drawing, while the collapse of air holes makes the dispersion of the MSF deviate from the original design. Furthermore, the fragility of the air-silica structure makes it a huge technical challenge to employ flattened dispersion MSFs for optical fiber communication systems that are hundreds of kilometers long. Multicore fibers (MCFs) are another kind of fiber attracting lots of attention recently. According to coupled-mode theory (CMT), the guiding modes of MCFs are supermodes [17; 18], which are formed due to the coupling of every individual core mode. In the formation of supermode, the propagation constant is also modulated, which further leads to a change in its dispersion. For example, by adjusting the coupling condition between the adjacent two cores, large negative dispersions as high as \(-4.5\times 10^{6}\,\mathrm{ps/(nm\cdot km)}\) were achieved at 1550 nm [19]. However, it is difficult to manage the dispersion of the MCFs in a broad band only using mode-coupling. Meanwhile, \(N\)-core MCFs commonly support at least \(N\) supermodes [20; 21; 22], and optical pulse broadening due to intermodal dispersion will severely deteriorate the performance of fiber-optic communication systems. Hence, the research on MCFs is mainly confined in the following four areas: low-cross-talk MCFs for space-division multiplexing (SDM) systems [23], large-mode-area MCFs for fiber lasers [24], MCF couplers [25], and MCF sensors [26]. To the best of our knowledge, there are still no reports regarding wideband nearly zero flattened dispersion MCFs with single-supermode transmission. In this paper, a trench-assisted multicore fiber (TA-MCF) with wideband nearly zero flattened dispersion is proposed. We noticed that, in recent years, a new kind of multicore fiber, proposed and optimized for the use of space-division multiplexing (SDM), was also named TA-MCF. However, we must state that those fibers and the TA-MCFs proposed in our paper belong to two totally different purposes for distinct areas. For the TA-MCFs proposed for SDM, the main purpose of the trenches is to suppress the overlap of the electric field distribution and reduce the cross-talk between different cores. Thus, each fiber cell in the TA-MCFs is encircled by a low-index trench. However, our proposed TA-MCFs are intended to find application in high-speed DWDM systems. Hence, the main purpose of our paper was to present a novel fiber structure to boost the flexibility of GVD management by combining the merits of both MCFs and MSFs.