## 1 Introduction  
The exponential growth of new data traffic, social networks, and rapid technological advancements has stimulated the need to expand the capacity of optical fiber communication [1, 2, 3]. However, the transmission capacity of traditional single-mode fiber has approached the nonlinear Shannon limit [4, 5]. The development of space division multiplexing (SDM) technology has been instrumental in overcoming the limitations of traditional single-mode fiber transmission [6, 7]. SDM, using few-mode fiber (FMF) and multi-core fiber (MCF), has been shown to significantly improve transmission system capacity [8, 9]. In particular, few-mode fibers (FMRs) in mode division multiplexing (MDM) systems are one of ways to break through the limitations of traditional single mode fiber. However, mode coupling-induced crosstalk remains a key challenge in MDM transmission using FMRs, necessitating a complex and costly multi-input multi-output (MIMO) digital signal processing (DSP) system for effective mitigation [10, 11, 12]. To reduce or even eliminate the need for MIMO-DSP, enlarging the mode effective refractive index difference between degenerate modes to larger than \(10^{-4}\) is a promising approach [13]. There have been several works on weakly coupled FMRs, including the ring core fiber [14], elliptical core fiber [15], elliptical ring core fiber [16, 17, 18], panda type fiber [19],bow-tie type fiber [20], and air hole fiber [21; 22; 23]. While the previously proposed weakly-coupled FMFs have demonstrated promising characteristics for mode division multiplexing, they still have limitations in terms of the number of supported eigenmodes and bandwidth. For example, [PERSON] et al. [24] proposed a panda-type separated-circles-formed elliptical ring core FMF capable of supporting 10 modes, with a min \(\Delta n_{\text{eff}}\) of 1.2 \(\times\) 10\({}^{-4}\) over the whole C+L band. [PERSON] et al. [25] proposed a segmented ring-core panda-type fiber, which also supports 10 modes, with a min \(\Delta n_{\text{eff}}\) of 3.5 \(\times\) 10\({}^{-4}\) over the whole C+L band. [PERSON] et al. [26] proposed a bow-tie ring-core FMF that can support 14 modes, with a min \(\Delta n_{\text{eff}}\) of 1.6 \(\times\) 10\({}^{-4}\) over the whole C+L band. [PERSON] et al. [27] proposed an M-type few-mode fiber that supports 10 modes, with a min \(\Delta n_{\text{eff}}\) of 5 \(\times\) 10\({}^{-3}\) over the C band. [PERSON] et al. [28] proposed a cladding rods-assisted depressed-core FMF that supports nine modes, with a min \(\Delta n_{\text{eff}}\) of 1 \(\times\) 10\({}^{-3}\) over the C band. However, the number of eigenmodes and bandwidth supported by the fiber need to be further improved. In this paper, we propose a weakly-coupled double bow-tie multi-ring elliptical core multi-mode fiber supporting 22 eigenmodes across the C+L+U band. The multi-ring elliptical core, bow-tie air holes, and symmetrical bow-tie stress-applying areas improve modal birefringence and effectively separate two adjacent degenerate modes. In particular, the introduction of a multi-ring elliptical fiber with a high-index ring and a trench improves the minimum effective refractive index difference between adjacent eigenmodes and reduces the bending loss. The implementation of a double bow-tie structure enhances birefringence, resulting in a greater separation of degenerate modes. We investigated the effects of various parameters of the proposed fiber on \(n_{\text{eff}}\) between adjacent eigenmodes, min \(\Delta n_{\text{eff}}\), \(A_{\text{eff}}\), \(B_{\text{m}}\), bending loss, and the number of supported eigenmodes at the wavelength of 1550 nm. Moreover, we evaluated \(n_{\text{eff}}\), min \(\Delta n_{\text{eff}}\), \(A_{\text{eff}}\), DMD, and dispersion of the C+L+U band. Simulation results demonstrate that the DBT-MREC-MMF effectively splits adjacent eigenmodes by combining bow-tie stress-applying, bow-tie air holes, and multi-ring elliptical core structures.