With the emerging social media age, mobile communications, and cloud computing, the ever-increasing consumer demand for network capacity has exploded, which has brought enormous challenges to optical fiber communication systems. To enhance data transmission and all-optical network switching capabilities, an upgrade of the optical fiber is imperative. Multiplexing technologies like wavelength-division multiplexing and time-division multiplexing are currently being employed to expand the capacity of traditional single-mode fiber (SMF) based optic-transmission systems. However, the SMF has merged all degrees of freedom of light (including amplitude, phase, frequency, and polarization), achieving the nonlinear Shannon limit’s maximal capacity. To solve the capacity barrier, space-division multiplexing (SDM) transmission has been studied in the recent decade. SDM technology has two efficient solutions: multicore fiber (MCF) and few-mode fiber (FMF). The MCF integrates numerous independent cores into a single fiber for parallel transmission and the multicore design increases manufacturing costs. Due to the intermode coupling, it is challenging to attain high core density. For FMF, each mode is a channel for independent signal transmission, which increases the single fiber’s transmission capacity. In particular, polarization-maintaining FMF supporting several orthogonally polarized modes with a large separation of effective indices would realize low-crosstalk high-capacity communications. In this study, we report an air hole-assisted polarization-maintaining FMF, where high birefringence is attained using air holes assisted structure. The numerical simulation results revealed that a low-index core fiber can realize 10 polarization-maintaining modes. In the future, the proposed fiber can find applications in large-capacity SDM communication technology.

The numerical model, i.e., the FMF’s cross-sectional structure, was established by exploiting COMSOL Multiphysics based on the complete vector finite element approach. The perfectly matched layer, which matched the fiber cladding’s wave impedance was placed to the periphery of the fiber to absorb electromagnetic waves at a boundary, simulating an infinite cladding size. In determining the size of the grid unit for a fiber cross-sectional structure’s discretization, comprehensive consideration of factors like solution accuracy, calculation time, and hardware conditions were required. To achieve accurate simulation results, the fiber cross-sectional area was divided into triangular mesh units with the maximum size of the mesh unit of one-fifth of the wavelength. Furthermore, Sellmeier’s dispersion formula was included in the numerical model to analyze the proposed fiber’s frequency-dependent performance, like, the dispersions over S, C, and L communication bands.

An air hole-assisted FMF model was proposed (Fig.1), in which an elliptical air hole was inserted in the center of the elliptical ring core, and four circular air holes of various sizes were placed in the horizontal and vertical directions, respectively. When appropriate values for structure parameters (the size and position of circular air holes, the size and ellipticity of ring-core, and the size and ellipticity of elliptical air hole) are selected at 1550 nm, numerical simulations shows that 10 modes can be transmitted with effective refractive index differences between two adjacent modes up to 10^-4, resulting in polarization-maintaining (Fig.3-Fig.10). Regarding the transmission performance in the S, C, and L communication bands (Fig.12), the dispersions for 10 modes range from 17.6 ps/(nm·km) to 51.3 ps/(nm·km), and the effective refractive index differences between 1520 nm and 1600 nm are no less than 10^-4 (the bandwidth is 80 nm). Furthermore, compared with other fibers reported in relevant literatures (Table 1), the proposed fiber has a larger bandwidth and the same level of dispersion. Further simulations showed that the high refractive index core would inevitably lead to a significant increase in the core doping cost while having less influence on the effective refractive index differences in the low-order modes (Fig.13). Consequently, to balance the number of modes and the manufacturing cost, a low refractive index core is employed.

In this study, an air hole-assisted polarization-maintaining FMF was proposed. The influences of the size (d_1,d₂) and position (d_x,d_y) of circular air holes, the size (c_x) and ellipticity (c_x/c_y) of ring-core, and the size (a_x/c_x) and ellipticity (a_x/a_y) of elliptical air holes on transmission performance were investigated exploiting numerical model. According to the simulation results, when d₁=7 μm, d₂=7.6 μm, d_x=14.5 μm, d_y=13 μm, c_x=7.2 μm, c_x/c_y=1.4, a_x/c_x=0.36, a_x/a_y=1.4, the proposed FMF supports 10 mode transmission between 1520 nm and 1600 nm (the bandwidth is 80 nm), and the effective refractive index differences between two adjacent modes are greater than 10^-4 to attain polarization-maintaining transmission. Meanwhile, the dispersions for 10 modes range from 17.6 ps/(nm·km) to 51.3 ps/(nm·km). The proposed FMF may find applications for large-capacity SDM communication technology in the future.