The emergence of new data services and the rapid development of cloud computing have put forward urgent demands for improving transmission capacity in optical interconnection networks. The capacity of traditional single-mode fibers is difficult to enhance due to the nonlinear Shannon limit. Space division multiplexing (SDM) technology has caught extensive attention for its ability to reach maximum capacity. Few-mode fibers are a typical design using SDM, and the mode crosstalk that occurs during the few-mode fiber transmission is a major problem. The multiple-input multiple-output (MIMO) digital signal processing technology is usually introduced at the receiver to solve this problem. However, as the number of guided modes increases, the system complexity will grow nonlinearly, resulting in significant power consumption. Therefore, the communication system should be simplified by suppressing the mode coupling from the root, which is separating the adjacent eigenmodes to the maximum extent. The polarization-maintaining few-mode fibers with special structures can improve the capacity and ensure a higher mode separation degree. However, the previous fiber designs ignore the contradiction between the lower-order mode separation and the higher-order mode separation. Thus, we take this as a breakthrough point to introduce concentric-circular stress-applying region, improving this contradiction relationship and increasing the effective refractive index difference between adjacent modes. Meanwhile, the polarization characteristics, mode effective area, wavelength dependence, mode dispersion, bending resistance, and other indicators are considered to yield better transmission performance and reliability.

To adapt to the development of optical interconnection networks with short-distance and large-capacity transmission, we propose a concentric-circular stress-applying region-assisted panda polarization-maintaining few-mode fiber. The prominent feature of this fiber is that this concentric-circular region is set around the elliptical-ring core. Since the core mainly determines the guided mode number, parameter optimization is first performed on the semi-major axis (b_x) and semi-minor axis (b_y) of the elliptical-ring core. Subsequently, the concentric-circular stress region with a lower refractive index is introduced between the core and the cladding to improve the effective refractive index difference between the higher-order modes and ensure the separation of fundamental modes. Comparison conducted on optical fibers with the same parameters without concentric-circular stress regions or stress regions of other shapes indicates that the concentric-circular stress region has an excellent ability to separate degenerate modes. Additionally, frequency sweeping is conducted at 1530-1570 nm to investigate the modal wavelength dependency and mode dispersion of the fiber. Finally, the beam propagation method (BPM) is adopted to simulate the fiber bending, and the bending resistance is analyzed. Our study provides ideas for the design of short-distance and large-capacity optical fibers.

Through the design and optimization of the fiber (Fig. 1), the results show that the proposed optical fiber can transmit 10 modes stably (Fig. 6). The introduction of concentric-circular stress-applying region in the structure can enhance the effective refractive index difference between higher-order modes by nearly an order of magnitude, balancing the lower-order mode separation and the higher-order mode separation (Fig. 4). The minimum effective refractive index difference between adjacent modes reaches 2×10^-4 at 1550 nm (Table 1) and not less than 1.8×10^-4 over the C-band. At 1530-1570 nm, mode dispersion is not higher than |-55.0219| ps·nm^-1·km^-1 (Fig. 7) and can be further reduced by increasing the semi-minor axis of the fiber core to better adapt to the short-distance transmission. In addition, the bending resistance of the fiber is analyzed. The results indicate that when the bending radius is no less than 9.5 cm, none of the 10 modes will be leaked into the cladding and the maximum bending-induced loss is in the order of 10^-7 dB/m.

We put forward a panda polarization-maintaining few-mode fiber with concentric-circular stress-applying region. The effects of elliptical-ring core size and concentric-circular stress region size on the effective refractive index of 10 modes and the effective refractive index difference between adjacent modes are studied. Numerical results show that by optimizing the parameters, this structure can separate the degenerate modes transmitted in the fiber. The mode characteristic changes before and after the introduction of the concentric-circular stress region are analyzed. It is proven that this low refractive index stress region can significantly improve the effective refractive index difference between higher-order modes. The bending resistance of the fiber is sound, with small mode dispersion. In addition, the fiber structure is expected to further increase the number of guided modes by changing the size of the core and other parts. The element of the concentric-circular stress-applying region is also suitable for designing other fiber structures. Our research has application value in the future optical interconnection transmission systems to provide a new idea for the development and design of polarization-maintaining few-mode fibers.