Objective In this paper, a composite 7-core photonic crystal fiber is designed and investigated theoretically. By combining the step-index structure and the photonic crystal fiber structure, the inter-core crosstalk of the proposed fiber can be effectively reduced and the core density could be greatly improved, which provides a new opportunity for the realization of large-capacity and long-distance optical fiber space division multiplexing technology. The parameters of the fiber structure are analyzed by the theoretical analysis and research. At the wavelength of 1550 nm, the crosstalk between the intermediate core and the peripheral core is lower than -60 dB/km, the effective mode field area is over 90 μm 2, and the core spacing is up to 31.7 μm. When increasing the core number to 31, the relative core multiplexing factor can reach 8.78, which effectively improves the core density. This work has a guiding significance for the design of multi-core photonic crystal fiber for space division multiplexing technology and can be used in the network system which demands low crosstalk and large capacity for long distance transmission.
Methods Based on the theory of mode coupling and power coupling, the finite element method is used to calculate the crosstalk characteristics of the fiber. The cross-section and refractive index distribution of the composite multi-core photonic crystal fiber proposed in this paper are shown in Fig. 1. In order to suppress the crosstalk and nonlinear effects in the fiber, analyze the influence of the fiber structure parameters on the crosstalk and the effective mode area of each core, and select a set of initial structure parameters, as shown in Table 1, the theoretical analysis is at the wavelength of 1550 nm, the crosstalk and the effective mode area variation law of L=1 km in fiber propagation. In order to show the advantages of composite multi-core photonic crystal fiber in suppressing XT when the core spacing is small, this paper compares the designed composite 7-core PCF with the traditional step multi-core fiber and trench assisted multi-core fiber with known structural parameters. In order to further increase the transmission capacity of the optical fiber, the composite multi-core photonic crystal fiber has the characteristics of low crosstalk and small core spacing to expand the number of cores in the optical fiber from 7 to 31 cores.
Results and Discussion For the composite 7-core photonic crystal fiber, increasing the effective mode field area of the fiber is very important to increase the transmission capacity and overcome the nonlinear effect. Under the condition of ensuring the fiber single-mode transmission, appropriately increase the core size and reasonably control the core doping concentration, Using the air holes periodically arranged around the core to restrain the beam, by optimizing the design of the air hole structure to suppress crosstalk, a larger effective mode field area can be obtained at a lower doping concentration(Fig. 2). For the composite 7-core photonic crystal fiber designed in this paper, choosing the appropriate air hole parameters can achieve the effect of the smaller the distance between the cores, the smaller the crosstalk, so as to overcome the mutual restriction of low crosstalk and high-density in traditional multi-core fibers (Fig. 4). In the 31-core example presented in this article, the relative core multiplexing factor can reach 8.78, which has great advantages over the reported high-density multi-core fiber (Fig. 7).
Conclusions The composite 7-core photonic crystal fiber proposed in this paper has the characteristics of low crosstalk, high-density, and flexible design. By analyzing the influence of fiber core size, core-cladding relative refractive index difference, air hole spacing, and air hole structure parameters on the optical performance of the 7-core photonic crystal fiber, three sets of balance parameters are obtained, it balances single-mode transmission, larger mode field area, and lower crosstalk. The restriction relationship between the three sets of balance parameters can be obtained, and the crosstalk can be lower than -60 dB/km at the wavelength of 1550 nm, and the effective mode field area can exceed 90 μm 2. Comparing the composite 7-core photonic crystal fiber with the conventional step multi-core fiber and trench assisted multi-core fiber that have been reported, the results show that the 7-core photonic crystal fiber has more advantages in reducing crosstalk and distance between cores. It can effectively alleviate the mutual restriction of low crosstalk and high-density. On this basis, the number of fiber cores is expanded to 31-core, the minimum value of outer cladding thickness is calculated to be 42.5 μm, and the relative core multiplexing factor reaches 8.78. The composite 7-core photonic crystal fiber designed in this paper has broad application prospects in the direction of high-density, long-distance, and large-capacity information transmission systems.
