The special wavelength position of terahertz waves makes them the link between microphotonics and macroscopic electronics. However, the terahertz wave transmission in free space is easily affected by the water vapor absorption in the air. The optical fiber structure is proposed to transmit terahertz waves and realize effective transmission control. Among them, the hollow-core THz fiber based on the anti-resonant principle can limit the wave transmission in the air fiber core, which greatly reduces the influence of material absorption. The optical fiber coupler is the key device for beam splitting and transmission tailoring. Due to the existence of absorption loss in THz fiber, the insertion losses of THz fiber couplers are usually large and affect their utilization. Although the transmission loss of THz fibers with hollow-core structures is low, the design of THz couplers with broadband beam splitting is generally difficult.

We propose a hollow-core anti-resonant fiber coupler based on the three-core symmetry structure. By employing its structure symmetry, broadband beam splitting can be realized, and the transmission loss of the coupler can be reduced via the hollow-core structure. The mode and coupling characteristics of the coupler are analyzed by the finite element method, and the relationship between the coupling length and the fiber structure parameters is verified. The mode loss characteristics of the coupler are analyzed, and then the beam splitting structure with low loss and wide bandwidth is obtained.

We design a hollow-core three-core anti-resonant fiber. At the frequency of 1 THz, the coupling length increases with the rising core distance (Fig. 4). As can be seen from the relation curve between D₁ and binding loss, since the even mode field expands more to the intermediate core, the increase in D₁ leads to the growing mode field, and the improvement in the binding ability of the core reduces the binding loss, while the odd mode is less affected by D₁ [Fig. 5(a)]. With the increasing D₁, the total loss of even mode obviously shows a downward trend, while the change of odd mode is not obvious. Compared with the mode loss of single-core fiber, the total odd-mode loss of the three-core structure is always smaller than that of the single-core structure in the shown interval. Therefore, using the three-core structure can actually obtain smaller mode losses. This is also consistent with the theory that the larger core size leads to lower mode losses [Fig. 5(c)]. The coupling length decreases slowly with the rising d_r when the separation hole spacing d is small. As d_r spacing increases, the coupling length decreases linearly (Fig. 6). Since the odd-mode field extends more to the intermediate core, the d_r increase is easier to increase its mode field, thereby reducing the mode binding loss, while the even-mode field is relatively independent of each core, and thus the change of d_r has little effect on it [Fig. 7(a)]. With the increasing d_r, the absorption loss shows an obvious downward trend, and the total mode loss is mainly determined by the absorption loss [Fig. 7(b)]. With the rising spacing d_r, the total loss of odd mode shows an obvious downward trend, while that of even mode mainly decreases under small d_r values, and the further increase in d_r value has little influence on it [Fig. 7(c)]. The relationship between the output optical power under the influence of mode transmission loss and transmission distance and coupling length is shown in Eq. (6) and Fig. 8. In all bandwidth ranges, the polarization-related loss is lower than 0.2 dB. The two polarization curves indicate that the coupler is not sensitive to polarization, the insertion loss is less than 3.5 dB in the range of 0.82-1.34 THz, and the bandwidth can reach 0.52 THz. This coupler is found to feature wide bandwidth and low loss transmission (Fig. 9). Transmission loss has less effect on the loss of the device (Fig. 10). The two-core coupler is in the range of 0.9-1.1 THz, the insertion loss is less than 3.45 dB, and the bandwidth is 0.2 THz. In the working bandwidth range, the two output ports cannot output the same power, and when the output power difference between the two ports is less than 0.1 dB, the corresponding bandwidth is 1 THz, with a large output power difference. The working bandwidth of the three-core structure coupler and the two-core structure coupler is relatively narrow (Fig. 12).

We design an anti-resonant air-core three-core terahertz fiber coupler with cycloolefin copolymer as the base material. The mode analysis and calculation of the new structure of the terahertz waveguide are carried out by COMSOL multi-physics simulation coupling software, and the mode field distribution among waveguides and the mode coupling characteristics between fiber cores are analyzed. Finite element analysis and full vector beam propagation method are employed to analyze the structural parameters, coupling length and loss effects, bandwidth, and other characteristics. The results show that the coupling length increases with the rising core spacing and decreases with the growing hole spacing, and the mode transmission loss has little effect on the insertion loss of the device. Due to the symmetry, the three-core structure can realize the uniform beam splitting of 1×2 light, the working bandwidth reaches 0.52 THz, and the insertion loss is less than 3.5 dB, with the polarization-related loss less than 0.2 dB.