This study designs a biconical axicon to overcome the limitation of a short non-diffraction distance of terahertz Bessel beams generated by a traditional axicon. The traditional axicon is widely used to generate terahertz Bessel beams because of its simple structure and high conversion efficiency. However, when the radius of the incident terahertz wave is fixed, the non-diffracting distance of the terahertz Bessel beam is inversely proportional to the base angle and refractive index of the axicon. At present, the materials used to make terahertz lenses are mostly high-density polyethylene (HDPE), and the refractive index changes little with the frequency of the terahertz wave. Most studies increase the non-diffracting distance of terahertz Bessel beams by reducing the base angle of the axicon. However, the small-angle axicon is prone to produce errors in the processing process, which affects the quality of terahertz Bessel beams. Hence, the traditional axicon has certain limitations in generating terahertz Bessel beams with a long non-diffracting distance, which restricts the application of terahertz Bessel beams in some fields such as large depth-of-focus imaging and nondestructive testing. Therefore, on the basis of the traditional axicon, we design a biconical axicon with a simple structure, which does not require complex optical path adjustment to generate terahertz Bessel beams with a long non-diffracting distance.

The theory of geometric optics is used to analyze the principle of generating terahertz Bessel beams by the biconical axicon and derives the expression of the non-diffracting distance of terahertz Bessel beams. Then, the optical field distribution of the terahertz wave after passing through the biconical axicon is analyzed with the integral theory of diffraction. The finite-difference time-domain method is employed to simulate and analyze the transmission characteristics of terahertz Bessel beams and the influence of different processing errors on the transmission characteristics. Finally, the function of the biconical axicon is verified by the transmission terahertz time-domain spectroscopy system.

By the finite-difference time-domain method, the traditional axicon with a base angle of 20° and the biconical axicon with base angles of γ₂=20°and γ₁=15° are simulated and calculated. The non-diffracting distance of the terahertz Bessel beams generated by the terahertz wave through the traditional axicon is 27.55 mm. Under the same parameter conditions, the biconical axicon can generate terahertz Bessel beams with a non-diffracting distance of 110.18 mm, namely that the non-diffracting distance is broadened by 82.63 mm (Fig. 5). The non-diffracting distance increases as the base angle of the biconical axicon grows (Fig. 6). Meanwhile, the simulations show that the vertex alignment error does not affect the non-diffracting distance of the terahertz Bessel beams when it does not exceed 0.2 mm (Fig. 8). The experimental results demonstrate that the terahertz Bessel beams with a non-diffracting distance of 110 mm can be generated by the terahertz wave through the biconical axicon with base angles of γ₂=20° and γ₁=15° (Fig. 11), which is consistent with the simulations.

In this paper, a biconical axicon capable of generating terahertz Bessel beams with a long non-diffracting distance is designed. The theory of geometrical optics is used to analyze the generation mechanism of terahertz Bessel beams, and the diffraction theory is applied to derive the expression of the optical field distribution of terahertz Bessel beams. The transmission characteristics of non-diffracting beams and the influence of processing errors on transmission characteristics are simulated and analyzed. The simulations show that compared with the traditional axicon, the biconical axicon can generate terahertz Bessel beams with a longer non-diffracting distance. The non-diffracting distance increases with the rise in the base angle of the conical bottom surface, and the vertex alignment error does not affect the non-diffracting distance of the terahertz Bessel beams when it does not exceed 0.2 mm. Meanwhile, a transmission terahertz time-domain spectroscopy system is built to verify the function of the biconical axicon. The biconical axicon with base angles of γ₂=20° and γ₁=15° is used to generate terahertz Bessel beams with a non-diffracting distance of 110 mm. Compared with the case of the traditional axicon, the non-diffracting distance is broadened by 82.63 mm. The experimental results are consistent with the theoretical simulations. The results indicate that the biconical axicon can generate terahertz Bessel beams with a long non-diffracting distance, which improves the practicability of terahertz Bessel beams in terahertz nondestructive testing.