Extended Projection Distance and Sidelobe Suppression of THz Bessel Beam With Combined Axicons

The Bessel beam, owing to its unique properties (keeping the intensity distribution unchanged during transmission, focusing energy on the central spot, self-reconstruction, etc.), has received extensive attention. In this study, a new type of combined axicon (CA) is designed that can generate a high-quality zero-order terahertz quasi-Bessel beam. The simulation and experimental results show that the quasi-Bessel beam generated by the CA has an obvious projection distance and sidelobe suppression effect. And the projection distance, length of the non-diffracting region, and sidelobe-suppression ratio of the beam can be modulated as required, which is not possible with ordinary axicon. The CA consists of a local negative axicon tightly bonded to an ordinary axicon. The quasi-Bessel beam generated by the CA exhibits enhanced intensity on the central axis. The influence of different parameters has been investigated, and the results have been explained using geometric optical. We believe that this element has broad application prospects in non-destructive testing, laser cutting, and microscopic imaging.