Changing the Parameters of Tightly Focused THz Laser Beams

The possibility for changing the parameters of the focal region of tightly focused laser beams excited by the modes of resonator of a THz laser based on a hollow circular dielectric waveguide was studied theoretically and experimentally. In the experiment, the focusing system was a short-focus lens with the central region covered by absorbing masks. The focal area parameters were changed by varying the size of the masks. The electric field components of the modes under study were theoretically calculated using the integral Rayleigh-Sommerfeld transformations under the non-paraxial approximation. The use of absorbing masks made it possible to reduce the beam diameter in the focal region and increase the depth of focus of the linearly polarized EH11-mode and azimuthally polarized TE01-mode. The results of this work can be implemented in medical diagnostics, high-speed communications, and environmental monitoring.