Selection and focusing of higher-order modes in a CW waveguide terahertz laser

Abstract

Background: The problems of selection and focusing of higher order modes of a dielectric waveguide laser are considered. The proposed and investigated scheme of mode selection in waveguide quasi-optical resonators can be used in the development and creation of new designs of laser systems with controlled characteristics for scientific and applied research - single-mode lasers with a given shape and polarization of the output beam. The results of laser beam focusing research can be used to solve problems related to the interaction of electromagnetic waves with matter: diagnostics of the surface of materials, thin films, biological objects, achieving subwavelength resolution of THz tomography, for radar and telecommunications applications, etc.

The aim of the work is to establish the physical laws of selection and focusing of wave laser beams of continuous radiation of the terahertz range with different spatial polarization.

Materials and methods: The matrix method was used to calculate the mode characteristics of a waveguide laser resonator with an inhomogeneous phase-stepped mirror. The Rayleigh-Sommerfeld vector theory was used to study the propagation and focusing of laser beams excited by modes of a waveguide quasi-optical resonator in different diffraction zones. Well-known methods of measuring the THz range were used for the experimental study of the studied phenomena.

Results: For the first time, a method of selection of higher EH_12q-mode in a terahertz laser resonator based on the placement of a groove 2.3 – 2.8 l wide on the surface of one of the resonator mirrors was proposed, theoretically substantiated and experimentally confirmed. This can significantly increase losses for all undesirable modes. At the same time, the losses for the higher EH_12q-mode remain practically unchanged, which creates conditions for its predominant excitation. Theoretical and experimental studies of moderate and sharp focusing in the free space of higher-order modes with different spatial polarizations of the dielectric waveguide resonator have been carried out.

Conclusion: It is shown that the proposed phase-stepped mirror with a groove effectively selects the required higher transverse mode. It is shown that the linearly polarized EH_12q -mode has the maximum field intensity in the focal region of the lens. For azimuthally polarized TE_02q - and TE_03q-modes, the central lobes have a maximum of the field, which is significantly shifted from the lens focus. With sharp focusing in the field distribution in the radially polarized TМ_02q - and TМ_03q-modes, an increase in axial intensity is observed. In this case, their central lobes, as in the higher TE_0nq -modes, are markedly offset from the focus of the lens.