Research On Terahertz Quasi-Qptical Mode Converter

The resources of the terahertz?THz?band in the electromagnetic spectrum have not been fully developed.As a fast-wave device,the gyrotron can generate continuous and stable terahertz waves.The mechanism is that the electron beams interact with the electric and magnetic field to radiate the electromagnetic waves.The emergence of the gyrotron solved the problem of lacking stable radiation source in the terahertz band and promoted research and application of terahertz waves.With the increase of microwave frequency and power,the working mode of the gyrotron gradually changed from low-order axisymmetric mode to high-order rotational polarization mode,which is not conducive to long-distance transmission.Therefore,a built-in quasi-optical mode converter is needed to transform the higher-order polarization mode into the quasi-Gaussian distribution.The quasi-optical mode converter constists mainly two parts,a launcher with irregular cut and several mirrors for the adjustment of the beam.The launcher can be further divided into the Vlasov type with a smooth waveguide wall and the Denisov type with small deformation on the waveguide wall.The perturbation structure of the Denisov-type launcher allows the working mode to couple out eight satellite modes.In addition to the working mode,the nine modes are superimposed on the spiral cut of the launcher according to a certain power ratio to generate a quasi-Gaussian beam.Compared with the Vlasov type,it can better optimize the mirror design and improve the conversion efficiency of the mode converter.The main work of the paper is as follows:1.Starting from Maxwell's equations,the field equation of the transverse electric wave in the circular waveguide is derived.Geometric description of the ray propagation in the cylindrical waveguide is given,and the method to make the launcher cut is clarified.By using the raised cosine function to approximate the two-dimensional Gaussian distribution and the coupled wave theory,the waveguide modes that can be superimposed into a quasi-Gaussian distribution are obtained.Furthermore,the relative power distribution between the waveguide modes is given.The perturbation structure of the cylindrical waveguide is obtained.2.Based on the geometric optics theory,the structure of the two reflective mirrors is designed.The coordinate parameters of the quasi-elliptical cylindrical mirror and the parabolic mirror in the global coordinate system are obtained.Using the vector diffraction theory,the field distributions on the mirror and the output windows are obtained.3.The 0.5THz-TE₈₅ mode Vlasov-type quasi-optical mode converter with a radius of 5mm is designed and verified by FEKO simulation software.The simulation results in FEKO are basically consistent with the numerical results in Matlab.Then the0.5THz-TE₈₅ mode Denisov-type quasi-optical mode converter with a radius of 3.8mm is designed and the perturbation structure is optimized.Finally,a launcher is designed based on the perturbation structure in which the relative power ratio of each mode is closest to the ideal situation.