Millimeter-wave Quasi-optical Mode Converter Test Study

At present almost 70% of the international gyrotrons utilize the built-in type of quasi-optical mode converter for horizontal output, while the domestic gyrotron's output mode is vertical, compact systems like in-vehicle system need an external quasi-optical mode converter to achieve the mode conversion from high order waveguide mode to quasi-optical Gauss mode. It's necessary to do research on the testing technology of quasi-optical mode converter for guiding its assembling and testing its performance. The study of quasi-optical mode converter testing technology focus on two parts, one is to design higher-order waveguide mode exciters to simulate the gyrotron's output to provide the necessary input conditions for the cold-test of optical mode converter; the other is the design of flat field distribution test system, implementation of accurate measurement of the exciter's output mode and the amplitude-phase distribution of quasi-optical mode converter beam. In-depth theoretical and experimental exploration of these two parts is presented in this paper.The prime methods of designing the exciter is quasi-optical conversion and gradual change including axial and radial change coupling of waveguide. The quasi-optical mode exciter with the method of quasi-optical conversion has high purity output mode, but its disadvantage is narrow bandwidth, low efficiency, complicated structure and many of its modules need high accuracy fabrication and assembling. And the exciter which outputs high order mode like TE₆₂ with the method of gradual change coupling of waveguide is too long for practical use, and its mode purity and conversion efficiency will be low as its many times change. Based on the coupling theory of the hole on the outer conductor wall of coaxial and the gradual change of circular waveguide, a 3mm waveband mode exciter of circular waveguide TE₆₂ is designed, which is mode converter of rectangular waveguide TE₁₀-circular waveguide TE₆₂, and its total length is 137.1mm. With the simulation of HFSS, the result is got, that the mode purity of TE₆₂ is 91.37% and its conversion efficiency is 91.27% at the center frequency of 94GHz, and the mode purity of TE₆₂ is higher than 87.63% and its conversion efficiency is higher than 87.38% in the frequency range from 93GHz to 95GHz. To sum up, the TE62 mode exciter has compact structure some bandwidth and high mode conversion, and its mode purity meets the requirements of practical use.Based on the method of scanning probe, a automatic testing system for millimeter microwave quasi-optical mode converter is designed, it can work on the condition of low or high power. This testing system gets the power of all points in a rectangular plane with the control of computer. The applying of the system reduces the personal test error and the man's workload, and it also greatly improves the testing accuracy and speed. 3mm and 8mm bandwidth quasi-optical mode converter was tested respectively with the testing system, and the testing results was in keeping with the results of theoretical calculation and simulation, so the testing system was proved to be precise and valid, and the performance of the quasi-optical mode converter also are got at the same time.