Quasi-Optical Mode Converter for a 0.42 THz TE17,4 Gyrotron

A quasi-optical mode converter (QOMC) for a 0.42 THz, TE17,4 mode pulsed gyrotron oscillator is designed, fabricated and tested. Based on the coupled-wave theory, geometric optics theory and vector diffraction theory, a computer code is developed to design and optimize the QOMC which converts the high order TE17,4 mode into a fundamental Gaussian distribution. The numerical results show that the scalar fundamental Gaussian mode content of the wave beam at the output window is about 98.1%. The experiment result shows that a well-focused Gaussian-like beam with 95.2 % scalar Gaussian mode content has been obtained by using an infrared video camera. The measured results agree well with the theoretical predictions.