A dual-frequency quasi-optical output system for a THz gyro-multiplier

This paper presents the investigation of a quasi-optical output system for a fourth-harmonic gyro-multiplier in the THz band through geometric optic analysis and three-dimensional full-wave simulation. The output system of the gyro-multiplier needs not only to transfer two high-order waveguide modes into the quasi-Gaussian beams, but also to separate the two beams. A dual-frequency quasi-optical mode converter (DQMC) and a frequency selective surface (FSS) are employed to achieve both goals within this system. Our study has shown that the optimal design of the DQMC requires a special mode selection in the gyro-multiplier. The high-pass FSS is designed, fabricated, and experimentally verified by a THz-TDS system. We have also demonstrated that the designed FSS is capable of handling the high-power output from the gyro-multiplier.