Design Study of a Fundamental Mode Input Coupler for a 372-GHz Gyro-TWA I: Rectangular-to-Circular Coupling Methods

The design of two fundamental mode rectangular-to-circular waveguide input couplers for a low-terahertz gyrotron-traveling wave amplifier (gyro-TWA) is presented. A T-junction input coupler with a Bragg reflector and a multiple-hole directional coupler were optimized for operation between 360 and 384 GHz, the proposed gyro-TWA bandwidth. The T-junction coupler and the multiple-hole coupler achieved the respective bandwidths of 10% and 35%. The benefits and potential limitations of the low-terahertz wave coupler topologies are discussed alongside the challenging manufacturing methods of the submillimeter-wave components.