Bidirectional overmoded thz coupler

Abstract

A low-loss bidirectional HEcoupler is disclosed with substantially higher directivity than prior HETHz couplers for overmoded waveguides operating in a frequency band centered about f, where fis between 0.03 to 1.5 THz, with mid-band free-space wavelength λ. The coupler comprises a main hollow-metal waveguide, aligned along the z axis, of inner radius r, where ris greater than 2λ, a first end of the main waveguide is identified as port-1 and its second end is identified as port-2. A first transverse lofted transition waveguide couples a port-3 of radius r, where r<r, to a substantially elliptical opening of mean radius greater than rin one side of the main waveguide. A similar second tapered lofted transition waveguide, aligned on the same axis as the first lofted transition, identified as the y axis, couples a port-4 to a substantially elliptical opening in the opposite side of the main waveguide. A substantially elliptical dielectric mirror, centered with respect to the intersection of the axes of the main waveguide and the transverse coupling waveguides, rotated at approximately 45° with respect to the xy plane about the x axis for HEE-field polarization in the y direction, supported on its +x and −x sides, has major and minor transverse dimensions sufficient to intersect a substantial fraction of the cross-sectional area of the beam in the main waveguide. The mirror is further characterized as having thickness greater than λ/10 and less than λ, where λis the wavelength in the mirror at f. The mirror dielectric is further characterized as typically having dielectric constant εless than 3.7 and loss tangent less than 0.02. The coupler is further characterized as having portions of the inside surfaces of waveguides near their intersections coated with a microwave absorptive material that is characterized as having loss tangent greater than 0.01 and thickness greater than λ/5, where λis the wavelength in the absorptive coating at f. Other inner surfaces of the waveguides may be bare metal, or corrugated, or laminate lined. A polarizer may be included at port-1 and/or port-2 to partially block s-polarization, here polarization with E field in the x direction. The polarizer may be formed by etching an array of narrow traces on copper-clad low-loss laminate of substrate thickness approximately λ/2, where λis the wavelength in the polarizer substrate at f. The traces are further characterized as having periodicity approximately λ/4, trace width less than λ/12, and being aligned in the x direction.