How do I design a quasi-optical system for terahertz signal routing and focusing?

A quasi-optical system for terahertz signal routing uses free-space Gaussian beam propagation with lenses, mirrors, and beam splitters instead of waveguides or transmission lines, because waveguide losses become prohibitively high at terahertz frequencies and conventional optical components are too large relative to the beam size. Quasi-optical design applies Gaussian beam theory, where the terahertz beam from a feedhorn propagates as a Gaussian beam with a defined waist size and divergence angle. The beam is routed and focused using off-axis parabolic mirrors (which are achromatic and have no material absorption), HDPE or TPX lenses (which have low terahertz absorption), and wire-grid polarizers or beam splitters for combining and separating signals. The key design parameters are the Gaussian beam waist location and size, which determine the mirror or lens focal lengths needed to relay the beam between components while maintaining beam truncation below 1% at each aperture.