Suspended Stripline on Silicon-Membrane for Integrated Terahertz Couplers

Journal article, 2026

Directional couplers and power-distribution networks are critical components in balanced circuit topologies. At terahertz frequencies, they are commonly implemented using hollow rectangular waveguides. However, these technologies are inherently difficult to scale to shorter wavelengths and to integrate with active devices. This paper describes the design, fabrication, and characterization of a 925 GHz planar branch-line hybrid coupler implemented in suspended stripline technology, with a 3-μm silicon membrane as the supporting substrate. An initial 3-dB coupler design, based on circuit theory, is fine-tuned using full-wave electromagnetic simulations. The coupler is fabricated using microfabrication techniques and characterized between 750 GHz and 1100 GHz in a WM250 waveguide housing using a vector network analyzer. The hybrid shows measured isolation better than 15 dB and amplitude balance better than 1.1 dB over an 18% bandwidth. Electromagnetic simulations of the waveguide-packaged suspended stripline coupler are in good agreement with the scattering-parameter measurements. The use of suspended stripline technology enables the integration of directional couplers with active devices for terahertz balanced-circuit topologies, such as sideband separating mixers.