Design and characterisation of terahertz planar hybrids

Abstract

Power splitting and combining are essential functions in electronic and optical instruments. These functions are typically implemented using a directional coupler, which is a passive component found in various forms across the electromagnetic spectrum. These couplers are used to combine power sources, distribute signals for balanced and sideband-separating mixers, or redirect portions of signals for monitoring. At terahertz frequencies, the multi-section branch guide coupler, implemented in E-plane split block waveguide technology, is the most common version. However, the fabrication of these couplers is constrained by tight tolerances and high aspect ratio features, making them scale poorly at higher frequencies. There is a need to revisit this simple component and explore alternative implementations for integration with semiconductor devices that are rapidly being developed at terahertz frequencies. In this work, three different planar hybrids operating in the 750 GHz to 1100 GHz frequency range are presented. Specifically, the branch-guide, broadside coupled line, and Lange couplers are evaluated through electromagnetic simulations. The 3-dB branch-guide coupler is fabricated and demonstrated on a 3-μm-thin silicon substrate using micro-fabrication and SOI technology. Gold beam leads are used to mechanically support the planar circuits in an E-plane split waveguide block. The branch-guide coupler, after de-embedding from the 18 mm long access waveguides, showed a measured bandwidth of 17% over which the isolation was better than 15 dB, with over 40 dB of isolation at the centre frequency of 934 GHz. Over this band, the through and coupled signals remained over -4 dB, with amplitude balance better than 0.25 dB. The measurements agree well with simulations. The broadside coupled line and Lange coupler designs, from simulations, promise a flatter amplitude response across the band and improved phase balance. These results demonstrate the promising potential of planar couplers and hybrids at terahertz frequencies, for their use in more advanced receiver and transmitter circuits.