Cryogenic Wideband Quadrature Hybrid Couplers Implemented in a Low Temperature Superconductor Multilayer Process

This paper presents the design, fabrication and testing of two cryogenic 90° hybrid couplers operating at Ka and Ku bands. The hybrid couplers use a CPW based tandem coupled line architecture and are fabricated using a MIT Lincoln Lab multilayer process for superconducting circuits. The first quadrature hybrid demonstrates a wideband coupling performance from 26 GHz to 44 GHz, with measured return loss and isolation better than 24 dB, and insertion loss less than 0.5 dB. The second tandem coupler offers an excellent coupling performance from 8 GHz to 18 GHz. The on-chip device dimensions for the two hybrid couplers are 0.92 mm × 0.16mm and 2.43 mm × 0.16mm, respectively. For further miniaturization, an alternative compact hybrid coupler design for operation at Ku band is proposed having a footprint of only 0.41 mm × 0.35mm, a 64% reduction in device area when compared to the first Ku band design.