A 550-ghz Dual Polarized Quasi-optical Sis Mixer

We describe the design, fabrication, and the performance of a low-noise dual-polarized quasi-optical superconductor insulator superconductor (SIS) mixer at 550 GHz. The mixer utilizes a novel crossslot antenna on a hyperhemispherical substrate lens, two junction tuning circuits, niobium trilayer junctions, and an IF circuit containing a lumped element 180 hybrid. The antenna consists of an orthogonal pair of twin-slot antennas, and has four feed points, two for each polarization. Each feed point is coupled to a two-junction SIS mixer, and therefore, for each polarization there are two IF outputs and four junctions, for a total of eight junctions on the chip. Due to the mixer structure and the use of series bias of the junction pairs, it turns out that the two IF outputs for a given polarization are 180 out of phase, requiring a 180 hybrid to combine the IF outputs. The hybrid is implemented using a combination of lumped element/microstrip circuit located inside the mixer block. Fourier transform spectrometer (FTS) measurements of the mixer frequency response show good agreement with computer simulations. The measured co-polarized and crosspolarized patterns for both polarizations also agree well with the theoretical predictions. The noise performance of the dual polarized mixer is excellent, giving uncorrected receiver noise temperature of better than 115K (DSB) at 528 GHz for both the polarizations.