Photonic local oscillator operating at 77 k for a 2 mm band SIS astronomical heterodyne receiver array

A single photomixer generated the local oscillator signal for a four element focal plane array of niobium superconducting heterodyne receivers working at 4 K. The photomixer incorporated an InGaAsP photodiode, driven by two 1.55 mum laser beams, that operated at an ambient temperature of 77 K. Its output signal propagated through stainless steel waveguide to the 4 K cryostat stage. Distribution of LO power to each superconducting junction was accomplished via broadwall waveguide couplers. Double sideband noise temperatures of 40plusmn5 K , integrated over the 3.85 to 7.85 GHz IF band, were measured at LO frequencies from 132 to 175 GHz.