Cryogenic photonic local oscillator for 2mm band SIS heterodyne astronomical receiver array

A single photomixer generated the local oscillator signal for a four-element focal plane array of niobium superconducting heterodyne receivers operating 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 a stainless steel waveguide to the 4 K cryostat stage. Distribution of LO power to each superconducting junction was accomplished via 17 dB broadwall waveguide couplers. Double sideband noise temperatures of the array were 60plusmn15K across the 129-174 GHz local oscillator band.