SIS Mixers as a Noise Detector for Optimization of Photonic Local Oscillators in Terahertz Range

We have demonstrated that a superconductor-insulator-superconductor (SIS) mixer is applicable for a power detector to evaluate the sideband noise radiated from a submillimeter-wave local oscillator (LO) under development. A simple analysis shows SIS mixers provide better resolution of LO noise than Schottky barrier diode mixers when the available LO power is less than ~ 0.4 mW. Systematic measurement of dependence of receiver noise temperature T _RX on components and operational conditions of a photonic LO (Ph-LO) is carried out, which provides a guide to their optimization, e.g., source-laser power, photodiode drive current, use of an optical filter, and selection of an optical amplifier. Noise temperature of the Ph-LO estimated from experimental results ranges from ~ 1 times 10³ K to ~ 5 times 10⁴ K as a function of operational conditions and parameters.