Abstract
In this report, Josephson junction detectors for millimeter and submillimeter waves are surveyed. Emphasis is placed on heterodyne detectors (mixers). Others are briefly referred to, such as video detectors, bolometers, quasiparticle mixers and parametric amplifiers. Also, three kinds of Josephson junctions for detector use are mentioned and brief comments are given on them. These are tunnel junctions, thin film microbridges and point contacts.
Thermally cyclable oxide barrier tunnel junctions have been obtained and used as low noise quasiparticle mixers at 115GHz. They have lower noise temperatures compared to Josephson effect mixers, but the receiver noise temperatures are not lowered owing to their lower conversion efficiencies. Thin film microbridges suffer from low junction resistances and also from heating effects caused by d.c. bias supply and r.f. incident power. There seems to exist many problems to be overcome in order to make good submillimeter wave detectors by tunnel junctions or microbridges.
Point contacts are now widely used as sensitive detectors for millimeter and submillimeter waves. Although there exist difficulties for making rugged and thermally cyclable point contacts, the techniques for making them have made considerable progress, and a new construction of detector mount has appeared, which brought a low noise mixer at 115GHz thermally cyclable.
It is shown that the sensitivity of video detectors decreases as proportional to the inverse square of frequency. So, for submillimeter waves, video detectors are inferior to superconducting bolometers.
Recent progress in heterodyne detectors (fundamental and harmonic mixers) are touched on. They are superior to Shottky diode mixers in that they have much smaller NEP's and much higher conversion efficiencies (especially in harmonic mixers), and also in that they operate with much smaller LO power. Experimental curves of IF output power v.s. bias voltage are given for harmonic mixers with harmonic number N=7 and N=10. It is noted that harmonic mixers with even harmonic number are preferable, because they can operate at zero bias voltage. Among them, low even numbers 2 or 4 are recommended in regards to good conversion efficiency.
By using Josephson junctions, a peculiar parametric operation is possible, namely doubly degenerate mode. This mode operates under zero d.c. bias by choosing pump frequency as 2ωp=ωs+ωi, ωp-ωs-ωi. Zero bias operation allows the adoption of microbridge arrays. A parametric amplifier with 160 microbridge arrays was built at 33GHz, and showed amplifier gain of 15dB and noise temperature of 20K.
