In this paper, we will introduce a dual-THz-band SIS (Superconductor-Insulator-Superconductor) heterodyne radiometer system developed for the atmospheric profiling synthetic observation system project (APSOS). This THz system is intended to have a durable and compact design to meet the challenging requirements of remote operation. The system as well as its major components such as antenna tipping, quasi-optics, cryogenics, SIS mixers and FFTS backend will be discussed thoroughly. Some scientific simulation focusing on the atmospheric profiling components at THz bands will also be investigated.
Proc. SPIE. 8562, Infrared, Millimeter-Wave, and Terahertz Technologies II
KEYWORDS: Signal to noise ratio, Sensors, Superconductors, Interference (communication), Amplifiers, Field programmable gate arrays, Signal processing, Terahertz radiation, Signal generators, Signal detection
Terahertz Superconducting Imaging Array (TeSIA) is a project for the development of a large THz direct-detection array for mapping observations. The prototype of TeSIA is an 8×8 pixel direct-detection array operating at the 850 μm band, incorporating THz superconducting detectors such as kinetic inductance detectors (KIDs) and transition edge sensors (TES). KIDs have the advantage that only a broadband low noise cryogenic amplifier is needed at low temperature for the readout system, and hundreds of pixels could be readout simultaneously by the frequency-division multiplexing (FDM) technique. The readout system for a KIDs array is composed of several parts like excitation signal generating, intermediate frequency (IF) circuit, baseband signal acquisition and processing. Excitation signal is a kind of comb signal carrying various frequencies corresponding simply to resonant frequencies of the detectors. It is generated in baseband with a bandwidth covering all KIDs and up-converted to microwave frequency (about several gigahertz) to feed the detectors. With THz radiation, the forward transmission coefficient (S21) of all KIDs varies and the variation can be measured through the comb signal. Fast Fourier transform (FFT) with pipeline structure will be used to process the baseband excitation signal in real time. Consequently, the radiation intensity can be estimated by monitoring the signal amplitude and phase of the corresponding frequency channels. In this paper we will present mainly the design ofan electronic readout system for the 8×8 pixel array.
As a new attempt to expand wireless transmission channel bandwidth, a submillimeter-wave communication experiment
has been carried out by using techniques of frequency multiplication of millimeter-wave signal source and high-sensitivity
superconducting SIS (Superconductor-Insulator-Superconductor) receiver. The transmitter consists of a 500-GHz frequency quintupler and a 93-GHz varactor-tunable Gunn oscillator with its output frequency modulated via a 10-14 MHz signal source, while the 500-GHz SIS receiver has an IF (Intermediate Frequency) band of 1.1-1.7 GHz and an
overall system noise temperature below 400 K. A theoretical analysis is given to prove the principle and engineering
feasibility of the experimental communication system. The measured spectra of the received FM (Frequency
Modulation) signals with modulation frequencies of 10-14 MHz on a downconverted IF carrier of 1.6 GHz show that
effective direct frequency modulation on a transmitting submillimeter-wave carrier of 466 GHz and the reception of the
submillimeter-wave FM signals by the 500-GHz SIS receiver have been successfully realized.
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AlO<sub>x</sub>/Nb junction and Nb/SiO<sub>2</sub>/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region.
Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (<i>J<sub>c</sub></i>) of 10kA/cm<sup>2</sup>. In this paper, we describe their design, fabrication and preliminary experimental results.