Qi-Jun Yao

Development of Superconducting Spectroscopic Array Receiver: A Multibeam 2SB SIS Receiver for Millimeter-Wave Radio Astronomy

We have developed a 3×3 multibeam sideband separation superconducting receiver-Superconducting Spectroscopic Array Receiver (SSAR)-for the 85-115 GHz frequency band. The receiver employs 2SB superconductor-insulator-superconductor (SIS) mixers with a typical single sideband (SSB) noise temperature of 60 K and image rejection ratio above 10 dB over the frequency band. Digital techniques are extensively applied in this receiver system for excellent operational stability and efficiency. They include fast Fourier spectrometers, digital LO and digital bias supplies. In our knowledge this is the first 2SB multibeam millimeter wavelength receiver in the world. This receiver has been successfully put into observation and its considerable enhancement of mapping speed has been demonstrated.

Conceptual design studies of the 5 m terahertz antenna for Dome A, Antarctica

As the highest, coldest and driest place in Antarctica, Dome A provides exceptionally good observing conditions for ground-based observations over terahertz wavebands. The 5m Dome A Terahertz Explorer (DATE5) has been proposed to explore new terahertz windows, primarily over wavelengths between 350 and 200 mu m. DATE5 will be an open-air, fully-steerable telescope that can function by unmanned operation with remote control. The telescope will be able to endure the harsh polar environment, including high altitude, very low temperature and very low air pressure. The unique specifications, including high accuracies for surface shape and pointing and fully automatic year-around remote operation, along with a stringent limit on the periods of on-site assembly, testing and maintenance, bring a number of challenges to the design, construction, assembly and operation of this telescope. This paper introduces general concepts related to the design of the DATE5 antenna. Beginning from an overview of the environmental and operational limitations, the design specifications and requirements of the DATE5 antenna are listed. From these, major aspects on the conceptual design studies, including the antenna optics, the backup structure, the panels, the subreflector, the mounting and the antenna base structure, are explained. Some critical issues of performance are justified through analyses that use computational fluid dynamics, thermal analysis and de-icing studies, and the proposed approaches for test operation and on-site assembly. Based on these studies, we conclude that the specifications of the DATE5 antenna can generally be met by using enhanced technological approaches.

Upgrade procedure for the Delingha 13.7-m telescope

The 13.7-m millimeter-wave radio telescope of Purple Mountain Observatory operates at 3200-m above the sea level near Delingha, Qinghai Province, China. Equipped with a superconducting SIS receiver, the telescope is used in the millimeter-wave band ranging from 85 to 115 GHz. An upgrade procedure is reported here which includes a superconducting SIS receiver, a new phase-locked local oscillator, a dedicated multi-line backend system, and a new control system based on industrial computer with PCI bus. With the dedicated multi-line backend system, the CO and isotopic lines around 110 GHz are obtained simultaneously. In recent years, scientific activities with this telescope have been focused on studies of Galactic molecular clouds and star formation regions, including surveys of molecular lines from IRAS sources and large-scale map of molecular clouds. Other programs include studies of the circumstellar envelope of late-type stars and interaction of Galactic supernova remnants with dense molecular gas.

Simulation of an integrated log-spiral antenna at terahertz

This paper presents the numerical simulation on the input impedance and beam pattern of a log-spiral antenna combined with an extended hemispherical lens. The effect of the lens on the input impedance of the antenna was studied by using HFSS (finite element method) and microwave studio CST (finite integration technique). A hybrid approach combining geometrical optics (GO) inside the lens and physical optics (PO) outside the lens was applied to calculate the far field pattern and Gaussian coupling efficiency of the integrated lens antenna. At 600 GHz the calculated maximum directivity of the hemispherical lens with an optimum extension length is about 37 dB and the corresponding Gaussian coupling efficiencies is about 76%. The off-axis performance of the integrated lens antenna was also examined.

The Superconducting Spectroscopic Array Receiver (SSAR) for Millimeter-wave Radio Astronomy

The studies on formation and evolution of interstellar molecular clouds and star formation give rise to high demands on imaging capability of millimeter-wave spectroscopic receiver. The Superconducting Spectroscopic Array Receiver (SSAR) of 3times3 pixels has been designed to work over 85-116 GHz. A number of new technological approaches are applied in the development of SSAR including of 4-K cooled superconducting SIS mixers using sideband separating scheme, automatically tuned digital bias, cooled SiGe HBT amplifiers, and digital FFT spectrometers. Development of the receiver system is undergoing. Upon completion, this instrument will enhance significantly the mapping capability in millimeter-wave line observations with high spectroscopic resolution. This paper introduces the main technological features applied in design and development of the receiver system.

Dependence of noise temperature of quasi-optical superconducting hot-electron bolometer mixers on bath temperature and optical-axis displacement

It is known that the increase of bath temperature results in the decrease of critical current of superconducting hot-electron bolometer (HEB) mixers owing to the depression of superconductivity, thus leading to the degradation of the mixers sensitivity. Here we report our study on the effect of bath temperature on the heterodyne mixing performance of quasi-optical superconducting NbN HEB mixers incorporated with a two-arm log-spiral antenna. The correlation between the bath temperature, critical current, LO power requirement and noise temperature is investigated at 0.5 THz. Furthermore, the heterodyne mixing performance of quasi-optical superconducting NbN HEB mixers is examined while there is an optical-axis displacement between the center of the extended hemispherical silicon lens and the superconducting NbN HEB device, which is placed on the back of the lens. Detailed experimental results and analysis are presented.

Atmospheric profiling synthetic observation system at THz

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.

A compact 500GHz SIS receiver system for POST

We introduced here a compact 500GHz SIS (Superconductor-Insulator-Superconductor) heterodyne receiver system developed for POST (POrtable Sub-millimeter Telescope). The system is proved to have a small volume to be integrated with the small Dewar, which is cooled by a compact close-cycled 100mW@ 4K cryo-cooler. Test results shows that a good LO coupling and low signal loss is achieved with a less than 250K overall receiver noise at 500GHz waveband.

A Fourier Transform Spectrometer for Site Testing at Dome A

Observations in tera-hertz astronomy can only be done at a site with good atmospheric transmission at millimeter and submillimeter wavelengths. With extremely dry weather and calm atmosphere resulted by high altitude and cold temperature, Dome A (or Dome Argus), Antarctica, is possibly the best site on this earth for THz astronomy. To evaluate the site condition there, we are constructing a Fourier Transform Spectrometer (FTS) based on Martin-Puplett interferometer to measure the atmospheric transmission in the frequency range of 0.75~15THz. The whole FTS system is designed for unattended and outdoor (temperatures even below -70 degrees Celsius) operation. Its total power consumption is estimated to be approximately 200W. This contribution will give a brief overview of this FTS development.

A design study on terahertz interferometry in Antarctica

Terahertz interferometry from high altitude site in Antarctica is proposed. Requirements for future terahertz astronomy are followed by comparison of interferometer technologies. Heterodyne interferometer can use matured technology and achieve high frequency resolution, whereas, direct detector interferometer can achieve observations in large bandwidth and wide field of view using focal plane arrays. As an example of direct detector interferometry multi-Fourier transform interferometer (MuFT) is introduced, which utilize Martin-Puplett type Fourier transform spectrometer, which is essential to realize multiplying interferometry to avoid atmospheric background fluctuation. As another example, photon counting interferometry is introduced. By implementing fast response FIR detectors to measure photon arrival time, visibility can be measured by the correlation of the photon statistics.