Submillimeter Wave Instrument Research Articles

1200 GHz High Spectral Resolution Receiver Front-End of Submillimeter Wave Instrument for JUpiter ICy Moon Explorer: Part I - RF Performance Optimization for Cryogenic Operation

The Submillimeter Wave Instrument (SWI) is a spectrometer/radiometer instrument on board of Jupiter Icy Moons Explorer (JUICE) mission operating in two submillimeter heterodyne bands around 530 μm (530 – 625 GHz) and 255 μm (1080 – 1275 GHz). The paper presents the results of acceptance testing and performance optimization carried out on the flight and spare model of the 1200 GHz receiver front-end. The safe operation conditions at 120-150 K ambient temperature, such as maximum authorized input RF power, applied DC bias voltage are determined through the definition of the Schottky junctions dissipated power and current density ranges. A tuning procedure that optimizes the sensitivity of the front-end while keeping it within safe limits at 120-150K temperature operation is discussed. In part II, we present a detailed analysis of reliability and endurance testing.

Transmission and Reflection Characterization of Polarizing Beam Splitters at Submillimeter Wavelengths

This paper presents a comparison of the transmission and reflection performance of different free-standing wire grids and photolithographic polarizers. This study has been carried out in the frame of the development work of the submillimeter-wave instrument (SWI) as part of the JUpiter and ICy moons Explorer mission of the European Space Agency. SWI is a passive heterodyne radiometer that is operating in the frequency bands from 530 to 625 GHz and from 1080 to 1275 GHz. The optical coupling network of SWI includes a polarizing beam splitter to separate the signal into the two channels. Simulations of the transmittance and reflectance in copolarization and in cross polarization have been performed at different incidence angles and polarizations to support the selection of the optimum beam splitter for SWI. The nominal and actual design parameters of the investigated polarizing beam splitters, measured with a confocal laser microscope, have been considered in the simulations. Measurements of the transmission and reflection losses in co- and in cross polarization have been performed at 636.5 GHz to validate these numerical calculations. The very low losses of the beam splitters in copolarization were accurately measured by means of a free-space cavity resonator setup. These measurements are in very good agreement with the simulations and radiometric measurements of the transmission loss which were performed with a breadboard model of the SWI receiver unit at 590.0 GHz. The lowest transmission losses among the studied beam splitters are observed for a free-standing wire grid with a single plane of gold-coated wires. The highest reflectance in copolarization and the lowest levels of cross-polarization leakage are determined for the photolithographic polarizers.

Reinforcement of Electromagnetic Wave Absorption Characteristics in PVDF-PMMA Nanocomposite by Intercalation of Carbon Nanofibers

With the recent developments in the millimeter and sub-millimeter wave instruments and devices, there is a need to develop electromagnetic (EM) wave absorbing materials in these frequency bands for applications like electromagnetic interference control, electromagnetic compatibility, etc. In this work, carbon nanofibers (CNF) were uniformly dispersed in a blend of poly(methyl methacrylate), polyvinylidene fluoride and cyanoacrylate for air spray coating a film on the cellulosic substrates. The samples were characterized for evaluation of their structure, morphology, electrical and EM absorption properties in 0.15–1.2 THz range by X-ray diffraction, field emission electron microscopy, I–V measurements and terahertz time domain spectroscopy. These coatings can conveniently be applied to the material surfaces by conventional air spray painting method, which makes this technique cost-effective as well as easy to deploy in various applications. The electrical conductivity enhancement in the samples has been attributed to the formation of conducting network by uniform distribution of CNFs in the insulating polymer matrix. As a result, the shielding effectiveness (SE) has been observed to improve with the increase in CNF’s loading in the polymer matrix. The SE is also a function of frequency, which is attributed to the increase in the skin depth. A SE of 20 dB has been estimated in these samples for the frequencies 1 THz and higher, which is of significant importance for the use of this technique in practical applications.

Optical Design and Analysis of the Submillimeter-Wave Instrument on JUICE

The submillimeter-wave instrument on the Jupiter icy moons explorer spacecraft is a passive dual-beam heterodyne radiometer operating in the frequency bands 530–625 and 1080–1275 GHz. The instrument will observe Jupiter's atmosphere as well as the atmosphere and surface properties of its moons. This paper presents the optical design and analysis of the instrument that has been carried out using Gaussian beam mode analysis and physical optics simulations. The optics consists of a 29-cm mechanically steerable off-axis Cassegrain telescope, relay optics, and two feedhorns. Frequency-independent operation of the 600-GHz channel is predicted by the simulations. The 1200-GHz channel shows some frequency dependency because of the selected feedhorn type. The steering of the telescope affects mainly its cross-polarization level. The manufacturing and mounting tolerances, as well as distortion and misalignment by thermoelastic effects, will cause the performance of a real instrument to deviate from that of an ideal one. Physical optics simulations combined with data from finite-element method simulations and measurements of optical surface profiles show that these factors affect, in particular, the instrument pointing. The induced pointing error is up to several arcminutes, whereas the specification is less than 0.5 arcmin. The pointing error can be reduced by adjusting the alignment of the telescope secondary mirror and the feedhorns.

THz Instruments for Space

Terahertz technology has been driven largely by applications in astronomy and space science. For more than three decades cosmochemists, molecular spectroscopists, astrophysicists, and Earth and planetary scientists have used submillimeter-wave or terahertz sensors to identify, catalog and map lightweight gases, atoms and molecules in Earth and planetary atmospheres, in regions of interstellar dust and star formation, and in new and old galaxies, back to the earliest days of the universe, from both ground based and more recently, orbital platforms. The past ten years have witnessed the launch and successful deployment of three satellite instruments with spectral line heterodyne receivers above 300 GHz (SWAS, Odin, and MIRO) and a fourth platform, Aura MLS, that reaches to 2520 GHz, crossing the terahertz threshold from the microwave side for the first time. The former Soviet Union launched the first bolometric detectors for the submillimeter way back in 1974 and operated the first space based submillimeter wave telescope on the Salyut 6 station for four months in 1978. In addition, continuum, Fourier transform and spectrophotometer instruments on IRAS, ISO, COBE, the recent Spitzer Space Telescope and Japan's Akari satellite have all encroached into the submillimeter from the infrared using direct detection bolometers or photoconductors. At least two more major satellites carrying submillimeter wave instruments are nearing completion, Herschel and Planck, and many more are on the drawing boards in international and national space organizations such as NASA, ESA, DLR, CNES, and JAXA. This paper reviews some of the programs that have been proposed, completed and are still envisioned for space applications in the submillimeter and terahertz spectral range.