For the telescope number and size of the PFI proposal (20 x 8m), the Q-band (around 25 μm) was shown to be an optimal complement to submm- and optical interferometry, to detect substructures in proto-planetary disks. Here, broadband medium-resolution heterodyne would be an appropriate option, considering recent experimental findings on a quantum-limit heterodyne detection (HD) scheme. Under high thermal background and the very small beam filling factors in interferometry, HD was shown to be more sensitive than direct detection for the same bandwidth for wavelengths larger 3-8 micron, depending on the conditions. A new class of heterodyne mixers based on graphene could extend the instantaneous IF-bandwidth up to hundred GHz, for which photonic correlators may be developed since a resolution of R=30000 would be sufficient at 10μm. Such large IF-bandwidths could then even be multiplied in “dispersed heterodyne receivers” using optical frequency combs and photonic techniques derived from the DWDM technology.
For interstellar spectral line observations in the mm- and submm-wavelength range, low-noise, broadband heterodyne receivers are needed. Using a Niobium-Aluminum Oxide-Niobium SIS mixer element with a broadband integrated tuning circuit in a single-backshort waveguide mixer, we achieved a receiver noise temperature <=100K DSB from 225-255GHz without any backshort tuning. The lowest noise temperature with optimized backshort is 58K DSB at 230GHz with a mixer noise contribution of 27K.
The 1.1 THz multi-pixel heterodyne receiver will be mounted in the Nasmyth A cabin of the 12 m APEX telescope on the Chajnantor plateau, 5000 meters altitude in northern Chile. The receiver will cover the spectral window of 1000 - 1080 GHz, where important spectral lines like CO 9-8 at 1036.9 GHz, a tracer of warm and dense gas and OH+ at 1033 GHz and NH+ at 1012.6 GHz, both important for the study of chemical networks in the ISM, are located. The multi-pixel receiver greatly enhances the science output under the difficult observing conditions in this frequency range. Two 9-pixel focal plane sub-arrays on orthogonal polarizations are installed in easily removable cartridges. We developed a new thermal link to connect the cartridges to the cryostat. Our thermal link is an all-metal design: aluminum and Invar. All the optics is fully reflective, thus avoiding the absorption and reflection losses of dielectric lenses and reducing standing waves in the receiver. To guaranty internal optics alignment, we employ a monolithic integrated optics approach for the cold optics and the Focal Plane Unit (FPU) optics modeled after the CHARM (Compact Heterodyne Array Receiver Module) concept. The receiver uses synthesizer-driven solid-state local oscillators (LO) and the mixers will be balanced SIS mixers, which are essentially based on the design of the on-chip balanced SIS mixers at 490 GHz developed in our institute. Singleended HEB mixers are used for the laboratory tests of the optics. The LO power distribution is accommodated behind the FPU optics. It is composed of the LO optics, which includes a collimating Fourier grating, and an LO distribution plate to supply LO signal to each of the 9 pixels of the sub-array. Different options for the LO coupling design and fabrication are being analyzed and will be based on in-house hybrid waveguide/planar technology. We summarize the receiver project with emphasis on the cryogenics and the optics and present laboratory test results of the cryogenics, including the thermal link's performance. Beam pattern measurements of the receiver optics are scheduled for the coming days, but unfortunately could not be included in the current paper.
We present the design of a broadband superconductor-insulator-superconductor (SIS) mixer operating near
700 GHz. A key feature of our design is the utilisation of a new type of waveguide to planar circuit transition
comprising a unilateral finline taper. This transition is markedly easier to design, simulate and fabricate than the
antipodal finline we employed previously. The finline taper and the superconducting circuitry are deposited on
a 15 μm thick silicon substrate. The employment of the very thin substrate, achieved using Silicon-On-Insulator
(SOI) technology, makes it easy to match the incoming signal to the loaded waveguide. The lightweight mixer chip
is held in the E-plane of the waveguide using gold beam leads, avoiding the need for deep grooves in the waveguide
wall. This new design yields a significantly shorter chip, free of serrations and a wider RF bandwidth. Since
tuning and all other circuits are integrated on the mixer chip, the mixer block is extremely simple, comprising
a feed horn and a waveguide section without any complicated mechanical features. We employ a new type
of smooth-walled horn which exhibits excellent beam circularity and low cross polarisation, comparable to the
conventional corrugated horn, and yet is easier to fabricate. The horn is machined by standard milling with
a drill tool shaped into the horn profile. In this paper, we describe the detailed design of the mixer chip
including electromagnetic simulations, and the mixer performance obtained with SuperMix simulations. We also
present the preliminary measurements of the smooth-walled horn radiation patterns near the mixer operating
frequencies.
We report on developments of submillimeter heterodyne arrays for high resolution spectroscopy with APEX. Shortly, we will operate
state-of-the-art instruments in all major atmospheric windows accessible from Llano de Chajnantor. CHAMP+, a dual-color 2×7 element heterodyne array for operation in the 450 μm and 350 μm atmospheric windows is in operation since late 2007. With its
state-of-the-art SIS detectors and wide tunable local oscillators, its cold optics with single sideband filters and with 3 GHz of processed IF bandwidth per pixel, CHAMP+ does provide outstanding observing capabilities. The Large APEX sub-Millimeter Array (LAsMA) is in the final design phase, with an installation goal in 2009. The receiver will operate 7 and 19 pixels in the lower submillimeter windows, 285-375 GHz and 385-520 GHz, respectively. The front-ends are served by an array of digital wideband Fast Fourier Transform spectrometers currently processing up to 32×1.5 (optionally 1.8) GHz of bandwidth. For CHAMP+, we process 2.8 GHz of instantaneous bandwidth (in 16.4 k channels) for each of the 14 pixels.
Pre-HEAT is a 20 cm aperture submillimeter-wave telescope with a 660 GHz (450 micron) Schottky diode heterodyne
receiver and digital FFT spectrometer for the Plateau Observatory (PLATO) developed by the University of New South
Wales. In January 2008 it was deployed to Dome A, the summit of the Antarctic plateau, as part of a scientific traverse
led by the Polar Research Institute of China and the Chinese Academy of Sciences. Dome A may be one of the best sites
in the world for ground based Terahertz astronomy, based on the exceptionally cold, dry and stable conditions which
prevail there. Pre-HEAT is measuring the 450 micron sky opacity at Dome A and mapping the Galactic Plane in the
13CO J=6-5 line, constituting the first submillimeter measurements from Dome A. It is field-testing many of the key
technologies for its namesake -- a successor mission called HEAT: the High Elevation Antarctic Terahertz telescope.
Exciting prospects for submillimeter astronomy from Dome A and the status of Pre-HEAT will be presented.
GREAT, the German REceiver for Astronomy at Terahertz frequencies, is a first generation SOFIA dual channel
heterodyne PI−instrument for high resolution spectroscopy. The system is developed by a consortium of German
research institutes. The receiver will allow simultaneous observations in two out of the following three far−infrared
frequency bands:
* a low−frequency (1.4−1.9 THz) channel for e.g. the fine-structure lines of ionized nitrogen [NII] at 205μm
and ionized carbon [CII] at 158μm;
* a mid−frequency (2.4−2.7 THz) channel for e.g. the 112μm transition of HD; and
* a high−frequency (4.7 THz channel) for the 63 μm fine−structure line of neutral atomic oxygen.
Hot electron bolometers (HEB) mixers provide state of the art sensitivity. A spectral resolving power of up to
108 is achieved with chirp transform spectrometers, and a total bandwidth of 4 GHz at 1 MHz resolution is
reached with wide band acousto-optical spectrometers. The modular concept of GREAT allows to observe with
any combination of two out of the three channels aboard SOFIA. A more complete frequency coverage of the
THz regime by adding additional GREAT channels is possible in the future. The adaptation of new LO−, mixer−
or backend−techniques is easily possible.
We describe details of the receiver and the results of first performance tests of the system at 1.9 THz. As an
outlook to future developments we show first results obtained with phase locking a quantum cascade laser, the
most promising option for future high power local oscillators in the Terahertz regime.
GREAT, the German REceiver for Astronomy at Terahertz frequencies, is a first generation SOFIA dual channel
heterodyne PI-instrument for high resolution spectroscopy. The system is developed by a consortium of German
research institutes. The receiver will allow simultaneous observations in two out of the following three far-infrared
frequency bands:
a 1.4-1.9 THz channel for e.g. the fine-structure line of ionized carbon [CII] at 158μm;
a 2.4-2.7 THz channel for e.g. the 112μm transition of HD; and
a 4.7 THz channel for the 63 μm fine-structure line of neutral atomic oxygen.
Hot electron bolometers (HEB) mixers provide state of the art sensitivity. A spectral resolving power of up to
108 is achieved with chirp transform spectrometers, and a total bandwidth of 4 GHz at 1 MHz resolution is
reached with wide band acousto-optical spectrometers. The modular concept of GREAT allows to observe with
any combination of two out of the three channels aboard SOFIA. A more complete frequency coverage of the
THz regime by adding additional GREAT channels is possible in the future. The adaptation of new LO-, mixer-
or backend-techniques is easily possible. We describe details of the receiver and the results of first performance
tests of the system at 1.9 THz.
Superconducting phonon-cooled hot-electron bolometer (HEB) mixers are promising heterodyne detectors for THz frequencies. KOSMA is working to develop a heterodyne receiver for the GREAT receiver on SOFIA for observation at 1.7-1.9 THz using waveguide mixers. Waveguide mixers at these frequencies require very thin device substrates. We report on recent progress in fabricating and characterizing mixers on 2μm thick silicon nitride membranes which are suspended in a substrate channel. Heterodyne measurements with receiver noise temperatures of 1000K at 800GHz RF and 1GHz IF, at a 4.2K bath temperature show that fabrication of phonon-cooled HEBs on membranes is possible with a good noise performance. The HEB device was fabricated at KOSMA and consists of a 3-5nm thin NbTiN film on an AlN buffer layer.
In November 2003 the heterodyne receivers WANDA (polarization diplexed 492/810 GHz) and PoleSTAR (2x2 810 GHz array) of AST/RO (Antarctic Submillimeter Telescope and Remote Observatory, located at the South Pole) were upgraded with new 810 GHz SIS (Superconductor-Insulator-Superconductor) waveguide mixers from KOSMA. Profiting from device development for the HIFI (Heterodyne Instrument for the Far-Infrared) Band 2 SIS mixers of the Herschel Space Observatory, a factor of approx. 2 improvement in receiver noise temperature (from 1100 K to 550 K DSB) was achieved with WANDA. The SIS mixer devices employ low-loss NbTiN-Al tuning circuits and are fabricated using electron beam lithographic junction area definition and CMP (Chemical Mechanical Polishing) of the tuning circuit dielectric.
With the South Pole being one of the best possible sites for ground-based submillimeter astronomy, the 1.7 m telescope currently makes AST/RO well suited for sensitive, large scale spectral line mapping at 810 GHz. Low atmospheric opacity (tau < 1) and, consequently, very low system noise temperatures (< 3000 K) are regularly achieved at 810 GHz, making AST/RO an extremely sensitive observatory at these frequencies.
"First light" astronomical measurements made with the upgraded 810 GHz channel of WANDA towards the galactic HII region NGC 3576 in CO J=7-6 (806.65 GHz) and the neutral carbon [CI] 3P2-3P1 (809.3 GHz) lines are presented.
We are developing a 1.4 THz receiver to explore the far infrared
universe, such as probing high mass star forming regions using,
e.g., the high J transitions of CO, investigating the warm
interstellar medium in N+ or probing cold and dense stellar cores in H2D+. Due to the poor atmospheric transmission at these frequencies we are planning to use this modular receiver on high altitude ground based observatories, for example the Atacama Pathfinder EXperiment (APEX), and as an additional channel for the German Receiver for Astronomy at Terahertz Frequencies (GREAT) on the Stratospheric Observatory For Infrared Astronomy (SOFIA) located on an airplane. To allow successful astronomical observations under poor atmosphere transmission a low receiver noise temperature and a high receiver stability are mandatory. To achieve a low receiver temperature the main effort is directed to develop phonon-cooled NbTiN HEB mixers. For optimum coupling with the telescope beam and easier alignment we are focussing on waveguide mixers. A phase locked Gunn (114-135 GHz) and three multipliers will be used as the Local Oscillator (LO) of 1370-1500 GHz. A liquid He Dewar will be used for operation on SOFIA and a closed-cycle system with a pulse-tube cooler on APEX. Initially, we are aiming for 1 GHz IF bandwidth (214 kms-1 at 1.4 THz) sufficient for galactic
observations. First tests and astronomical observations with a similar but lower frequency HEB at 800 GHz have yielded encouraging results.
We have proposed to develop a prototype 0.5-meter far-infrared telescope and heterodyne receiver/spectrometer system for fully-automated remote operation at the summit of Dome A, the highest point on the Antarctic plateau. The unparalleled stability, exceptional dryness, low wind and extreme cold make Dome A a ground-based site without equal for astronomy at infrared and submillimeter wavelengths. HEAT, the High Elevation Antarctic Terahertz Telescope, will operate in the atmospheric windows between 150 and 400 microns, in which the most crucial astrophysical spectral diagnostics of the formation of galaxies, stars, planets, and life are found. At these wavelengths, HEAT will have high aperture efficiency and excellent atmospheric transmission most of the year. The proposed superheterodyne receiver system will be comprised of 0.8, 1.4 and 1.9 THz channels which will observe the pivotal J=7-6 line of CO, the J=2-1 line of atomic carbon, and the far-infrared fine structure lines of N+ and C+, the brightest emission lines in the entire Milky Way Galaxy. When combined with the HEAT telescope, the receiver system represents a uniquely powerful instrument for reconstructing the history of star formation in our Galaxy, with application to the distant Universe. The receiver system itself serves as a valuable testbed for heterodyne Terahertz components, using leading-edge mixer, local oscillator, low-noise amplifier, cryogenic, and digital signal processing technologies that will play essential roles in future Terahertz observatories. The proposed study will pave the way for future astronomical investigations from Dome A.
We present the concept for KOSMA's 16 element 1.9 THz heterodyne array
STAR (SOFIA Terahertz Array Receiver) which is being developed for
SOFIA. The instrument will consist of two interleaved sub-arrays of 8
pixels each. Together we will have a 4 × 4 pixel array with a beam spacing on the sky of approximately 1.5 times the beam size of 15 arcsec (FWHM). The receiver is mainly targeted at measuring the fine structure transition of ionized atomic carbon at 1.9 THz (158 microns). STAR's optics setup is modeled after the successful design used in KOSMA's SMART receiver. It will contain a K-mirror type beam rotator, a Martin-Puplett diplexer for LO coupling and an LO multiplexer using imaging Fourier gratings. Complete optical sub-assemblies will be machined monolithically as integrated optics units, to reduce the need for optical alignment. STAR will probably use waveguide mixers with diffusion cooled hot electron bolometers, which are being developed at KOSMA. The receiver backends will be KOSMA Array-AOSs. Local oscillator power will be provided by a backward wave oscillator (BWO), followed by a frequency tripler.
GREAT - a heterodyne instrument for high-resolution spectroscopy aboard SOFIA is developed by a consortium of German research institutes. The first-light configuration will allow parallel observations in two far-infrared frequency bands. We will have a choice of back-ends, including a broad-band acousto-optical array and a high-resolution chirp transform spectrometer. We describe the structural and quasi-optical design of the receiver, update on the front-end and back-end developments and discuss the data acquisition system.
We report the successful operation of a 700 GHz SIS finline mixer employing a Nb tunnel junction and Nb transmission lines. In particular, we discuss the properties of a new mixer feed and the influence of tuning on the mixer performance. Experimental and simulation work shows that the performance of the mixer below the superconducting gap is strongly dependent on the electrical properties of the tuning stub, while at frequencies above the gap the mixer performance is dominated by both tuning and transmission line losses.
We present the first results obtained with our new dual frequency SIS array receiver SMART The instrument is operational since September 2001 at the KOSMA 3m telescope on Gornergrat near Zermatt/Switzerland. The receiver consists of two 2×4 pixel subarrays. One subarray operates at a frequency of 490 GHz, the other one at 810 GHz. Both subarrays are pointed at the same positions on the sky. We can thus observe eight spatial positions in two frequencies simultaneously. For the first year of operation we installed only one half of each subarray, i.e. one row of 4 mixers at each frequency.
The receiver follows a very compact design to fit our small observatory. To achieve this, we placed most of the optics at ambient temperature, accepting the very small sensitivity loss caused by thermal emission from the optical surfaces. The optics setup contains a K-mirror type image rotator, two Martin-Puplett diplexers and two solid state local oscillators, which are multiplexed using collimating Fourier gratings. To reduce the need for optical alignment, we machined large optical subassemblies monolithically, using CNC milling techniques. We use the standard KOSMA fixed tuned waveguide SIS mixers with Nb junctions at 490 GHz, and similar Nb mixers with Al tuning circuits at 810 GHz.
We give a short description of the front end design and present focal plane beam maps, receiver sensitivity measurements, and the first astronomical data obtained with the new instrument.
A consortium of German research laboratories has been established for the development of a modular dual-channel heterodyne instrument (GREAT: German Receiver for Astronomy at Terahertz Frequencies) for high-resolution spectroscopy aboard SOFIA. The receiver is scheduled to be available in time for SOFIA's very first astronomical mission in late 2002. The first-flight version will offer opportunities for parallel observations in two frequency bands. We will have a choice of backends, including an acousto-optic array (4 X 1 GHz) and a high-resolution chirp transform spectrometer.
We describe the receiver concept for KOSMA's planned second generation SOFIA instrument STAR (SOFIA Terahertz Array Receiver). The receiver will contain a 4 X 4 element heterodyne mixer array for the frequency range from 1.7 to 1.9 THz (158 to 176 microns). Its main scientific goal is large scale mapping of the 158 micron fine structure transition of singly ionized carbon. The design frequency range covers this line out to moderate red shifts and also allows to observe a variety of other spectral lines.
We describe the preliminary design of the proposed Heterodyne Instrument for FIRST (HIFI). The instrument will have a continuous frequency coverage over the range from 480 to 1250 GHz in five bands, while a sixth band will provide coverage for 1410 - 1910 GHz and 2400 - 2700 GHz. The first five bands will use SIS mixers and varactor frequency multipliers while in the sixth band a laser photomixer local oscillator will pump HEB mixers. HIFI will have an instantaneous bandwidth of 4 GHz, analyzed in parallel by two types of spectrometers: a pair of wide-band spectrometers (WBS), and a pair of high- resolution spectrometer (HRS). The wide-band spectrometer will use acousto-optic technology with a frequency resolution of 1 MHz and a bandwidth of 4 GHz for each of the two polarizations. The HRS will provide two combinations of bandwidth and resolution: 1 GHz bandwidth at 200 kHz resolution, and at least 500 MHz at 100 kHz resolution. The HRS will be divided into 4 or 5 sub-bands, each of which can be placed anywhere within the full 4 GHz IF band. The instrument will be able to perform rapid and complete spectral line surveys with resolving powers from 103 up to 107 (300 - 0.03 km/s) and deep line observations.
We describe frontend concepts for the future heterodyne array instruments of the KOSMA 3-m telescope and for SOFIA. For KOSMA we are currently developing a dual frequency (400 - 500, 800 - 900 GHz) SIS mixer array of four elements per frequency band. For SOFIA, we are planning an up to 4 X 4 element array for 1.6 - 2.0 THz using superconducting hot-electron bolometers. The small number of pixels allows us to keep the optics relatively compact. For the same reason, a single sideband filter is not included. The local oscillator power will be distributed using Dammann gratings. Motivated by the excellent beam characteristics of waveguide horns we are planning to extend the range of our waveguide mixers to 2 THz. The mixers are based on the wideband tunerless mixers that have been successfully used in single element telescope receivers at KOSMA. The mixers will be standard building blocks mounted at the back of waveguide horns integrated into the optical setup. Local oscillators for 400 - 900 GHz are solid state sources, for the Terahertz array we are developing several alternative local oscillator concepts.
For low noise performance of SIS junctions as heterodyne mixing elements a common requirement is an coRC of the junction of around 3-4. For R=505), co=27t•230 GHz, and a specific capacitance of 60 fF/µ,m2, this means a junction area of 0.69 ptm2, i.e. submicron area junctions. Junctions of these dimensions are difficult to fabricate. Using arrays of junctions to reduce the effective capacitance results in the necessity of high current densities and requires good junction uniformity. Using an integrated superconducting impedance matching structure, we were able to get good performance at 230 GHz with single junctions as large as 2.5 IA m2 (Nb-A10x-Nb junctions fabricated at KOSMA, University of Cologne). An additional advantage of the large size is the low magnetic field necessary to suppress the Josephson currents. The matching structure is described in a companion paper [1]. It consists of a loaded stub compensating for the junction capacitance and two ',za /4 transformer sections which transform the real part of the resulting impedance. The waveguide mixer uses a backshort and an E-plane tuner in full height waveguide and was optimized together with the RF filter by scale modeling to give a maximum impedance tuning range [2]. A sketch of the test receiver is shown in Fig. 1. The IF HEMT amplifier has a noise temperature of 8K and was developed in house.
A monolithic grid oscillator has been fabricated that oscillates in Ka-band. The grid consists of 36 heterojunction bipolar transistors (HBT's) at intervals of 1 mm. The grid was built on a 740 pm thick GaAs substrate with gold evaporated on the back side. The oscillation frequency is 34.7 GHz, with an effective radiated power (ERP) of 170 mW at a DC bias power of 370 mW. The is the first report of a successful monolithic grid oscillator.
RF coupling to Superconductor-Isolator-Superconductor (SIS) junctions suffers from their large geometric capacitance (45 to 60 fF/μm2 for Nb-Al2O3-Nb junctions). Junction areas smaller than 1 μm2 would be necessary for submillimeter wavelengths but pose considerable difficulties in fabrication. We show mixer performance calculations for moderately sized junctions using an integrated open circuited microstrip stub to tune out the junction capacitance.
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