The Atacama Cosmology Telescope (ACT) is a new, 6-meter aperture, millimeter-wave telescope that will be located at an altitude of 5200 meters in the Chilean Andes. ACT will be fielded with the Millimeter Bolometric Array Camera (MBAC). The MBAC will incorporate arrays of "pop-up" bolometers using multiplexed Transition Edge Sensors (TES), developed at NASA/GSFC and NIST. The camera will consist of three arrays for multi-frequency ground-based millimeter observations near 150, 220 and 270 GHz. The goal is to map the cosmic microwave background and discover Sunyaev-Zel'dovich clusters. Associated follow-up observations of the clusters will be taken at optical and X-ray wavelengths.

A new 10 meter diameter telescope is being constructed for deployment at the NSF South Pole research station. The telescope is designed for conducting large-area millimeter and sub-millimeter wave surveys of faint, low contrast emission, as required to map primary and secondary anisotropies in the cosmic microwave background. To achieve the required sensitivity and resolution, the telescope design employs an off-axis primary with a 10 meter diameter clear aperture. The full aperture and the associated optics will have a combined surface accuracy of better than 20 microns rms to allow precision operation in the submillimeter atmospheric windows. The telescope will be surrounded with a large reflecting ground screen to reduce sensitivity to thermal emission from the ground and local interference. The optics of the telescope will support a degree field of view at 2mm wavelength and will feed a new 1000-element micro-lithographed planar bolometric array with superconducting transition-edge sensors and frequency-multiplexed readouts. The first key project will be to conduct a survey over &dbigwig;4000 degrees for galaxy clusters using the Sunyaev-Zel'dovich Effect. This survey should find many thousands of clusters with a mass selection criteria that is remarkably uniform with redshift. Armed with redshifts obtained from optical and infrared follow-up observations, it is expected that the survey will enable significant constraints to be placed on the equation of state of the dark energy.

A new Combined Array for Research in Millimeter-wave Astronomy (CARMA) interferometer is being assembled from the existing Owens Valley Radio Observatory (OVRO), the Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometers and the new Sunyaev?Zeldovich Array (SZA) at Cedar Flat, a site at 2,200 m altitude in the Inyo Mountains east of OVRO. The array will consist of 23 antennas of three different diameters, 3.5, 6.1 and 10.4 m, and will support observations in the 1 cm, 3 mm and 1.3 mm bands. The fist-light correlator is a flexible FPGA based system that will process up to 8 GHz of bandwidth on the sky for two subarrays consisting of 8 and 15 elements. The array configurations will offer antenna spacings from 5 m to 1.9 km allowing unprecedented high resolution and wide field imaging at millimeter wavelengths. Radiometers observing the 22 GHz water vapor emission line will be used to measure and correct for the water vapor induced path delay along the line of sight for each telescope and thereby minimize the time lost to “bad seeing”. This university based facility will emphasize technology development and student training along with leading edge astronomical research in areas ranging from Sunyaev-Zeldovich effect galaxy cluster surveys to studying protoplanetary disks.

Advances in bolometric detector technology over the past decade have allowed submillimeter wavelength measurements to contribute important data to some of the most challenging questions in observational cosmology. The availability of large format bolometer arrays will provide observations with unprecedented image fidelity. The Balloon-borne Large Aperture Submillimeter Telescope (BLAST) will be one of the first experiments to make full use of this new capability. The high altitude (~35$ km) of the balloon platform allows for high-sensitivity measurements in the 250, 350 and 500 micron bands with a total of 260 detectors.

Cornell and Caltech are undertaking a two year conceptual design study for a 25-m class sub-mm telescope. The nominal location for this facility will be the high Atacama Desert of Northern Chile. The baseline design is a segmented mirror telescope optimized for operation at wavelengths longer than 200 microns to take advantage of a low precipitable water vapor at the site. We discuss science drivers and their implications for telescope design and technical requirements, and planned technical study areas.

SCUBA-2, which replaces SCUBA (the Submillimeter Common User Bolometer Array) on the James Clerk Maxwell Telescope (JCMT) in 2006, is a large-format bolometer array for submillimeter astronomy. Unlike previous detectors which have used discrete bolometers, SCUBA-2 has two dc-coupled, monolithic, filled arrays with a total of ~10,000 bolometers. It will offer simultaneous imaging of a 50 sq-arcmin field of view at wavelengths of 850 and 450 microns. SCUBA-2 is expected to have a huge impact on the study of galaxy formation and evolution in the early Universe as well as star and planet formation in our own Galaxy. Mapping the sky to the same S/N up to 1000 times faster than SCUBA, it will also act as a pathfinder for the new submillimeter interferometers such as ALMA. SCUBA-2's absorber-coupled pixels use superconducting transition edge sensors operating at 120 mK for performance limited by the sky background photon noise. The monolithic silicon detector arrays are deep-etched by the Bosch process to isolate the pixels on silicon nitride membranes. Electrical connections are made through indium bump bonds to a SQUID time-domain multiplexer (MUX). We give an overview of the SCUBA-2 system and an update on its status, and describe some of the technological innovations that make this unique instrument possible.

Bolocam is a millimetre-wave (1.1 and 2.1 mm) camera with an array of 119 bolometers. It has been commissioned at the Caltech Submillimeter Observatory in Hawaii and is now in routine operation. Here we give an overview of the instrument and the data reduction pipeline. We discuss models of the sensitivity of Bolocam in different observing modes and under different atmospheric conditions. We briefly discuss observations of star-forming Galactic molecular clouds, a blank field survey for sub-millimeter galaxies, preliminary results of a blank-field CMB secondary anisotropy survey and discuss observations of galaxy clusters using the Sunyaev-Zel'dovich effect.

We describe the development of an antenna-coupled bolometer array for use in a Cosmic Microwave Background polarization experiment. Prototype single pixels using double-slot dipole antennas and integrated microstrip band defining filters have been built and tested. Preliminary results of optical testing and simulations are presented. A bolometer array design based on this pixel will also be shown and future plans for application of the technology will be discussed.

The last decade has seen a number of technological advancements that have now made it possible to implement fully solid state local oscillator chains up to 2 THz. These chains are composed of cascaded planar multiplier stages that are pumped with W-band high power sources. The high power W-band sources are achieved by power combining MMIC amplifiers and can provide in access of 150 mW with about 10% bandwidth. Planar diode technology has also enabled novel circuit topologies that can take advantage of the high input power and demonstrate significant efficiencies well into the THz range. Cascaded chains to 1.9 THz have now been demonstrated with enough output power to successfully pump hot-electron bolometer mixers in this frequency range. An overview of the current State-of-the-Art of the local oscillator technology will be presented along with highlighting future trends and challenges.

The KOSMA group at the I. Physikalisches Institut der Universitat zu Koln combines expertise throughout a large part of the instrumentation and technologies as well as procedures and operations relevant for astronomical observations in the submm- and THz-spectral regime. This expertise is based on the long term heritage of operating the Kolner Observatorium fur Submillimeter Astronomie (KOSMA) with its 3m-telescope (Fig. 1) on Gornergrat near Zermatt, CH (since the year 2000, the observatory is operated jointly by the Universitat zu Koln and the Radioastronomisches Institut, Universitat Bonn). The experience gained by building and operating high frequency radio-astronomy equipment for KOSMA has made the Cologne group a competent partner in the instrumentation program for the airborne observatory SOFIA and for the space observatories SWAS and more recently Herschel/HIFI.

Improved and reproducible heterodyne mixing (noise temperatures of 950 K at 2.5 THz) has been realized with NbN based hot-electron superconducting devices with low contact resistances. A distributed temperature numerical model of the NbN bridge, based on a local electron and a phonon temperature, has been used to understand the physical conditions during the mixing process. We find that the mixing is predominantly due to the exponential rise of the local resistivity as a function of electron temperature.

Atacama Pathfinder EXperiment (APEX) submillimeter telescope is currently under completion on Chajnator, at an altitude of 5050 m on the Atacama Desert, in the Northern Chile. The telescope facility heterodyne receivers should have 3 bands covering 211-500 GHz. We present design of a 275-370 GHz SIS mixer to be used as a first light APEX Band 2 receiver. A novel waveguide-to-microstrip transition with integrated bias-T is used in this mixer. This structure allows coupling of the RF signal from a full height waveguide to a thin-film superconducting line via E-probe. The wide side of the probe is connected to another port via a specially shaped high impedance line that provides RF/DC isolation. This port is used to extract the IF signal and to inject a DC current that creates a local magnetic field parallel to the plane of the SIS junction to suppress the Josephson effect. The main advantage of this type of Josephson suppression circuit is its compactness as it uses the existing superconducting lines from the SIS integrated tuning circuitry. The entire structure with the probe, SIS junction with its tuning circuitry is placed on a quartz substrate. For more advanced designs, as a sideband separating or balanced mixer that we intend to have for the final version of the APEX telescope heterodyne receiver, the SIS junctions of two balanced or quadrature mixers will be at a very close distance. The standard solution of using superconducting coils to suppress Josephson effect is very difficult to implement and, therefore, this new structure should be of a great advantage.

In this paper recent developments of Hot Electron Bolometric receivers performed at Chalmers are summarized. This comprises progress on the mixers for HIFI and membrane HEB. All devices are modelled using Hot Spot model taking Andreev reflection at the interface between the normal conductor and the superconductor into account.

The Quantum Theory of Mixing developed by Tucker provides a solid framework for understanding the behaviour of SIS mixers, and subsequent developments allow the simulation of complete mixer circuits. These methods operate, however, under the assumption of small signal levels, and so neglect the non-linear behaviour of the signal path. The non-linearity of the mixer's response to applied signals is of vital importance to the calibration of SIS receiver systems. We have previously reported a procedure for calculating the full quantum behaviour of tunnel junction circuits under multiple high-level signals, allowing the accurate prediction of the saturation characteristics of SIS mixers. In this paper, we apply our procedure to both an idealized SIS mixer and one of our previously tested 700 GHz finline mixers. We find that the small signal behaviour predicted by our procedure agrees well with other simulation methods, and that the saturation properties of both of these mixers differ from that predicted by previous estimates of saturation behaviour.

NbN hot electron bolometer (HEB) mixers are at this moment the best heterodyne detectors for frequencies above 1 THz. However, the fabrication procedure of these devices is such that the quality of the interface between the NbN superconducting film and the contact structure is not under good control. This results in a contact resistance between the NbN bolometer and the contact pad. We compare identical bolometers, with different NbN - contact pad interfaces, coupled with a spiral antenna. We find that cleaning the NbN interface and adding a thin additional superconductor prior to the gold contact deposition improves the noise temperature and the bandwidth of the HEB mixers with more than a factor of 2. We obtain a DSB noise temperature of 950 K at 2.5 THz and a Gain bandwidth of 5-6 GHz. For use in real receiver systems we design small volume (0.15x1 micron) HEB mixers with a twin slot antenna. We find that these mixers combine good sensitivity (900 K at 1.6 THz) with low LO power requirement, which is 160 - 240 nW at the Si lens of the mixer. This value is larger than expected from the isothermal technique and the known losses in the lens by a factor of 3-3.5.

Since 1997, CEA/SAP and CEA/LETI/SLIR have been developing monolithic Si bolometer arrays sensitive in the far infrared and submillimiter range for space observations. Two focal planes, 32x64 and 16x32 pixel arrays, are designed and manufactured for the PACS (Photodetector Array Camera and Spectrometer) instrument of the Herschel observatory, to be launched in 2007. The two arrays cover respectively the 60-130 μm and 130-210 μm ranges. The goal of these large bolometer arrays is to achieve observations in a Background limited NEP around 10^-16 W.Hz^-1/2. The detector physics and manufacture techniques of the different stages of these arrays are first presented. Then we describe the read-out and multiplexing cold electronics (300mK) that make possible several functional modes (temporal and fixed pattern noise reduction,...). The latest experimental measurements carried out with the complete detector system at the nominal temperature are presented and performances are discussed.

We describe the development, construction, and testing of two 384 element arrays of ion-implanted semiconducting cryogenic bolometers designed for use in far-infrared and submillimeter cameras. These two dimensional arrays are assembled from a number of 32 element linear arrays of monolithic Pop-Up bolometer Detectors (PUD) developed at NASA/Goddard Space Flight Center. PUD technology allows the construction of large, high filling factor, arrays that make efficient use of available focal plane area in far-infrared and submillimeter astronomical instruments. Such arrays can be used to provide a significant increase in mapping speed over smaller arrays. A prototype array has been delivered and integrated into a ground-based camera, the Submillimeter High Angular Resolution Camera (SHARC II), a facility instrument at the Caltech Submillimeter Observatory (CSO). A second array has recently been delivered for integration into the High-resolution Airborne Widebandwidth Camera (HAWC), a far-infrared imaging camera for the Stratospheric Observatory for Infrared Astronomy (SOFIA). HAWC is scheduled for commissioning in 2005.

We report the performance of the flight bolometer arrays for the Spectral and Photometric Imaging REceiver (SPIRE) instrument to be on board of the Herschel Space Observatory (HSO). We describe the test setup for the flight Bolometric Detector Assembly (BDA) that allows the characterization of its performance, both dark and optical, in one instrument's cool down. We summarize the laboratory procedure to measure the basic bolometer parameters, optical response time, optical efficiency of bolometer and feedhorn, dark and optical noise, and the overall thermal conductance of the BDA unit. Finally, we present the test results obtained from the two flight units, Spectroscopic Long Wavelength (SLW) and Spectroscopic Short Wavelength (SSW).

In order to provide high sensitivity rapid imaging at 3.3 mm (90 GHz) for the Green Bank Telescope - the world's largest steerable aperture - a camera is being built by the University of Pennsylvania, NASA/GSFC, and NRAO. The heart of this camera is an 8x8 close-packed, Nyquist-sampled detector array. We have designed and are fabricating a functional superconducting bolometer array system using a monolithic planar architecture. Read out by SQUID multiplexers, the superconducting transition edge sensors will provide fast, linear, sensitive response for high performance imaging. This will provide the first ever superconducting bolometer array on a facility instrument.

A breakthrough in the packaging of cryogenic millimeterwave circuits enables mass production of radiometer or polarimeter array elements, leading to low cost, high performance, and straightforward scaling to arbitrarily large arrays. The Q/U Imaging ExperimenT (QUIET) will measure the polarization of the cosmic microvave background from the ground using such large arrays of coherent polarimeters. With two different optical systems, angular scales from ten degrees to 4' can be covered.

The redshift (z) and Early Universe Spectrometer (ZEUS) is an echelle grating spectrometer designed to study the history of star formation in the Universe from about 2 billion years after the Big Bang to the present by observing submillimeter and far-infrared spectral lines from distant dusty galaxies. ZEUS has moderate resolving power (R~1000), and large spectral coverage so as to optimize extragalactic point source sensitivity in the telluric submillimeter (350, 450, and 610 um) windows. When completed, ZEUS will have a 4 x 64-element array of TES PUD bolometers delivering an instantaneous 64-element spectrum for each of 4 spatial positions on the sky. ZEUS is designed for use on the 15 m JCMT telescope on Mauna Kea. We also plan to use it on the 12 m APEX telescope at the Chajnantor site in northern Chile. Our scientific goals include (1) investigating star formation in the early Universe by measuring the redshifted fine-structure lines from distant (z ~1 to 4) (proto-) galaxies, (2) measuring the redshifts of optically obscured submillimeter galaxies by detecting their bright 158 um [CII] line emission, and (3) investigating the properties of starburst and ultraluminous galaxies in the local Universe by observing their [CI] and mid-J CO rotational line emission.

The Large Millimeter telescope (LMT) is a 50 m diameter telescope currently under construction in Mexico. We describe the first generation continuum instruments for the LMT - BOLOCAM II, a 144 pixel array receiver, and SPEED, a 4 pixel array of frequency selective bolometers. Together, these two instruments form a complementary set of continuum receivers designed for efficient mapping and source follow-up at mm-wavelengths. An overview of the instruments, the telescope, and the key initial science is given.

We present the design, integration, and first ryogenic testing of our new broad-band millimeter-wave spectrometer, Z-Spec. Z-Spec uses a novel architecture called WaFIRS (Waveguide Far-IR Spectrometer), which employs a curved diffraction grating in a parallel-plate waveguide propagation medium. The instrument will provide a resolving power betwee 200 and 350 across an instantaneous bandwidth of 190-310 GHz, all packaged within a cryostat that is of order 1 meter in size. For background-limited astronomical observations in the 1mm terrestrial window, Z-Spec uses 160 silicon nitride micro-mesh bolometers and the detectors and waveguide grating are cooled to ~0.1 K. Our first cryogenic measurements at 225 GHz show resolving power greater than 200, and the end-to-end throughput is estimated to be greater than 30%, possibly as high as 40%. Z-Spec represents the first systematic approach to cosmological redshift measurement that is not based on optical or near-IR identifications. With its good sensitivity and large bandwidth, Z-Spec provides a new capability for millimeter-wave astrophysics. The instrument will be capable of measureing rotational carbon monoxide line emission from bright dusty galaxies at redshifts of up to 4, and the broad bandwidth insures that at least two lines will be simultaneously detected, providing an unambiguous redshift determination. In addition to Z-Spec's observations over the next 1-3 years, the WaFIRS spectrometer architecture makes an excellent candidate for mid-IR to millimeter-wave spectrometers on future space-borned and suborbital platforms such as SPICA and SAFIR. The concept is dramatically more compact and lightweight than conventional free-space grating spectrometers, and no mirrors or lenses are used in the instrument. After the progress report on Z-Spec we highlight this capability.

We describe the current status of the HIFI mixer units for Band 3 and Band 4. The mixer units cover the 800-960 GHz and 960-1120 GHz frequency range and have a 4-8 GHz IF frequency band. The major requirements and the design strategy are described. Functional tests of the magnet, the de-flux heater, IF-circuit, and the corrugated horn were performed. Details of the design of the mixer units and the performance status are presented. The DSB receiver noise performance ranges from 210 K at 850 GHz to 430 K at 1075 GHz.

The Submillimeter High Angular Resolution Camera II (SHARC-II) is a 32 x 12 pixel submillimeter camera that is used with the ten-meter diameter Caltech Submillimeter Observatory (CSO) on Mauna Kea. SHARC-II can be operated at either 350 or 450 microns. We are developing an optics module that we will install at a position between the SHARC-II camera and the focus of the CSO's secondary mirror. With our module installed, SHARC-II will be converted into a sensitive imaging polarimeter. The basic idea is that the module will split the incident beam coming from the secondary into two orthogonally polarized beams which are then re-imaged onto opposite ends of the “long and skinny” SHARC-II bolometer array. When this removable polarimetry module is in use, SHARC-II becomes a dual-polarization 12 x 12 pixel polarimeter. (The central 12 x 8 pixels of the SHARC-II array will remain unused.) Sky noise is a significant source of error for submillimeter continuum observations. Because our polarimetry module will allow simultaneous observation of two orthogonal polarization components, we will be able to eliminate or greatly reduce this source of error. Our optical design will include a rotating half-wave plate as well as a cold load to terminate the unused polarization components.

We present the first astronomical results from DesertSTAR, a 7 pixel heterodyne array receiver designed for operation in the astrophysically rich 345 GHz atmospheric window. DesertSTAR was constructed for the 10m Heinrich Hertz Telescope located at 3150m elevation on Mt. Graham, Arizona. This receiver promises to increase mapping speed at the HHT by a factor of ~15 over the facility's existing single beam, dual polarization receiver. DesertSTAR uses tunerless, single-ended waveguide SIS mixers to achieve uncorrected receiver noise temperatures of ~60K. The instantaneous bandwidth is 2 GHz, with a 5 GHz Intermediate Frequency, offering 1600 km/s of velocity coverage. Cryogenic isolators are employed between the mixers and low noise amplifiers to assure a flat IF passband. The system uses a Joule-Thompson closed-cycle refrigerator with 180W capacity at 70K and 1.8W capacity at 4K. A novel reflective phase grating is used for Local Oscillator multiplexing, while a simple Mylar beamsplitter is used as an LO diplexer. Optics include only polyethelene mixer lenses and a single, cold, flat mirror, maximizing simplicity for high efficiency and easy optical alignment. The computer controlled bias system provides low noise bias for the SIS junctions, magnets and LNAs through a modular and hardware independent GUI interface, and allows remote operation and monitoring. We present measurements of receiver noise, beam quality, efficiency and stability in addition to astronomical observations obtained during engineering runs at the HHT.

We describe a procedure for modelling the behaviour of multi-mode astronomical interferometers. The procedure is based on the concept of eigenfields. The input and output eigenfields are those field distributions on the sky and at the detector to which the individual telescopes of an interferometer can couple. The eigenfields of different telescopes are orthogonal, and therefore provide, when combined, a suitable basis set for propagating the second-order statistical properties of the field from a source through the telescopes, through the beam combiners, and onto the detectors. The scheme can be used at any wavelength, with any configuration of optical components (Michelson, Fizeau, etc.), with a source in any state of coherence and polarisation, and with any kind of detector.

In this paper the millimeter-wave passive components developed for the Ka-band Bar-SPOrt (Balloon-borne Radiometer for Sky Polarization Observatory) correlation radiometer are described. Comparison between numerical and experimental results are reported for all the building blocks of the radiometer: marker injector, polarizer, ortho-mode transducer, filtering sections and correlation unit. Due to the very low level of the polarized sky emission to be measured, all the components were designed and manufactured in order to achieve a very high level of sensitivity.

Optical design in the terahertz (THz) waveband suffers from a lack of dedicated software tools for modelling the range of electromagnetic and quasi-optical propagation conditions encountered in typical systems. Optical engineers are forced to use packages written for very different wavelength systems and there is often a lack of confidence in the results because of possible inappropriate underlying physical models. In this paper we describe the analytical techniques and dedicated CAD software tools (MODAL) that we are developing for long-wavelength design and analysis in the THz waveband. Our basic approach to modelling long-wavelength propagation is the application of modal analysis appropriate to the problem under investigation. We have extended this to include the efficient description of common off-axis (tilted) components such as simple curved reflectors. In earlier research we have investigated the conditions under which approximate methods (ray tracing, paraxial modes) can provide extremely efficient and accurate solutions and situations where a more rigorous approach is required. As a rigorous model of electromagnetic wave propagation, physical optics can be used to characterize complete systems to high accuracy. However, the straightforward approach is computationally intensive and, therefore, not suitable for the initial design or preliminary analysis of large multi-element optical systems. In order to improve the computational efficiency of the usual PO approach we have developed fast physical optics software, initially for the analysis of the ESA PLANCK system. The MODAL code is modular and multi-platform, and different propagation models can be used within the same framework. Distributed parallel computing enables significant reduction of the time needed to perform the calculations. We present the new software and analyses of the QuaD and Herschel (HIFI) telescope systems.

Balanced receivers are under development at the Caltech Submillimeter Observatory (CSO) for the 230/460 GHz and 345/660 GHz atmospheric windows. The mixers are tunerless, implemented in a balanced configuration, have a 4-8 GHz IF, and can be used in dual frequency observation mode. As shall be seen, the balanced arrangement provides a high level of amplitude noise immunity and allows all of the available LO power to be used. In turn, this permits complete automation of the receivers by means of synthesized LO source(s). A disadvantage of balanced mixers is, perhaps, that the sidebands at the IF remain convolved (DSB), unlike sideband separating (2SB) receivers. The latter, however are unbalanced and do not have the noise and LO injection advantages of balanced mixers. For the CSO, balanced mixers covering the range 180-720 GHz were judged most promising to facilitate many of the astrophysical science goals in the years to come. In parallel, a dual polarization 280-420~GHz continuous comparison (correlation) receiver is in an advanced state of development. The instrument has two beams on the sky; a reference and a signal beam. Using only cooled reflecting optics, two polarizing grids, and a quadrature hybrid coupler, the sky beams are coupled to four tunerless SIS mixers (both polarizations). The 4-12 GHz mixer IF outputs are, after amplification, correlated against each other. In principle, this technique results in flat baselines with very low RMS noise, and is especially well suited for high redshift Galaxy work. Not only do these changes greatly enhance the spectroscopic capabilities of the CSO, they will also enable the observatory to be integrated into the Harvard-Smithsonian Submillimeter Array (SMA), as an additional telescope.

A novel concept of the Cold-Electron Bolometer (CEB) with strong electrothermal feedback has been proposed. The concept is based on direct electron cooling of the absorber that serves as negative electrothermal feedback for incoming signal. This feedback is analogous to TES (transition-edge sensor) but additional dc heating is replaced by deep electron cooling to minimum temperature. It could mean a principle breakthrough in realization of supersensitive detectors. Noise properties are considerably improved by decreasing the electron temperature. The loop gain of electrothermal feedback could exceed 1000. The response time is reduced by electrothermal feedback to 10ns in comparison with the intrinsic e-ph time constant of 10ms. The CEB gives opportunity to increase dynamic range by removing all incoming power from supersensitive absorber to the next stage of readout system (SQUID) with higher dynamic range. Saturation problems are not so severe for CEB as for TES: after exceeding the cooling power there is only slight deviation from linear dependence for voltage response. The full saturation comes at the level of 100pW when temperature of absorber achieves Tc of Al. Ultimate performance of the CEB is determined by shot noise of the signal readout. For relatively low background load P0 =10fW and quantization level T_e= 50mK, the limit NEP is equal to 10^-19W/Hz^1/2. The estimations show that it is realistic to achieve ultimate NEP at 100 mK with SQUID readout system and NEP=10^-18W/Hz^1/2 at 300mK for background load of 10fW. Applicability of the CEB to post-Herschel missions looks very promising.

In this paper we present a novel design of an antenna coupled TES direct detector for high performance applications. In particular, the design of the detector has been optimised to be suitable for the measurement of the weak B-mode signal in the CMB polarization. An important feature of this design is that it employs corrugated horn antennas for coupling the astronomical signal to the detector. This allows us to feed the telescope with a well collimated beam with low sidelobes and cross polarization. The paper contains simulations demonstrating the suitability of individual electromagnetic components to be used in the instrument.

SIS photon detectors are niobium-based superconducting direct detectors for submillimeter-wave that show superior performance when compared with bolometric detectors for ground-based observations. We present the design and development of the SIS photon detectors together with optical and cryogenic components for wide field continuum observation system on Atacama Submillimeter Telescope Experiment (ASTE). Using antenna coupled distributed junctions, SIS photon detectors give wide band response in a 650-GHz atmospheric window as well as high current sensitivity, shot noise limited operation, fast response and high dynamic range. Optical noise equivalent power (NEP) was measured to be 1.6x10^-16 W/Hz^0.5 that is less than the background photon fluctuation limit for ground based submillimeter-wave observations. Fabrication of focal plane array with 9 detector pixels is underway to install in ASTE. Readout electronics with Si-JFETs operating at about 100 K will be used for this array. Development of readout electronics for larger array is based on GaAs-JFETs operating at 0.3 K. For the purpose of installing 100 element array of SIS photon detectors, we have developed remotely operable low-vibration cryostat, which now cools bolometers for 350, 450, 850-µm observations down to 0.34 K. GM-type 4-K cooler and He3/He4 sorption cooler is used, which can be remotely recycled to keep detectors at 0.34 K. Since we have large optical window for this cryostat, sapphire cryogenic window is used to block infrared radiation. The sapphire window is ante-reflection coated with SiO₂ by chemical vapor deposition (CVD). The transmittance of the cryogenic window at 650 GHz is more than 95%.

We are developing a new type of detector for observational cosmology and astrophysical research. Incoming radiation from the sky is coupled to a superconducting microstrip transmission line that terminates in a thin film absorber. At sub-Kelvin temperature, the thermal isolation between the electrons and the lattice makes it possible for the electrons in the small absorber (100's of cubic micro-meter) and superconducting bilayer (Transition Edge Sensor) to heat up by the radiation absorbed by the electrons of the normal absorbing layer. We call this detector a Transition-edge Hot-electron Micro-bolometer (THM). THMs can be fabricated by photo lithography, so it is relatively easy to make matched detectors for a large focal plane array telescope. We report on the thermal properties of Mo/Au THMs with Bi/Au absorbers.

We have investigated the noise performance of MoAu-bilayer TES bolometers designed for infrared detectors. A set of devices with variations in geometry were fabricated at the NASA/GSFC detector development facility. These detectors have different bilayer aspect ratios and have varieties of normal metal regions deposited on top of the bilayer to study the effects of geometry on noise. These normal metal regions are oriented either parallel or transverse to the direction of current flow, or both. The lowest noise detectors are found to have normal metal regions oriented transversely. Our detectors with the most favorable design feature negligible excess noise in the in-band region, only slight excess noise in the out-of-band region, and low 1/f noise. The detectors are successfully used in the Submillimeter Broadband Spectrometer FIBRE which is used for astronomical observations at the Caltech Submillimeter Observatory.

QUaD is a ground-based high-resolution (up to l ≈ 2500) instrument designed to map the polarisation of the Cosmic Microwave Background and to measure its E-mode and B-mode polarisation power spectra. QUaD comprises a bolometric array receiver (100 and 150 GHz) and re-imaging optics on a 2.6-m Cassegrain telescope 2. It will operate for two years and begin observations in 2005. CMB polarisation measurements will require not only a significant increase in sensitivity over earlier experiments but also a better understanding and control of systematic effects particularly those that contribute to the polarised signal. To this end we have undertaken a comprehensive quasi-optical analysis of the QUaD telescope. In particular we have modelled the effects of diffraction on beam propagation through the system. The corrugated feeds that couple radiation from the telescope to phase-sensitive bolometers need to have good beam symmetry and low sidelobe levels over the required bandwidth. It is especially important that the feed horns preserve the polarisation orientation of the incoming fields. We have used an accurate mode-matching model to design such feed horns. In this paper we present the diffraction analysis of the QUaD front-end optics as well as the electromagnetic design and testing of the QUaD corrugated feeds.

The realization of a large (40x32) pixel sub-array on a 3-inch silicon wafer brings unique challenges involving the integration of a variety of microfabrication techniques. Design, development and fabrication procedures are described, with conventional MEMS techniques in silicon being used where possible. High resolution imaging in the sub-millimetre range requires a pixel size of the order of one millimetre with a high signal/noise ratio detector, which must be addressed at cryogenic temperatures via a very low noise amplifying system. This has been realized using a combination of Transition Edge Sensors (TES) with amplification and multiplexing (MUX) by Superconducting Quantum Interference Devices (SQUID), which imposes particular requirements in the method of construction. This paper describes the details of the technologies used to overcome the conflicting demands of the different elements. The need to operate at millikelvin temperatures limits the materials that can be selected. Particular attention has been paid to the stresses induced in the structure by overlying films, bump bonding and any thermal processing.

SCUBA-2 is the next-generation replacement for SCUBA (Sub-millimetre Common User Bolometer Array) on the James Clerk Maxwell Telescope. Operating at 450 and 850 microns, SCUBA-2 fills the focal plane of the telescope with fully-sampled, monolithic bolometer arrays. Each SCUBA-2 pixel uses a quarter-wave slab of silicon with an implanted resistive layer and backshort as an absorber and a superconducting transition edge sensor as a thermometer. In order to verify and optimize the pixel design, we have investigated the electromagnetic behaviour of the detectors, using both a simple transmission-line model and Ansoft HFSS, a finite-element electromagnetic simulator. We used the transmission line model to fit transmission measurements of doped wafers and determined the correct implant dose for the absorbing layer. The more detailed HFSS modelling yielded some unexpected results which led us to modify the pixel design. We also verified that the detectors suffered little loss of sensitivity for off-axis angles up to about 30 degrees.

The Stratospheric Observatory For Infrared Astronomy's (SOFIA's) High resolution Airborne Wideband Camera (HAWC) will use an ion-implanted silicon bolometer array developed at NASA's Goddard Space Flight Center (GSFC). The GSFC Pop-Up Detectors (PUDs) use a unique "folding" technique to enable a 12 x 32 element close-packed array of bolometers with a filling factor greater than 95%. The HAWC detector uses a resistive metal film on silicon to provide frequency independent, ~50% absorption over the 40 - 300 micron band. The silicon bolometers are manufactured in 32-element rows within silicon frames using Micro Electro Mechanical Systems (MEMS) silicon etching techniques. The frames are then cut, "folded", and glued onto a metallized, ceramic, thermal bus "bar". Optical alignment using micrometer jigs ensures their uniformity and correct placement. The rows are then stacked side-by-side to create the final 12 x 32 element array. A kinematic Kevlar suspension system isolates the 200 mK bolometer cold stage from the rest of the 4K detector housing. GSFC - developed silicon bridge chips make electrical connection to the bolometers, while maintaining thermal isolation. The Junction Field Effect Transistor (JFET) preamplifiers for all the signal channels operate at 120 K, yet they are electrically connected and located in close proximity to the bolometers. The JFET module design provides sufficient thermal isolation and heat sinking for these, so that their heat is not detected by the bolometers. Preliminary engineering results from the flight detector dark test run are expected to be available in July 2004. This paper describes the array assembly and mechanical and thermal design of the HAWC detector and the JFET module.

The Fabry-Perot Interferometer Bolometer Research Experiment FIBRE, a protoype submillimeter spectrometer for astronomical observations, is based on a helium-cooled scanning Fabry-Perot and superconducting transition edge sensor bolometers (TES). The TES design takes advantage of a recently discovered method of excess noise reduction by depositing lateral normal metal bars on these devices. A SQUID multiplexer is used to read out the individual detector pixels. The spectral resolving power of the instrument is provided by a Fabry-Perot spectrometer. The outgoing light from the Fabry-Perot passes onto a low resolution grating for order sorting. A linear bolometer array consisting of 16 elements detects this dispersed light, capturing 5 orders simultaneously from one position on the sky. With tuning of the Fabry-Perot over one free spectral range, a spectrum covering Δλ/λ =1/7 at a resolution of ~1/1200 can be achieved. This spectral resolution is sufficient to resolve doppler broadened line emission from external galaxies. FIBRE operates in the 350 μm and 450 μm bands. These bands cover line emission from the important PDR tracers neutral carbon [CI] and carbon monoxide CO. The spectrometer is used at the Caltech Submillimeter Observatory for astronomical observations.

The SCUBA-2 instrument is a new wide field submillimeter imager currently being designed for the James Clerk Maxwell telescope on Mauna Kea in Hawaii. The instrument will observe simultaneously in the 450 and 850 micron bands and has a field of view of approximately 50 square arcminutes. To meet the performance requirements the detectors require a heat sink at a temperature of 50 mK or lower, and must be surrounded by an enclosure at a temperature of 1.1 K or below. Cooling is provided by the mixing chamber and still of a cryogen-free dilution refrigerator (DR), via thermal links of the order of a metre in length. A challenging set of requirements result from the need for a small temperature drop between the detectors and the refrigerator insert despite the large distance between them, the need to provide flexibility in the links to allow for movement during thermal contraction, and the need to allow for the detectors to be removed from the cryostat. Further, the arrays require a mounting structure which provides rigid mechanical support from the 1-K stage yet causes a very small heat input to millikelvin stage. This paper describes the design which has been evolved to meet these difficult (and often conflicting) requirements.

A wideband correlator system with a bandwidth of 16 GHz or more is required for Array for Microwave Background Anisotropy (AMiBA) to achieve the sensitivity of 10μK in one hour of observation. Double-balanced diode mixers were used as multipliers in 4-lag correlator modules. Several wideband modules were developed for IF signal distribution between receivers and correlators. Correlator outputs were amplified, and digitized by voltage-to-frequency converters. Data acquisition circuits were designed using field programmable gate arrays (FPGA). Subsequent data transfer and control software were based on the configuration for Australia Telescope Compact Array. Transform matrix method will be adopted during calibration to take into account the phase and amplitude variations of analog devices across the passband.

The German REceiver for Astronomy at Terahertz frequencies (GREAT) is a first generation PI instrument for the SOFIA telescope, developed by a collaboration between the MPIfR, KOSMA, DLR, and the MPAe. The first three institutes each contribute one heterodyne receiver channel to operate at 1.9, 2.7 and 4.7 THz, respectively. A later addition of a e.g. 1.4 THz channel is planned. The GREAT instrument is developed to carry two cryostats at once. That means that any two of the three frequencies can be observed simultaneously. Therefore, we need to be able to quickly exchange the optics benches, the local oscillator (LO) subsystems, and the cryostats containing the mixer devices. This demands a high modularity and flexibility of our receiver concept. Our aim is to avoid the need for realignment when swapping receiver channels. After an overview of the common GREAT optics, a detailed description of several parts (optics benches, calibration units, diplexer, focal plane imager) is given. Special emphasis is given to the LO optics of the KOSMA 1.9 THz channel, because its backward wave oscillator has an astigmatic output beam profile, which has to be corrected for. We developed astigmatic off-axis mirrors to compensate this astigmatism. The mirrors are manufactured in-house on a 5 axis CNC milling machine. We use this milling machine to obtain optical components with highest surface accuracy (about 5 microns) appropriate for these wavelengths. Based on the CNC machining capabilities we present our concept of integrated optics, which means to manufacture optical subsystems monolithically. The optics benches are located on three point mounts, which in conjunction with the integrated optics concept ensure the required adjustment free optics setup.

We present the opto-mechanical layout of KOSMA's (Kolner Observatorium fur SubMillimeter Astronomie) submillimeter and terahertz heterodyne array receiver STAR (SOFIA Terahertz Array Receiver) which is derived from SMART (Sub-Millimeter Array Receiver for Two frequencies). To reduce the alignment effort, SMART, for the first time, uses an integrated optics concept with no adjustable optical components inside the receiver dewar. For STAR this successful design concept will be extended and adapted for 1.9 THz and for use aboard SOFIA (Stratospheric Observatory For Infrared Astronomy). The design of STAR's cryostat and cryogenic optics is described. Emphasis is laid on the required accuracy for 4x4 spatially multiplexed Terahertz heterodyne receivers. The proposed design of the local oscillator, a frequency-tripled BWO (Backward Wave Oscillator), is outlined. The presentation comprises the scheme for multiplexing the local oscillator, dense arrangement of mixer elements in a cryogenic focal plane and manufacturing techniques of integrated optics units for reduction of optical adjustment efforts in astronomical submillimeter and terahertz receivers.

None of the existing spectrometer technologies currently fulfil the demands for larger IF-bandwidth up to 10 GHz for future THz heterodyne systems without using hybrid-solutions. At KOSMA (Koelner Observatorium fuer Sub-Millimeter Astronomie) we are investigating the idea of a laser side-band-spectrometer. We use laser modulation method to generate optical sidebands near of the laser frequency, which can then be frequency analyzed by standard optical methods like a high finesse Fabry-Perot (FP) etalon. First laboratory results of a prototype setup with 9 GHz Bandwidth will be presented.

The Heterodyne Instrument for Far Infrared (HIFI) on ESA's Herschel Space Observatory utilizes a variety of novel RF components in its five SIS receiver channels covering 480- 1250 GHz and two HEB receiver channels covering 1410-1910 GHz. The local oscillator unit will be passively cooled while the focal plane unit is cooled by superfluid helium and cold helium vapors. HIFI employs W-band GaAs amplifiers, InP HEMT low noise IF amplifiers, fixed tuned broadband planar diode multipliers, high power W-band Isolators, and novel material systems in the SIS mixers. The National Aeronautics and Space Administration through the Jet Propulsion Laboratory is managing the development of the highest frequency (1119-1250 GHz) SIS mixers, the local oscillators for the three highest frequency receivers as well as W-band power amplifiers, high power W-band isolators, varactor diode devices for all high frequency multipliers and InP HEMT components for all the receiver channels intermediate frequency amplifiers. The NASA developed components represent a significant advancement in the available performance. This paper presents an update of the performance and the current state of development.

This paper describes the system configuration and the some results of the prototype wide-band digital autocorrelation spectrometer for radio astronomy observation, which will be used as back-end spectrometer for the 2/3mm dual channels SIS receiver at Taeduk Radio Astronomy Observatory. This spectrometer consists of high-speed sampler module, circular memory buffer module, and correlator module based on QUAINT correlator chip. The developed digital autocorrelator shows good performance results at 50MHz and 100MHz mode with the 10MHz CW source input. After some calibration procedure, this spectrometer can be use as back-end system at TRAO

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.

This is to report on our development for a dual-polarization receiver to detect the cosmic microwave background (CMB) in 85 to 105 GHz band. The receiver is based on a MMIC, HEMT-based LNA developed in the Jet Propulsion Laboratory. A W-band, orthomode transducer (OMT) is used for polarization separation. Most of the RF front-end is located in cryogenics environment at 20K. We have developed a MMIC sub-harmonically pumped diode mixer, operating at 42 GHz, for signal down-conversion. The entire base-band, 2 to 18 GHz, is correlated in a lag-correlator system. The receiver design details and the lab test results will be described in this report.

A new type of geometry for terahertz traveling-wave photomixers, vertically pumped free-space by two detuned continuous-wave diode lasers, is proposed and experimentally verified for devices based on low-temperature-grown GaAs (LT-GaAs). It combines the advantages of conventional interdigitated small area structures and traveling-wave devices. An output power of 1 μW at the mixing frequency of 1 THz was measured as a first result, which meets power requirements for superconducting heterodyne mixer devices.

We report on photonic technologies developed at the MPI fur Radioastronomy and the Research Center Julich to generate Terahertz Local Oscillator reference signals for use on e.g. ALMA/APEX and SOFIA. The principle is to mix two (NIR) laser colours in a biased LTG-GaAs layer, thus creating a high-frequency beat (difference) frequency signal. This output signal is coupled to free space through an antenna. In this work a systematic study of the photomixer design, in order to optimize the RF power, is presented. Part of the experiments were done with photomixers integrated to with a broadband spiral antenna designed for frequencies up to 1 THz. The LT GaAs photomixers are prepared on materials with various growth temperatures as well as using resonant cavity material structures and various finger contact geometries. An improvement in the output power up to around 3 μW of submillimeter radiation (0.5 THz) is demonstrated.

The next generation of hot electron bolometric (HEB) mixer receivers for terahertz frequencies is under development. In order to improve sensitivity and integration time, terahertz focal plane arrays with HEB elements are required. We have designed, fabricated, and tested a three-element focal plane array with HEB devices. We implemented a quasi-optical power coupling scheme using three elliptical silicon lenses. Recently developed wideband (0.5 GHz to 12 GHz) MMIC low noise amplifiers were directly integrated with HEB devices in a single block. The array was tested using an FIR laser as the LO source and a side band generator as the signal source. This is the first heterodyne array for a frequency above 1 THz, and the suitability of HEB elements in a terahertz FPA has thus been demonstrated. This development is also geared toward investigating new architectures for much larger arrays utilizing HEB elements. Additional issues to be resolved include an improved antenna design for efficient LO injection, compact and low power IF amplifiers, and cryogenic optimization.

We report on attempts to broaden the IF bandwidth of the BIMA 1mm SIS receivers by cascading fixed tuned double-sideband (DSB) SIS mixers and wideband MMIC IF amplifiers. To obtain the flattest receiver gain across the IF band we tested three schemes for keeping the mixer and amplifier as electrically close as possible. In Receiver I, we connected separate mixer and MMIC modules by a 1" stainless steel SMA elbow. In Receiver II, we integrated mixer and MMIC into a modified BIMA mixer module. In Receiver III, we devised a thermally split block where mixer and MMIC can be maintained at different temperatures in the same module. The best average receiver noise we achieved by combining SIS mixer and MMIC amplifier is 45 -50 K DSB for νLO = 215 - 240 GHz and below 80 K DSB for νLO = 205 - 270 GHz. The receiver noise can be made reasonably flat over the CARMA IF band (νIF = 1 - 5 GHz). Noise temperatures for all three receivers are comparable to or better than those obtained with the BIMA receiver.

The Combined Array for Research in Millimeter-wave Astronomy (CARMA) requires a flexible correlator to process the data from up to 23 telescopes and up to 8GHz of receiver bandwidth. The Caltech Owens Valley Broadband Reconfigurable Array (COBRA) correlator, developed for use at the Owens Valley millimeter-wave array and being used by the Sunyaev-Zeldovich Array (SZA), will be adapted for use by CARMA. The COBRA correlator system, a hybrid analog-digital design, consisting of downconverters, digitizers and correlators will be presented in this paper. The downconverters receive an input IF of 1-9GHz and produce a selectable output bandwidth of 62.5MHz, 125MHz, 250MHz, or 500MHz. The downconverter output is digitized at 1Gsample/s to 2-bits per sample. The digitized data is optionally digitally filtered to produce bands narrower than 62.5MHz (down to 2MHz). The digital correlator system is a lag- or XF-based system implemented using Field-Programmable Gate Arrays (FPGAs). The digital system implements delay lines, calculates the autocorrelations for each antenna, and the cross-correlations for each baseline. The number of lags, and hence spectral channels, produced by the system is a function of the input bandwidth; with the 500MHz band having the coarsest resolution, and the narrowest bandwidths having the finest resolution.

Heterodyne receivers for applications in astronomy and planetary research need quantum limited sensitivity. In instruments which are currently build for SOFIA and Herschel superconducting hot electron bolometers (HEB) will be used to achieve this goal at frequencies above 1.4 THz. The local oscillator and the mixer are the most critical components for a heterodyne receiver operating at 3-5 THz. The design and performance of an optically pumped THz gas laser optimized for this frequency band will be presented. In order to optimize the performance for this frequency hot electron bolometer mixers with different in-plane dimensions and logarithmic-spiral feed antennas have been investigated. Their noise temperatures and beam patterns were measured. Above 3 THz the best performance was achieved with a superconducting bridge of 2.0 x 0.2 μm² incorporated in a logarithmic spiral antenna. The DSB noise temperatures were 2700 K, 4700 K and 6400 K at 3.1 THz, 4.3 THz and 5.2 THz, respectively. The results demonstrate that the NbN HEB is very well suited as a mixer for THz heterodyne receivers up to at least 5 THz.

We present results of the development and measurements of a heterodyne sideband separating SIS mixer for 85-115 GHz band. The sideband separation is achieved by using a quadrature scheme where a local oscillator (LO) pumps two identical mixer junctions with 90° phase difference. A key component in the mixer is a waveguide to microstrip double probe transition used as a power divider to split the input RF signal and to provide transition from waveguide to microstrip line. The double probe transition enables the integration of all mixer components on a single compact substrate. The design also involves coupled lines directional couplers to introduce the LO power to the mixer junctions. An additional pair of SIS junctions is used to provide termination loads for the idle ports of the couplers. Several mixer chips were tested and similar and consistent performance was obtained. The best single sideband noise temperature is below 40 K with IF bandwidth 3.4-4.6 GHz. The sideband suppression ratio is better than 12 dB for both sidebands across the entire RF band. The mixer was also successfully tested with 4-8 GHz IF band. In this paper we present complete mixer characterization data.

Photomixing is a flexible and efficient method of providing both local oscillator signals for heterodyne receivers and high frequency phase reference signals. Ultrafast, 70 GHz bandwidth, λ = 1.55 μm, photodiodes from u²t Photonics AG have been incorporated into three designs of mm-wave waveguide mounts. The photomixers utilise a thin freestanding gold foil, or a gold on dielectric, probe to couple power into the waveguide and to deliver the photodiode bias. The frequency coverage of the designs is from 70 GHz to 300 GHz. A method of rapidly characterizing the frequency response of these photomixers using spontaneous-spontaneous beating of light from an EDFA is described. Recent work has been directed at increasing the degree of integration of the photodiode with the waveguide probe and choke filter to reduce the frequency dependence of the output power. A simplified photomixer block manufacturing process has also been introduced. A combined probe and filter structure, impedance matched to both the coplanar output line on the photodiode chip and to 0.4 height milled waveguide, is presented. This matching is achieved over the W-band with a fixed waveguide backshort. We present modelled and experimental results showing the increased efficiency and smoother tuning. The design and frequency response of such a probe is reported. We also present the performance of a simpler mount, operating in the frequency range from 160 GHz to 300 GHz, which generates powers of around 10 μW up to 250 GHz.

Stability of a hot-electron bolometer (HEB) heterodyne receiver was investigated at frequencies from 0.6THz to 1.9THz. The Allan variance was measured as a function of the integration time and the Allan time was obtained for HEB mixers of different size, as well as with different types of the local oscillator: FIR laser, multiplier chain, and BWO. We have found that due to stronger dependence of the mixer gain and noise vs mixer bias voltage and current the Allan time is shorter for smaller mixers. At 1.6THz the Allan time is 3 sec for 4x0.4μm² bolometer, and 0.15-0.2 sec for 1x0.15μm² bolometer. Obtained stability apears to be the same for the FIR laser and the mulitplier chain. The Allan time for smaller bolometers increases to 0.4-0.5sec at 0.6-0.7THz LO frequencies. The influence of the IF chain on the obtained results is also analyzed.

We present a low noise SIS mixer developed for the 1.2 THz band of the heterodyne spectrometer of the Herschel Space Observatory. With the launch of the Herschel SO in 2007, this device will be among the first SIS mixers flown in space. This SIS mixer has a quasi-optical design, with a double slot planar antenna and an extended spherical lens made of pure Si. The SIS junctions are Nb/AlN/NbTiN with a critical current density of about 30 KA/cm2 and with the junction area of a quarter of a micron square. Our mixer circuit uses two SIS junctions biased in parallel. To improve the simultaneous suppression of the Josephson current in each of them, we use diamond-shaped junctions. A low loss Nb/Au micro-strip transmission line is used for the first time in the mixer circuit well above the gap frequency of Nb. The minimum uncorrected Double Sideband receiver noise is 550 K (Y=1.34). The minimum receiver noise corrected for the local oscillator beam splitter and for the cryostat window is 340 K, about 6 hv/k, the lowest value achieved thus far in the THz frequencies range.

IMEC has designed, in the framework of the PACS project (for the European Herschel Space Observatory) the Cold Readout Electronics (CRE) for the Ge:Ga far-infrared detector array. Key specifications for the CRE were high linearity (3 %), low power consumption (80 μW for an 18 channel array), and very low noise (200 e-) at an operating temperature of 4.2 K (LHT - Liquid Helium Temperature). IMEC has implemented this circuit in a standard CMOS technology (AMIS 0.7 μm), which guarantees high production yield and uniformity, relatively easy availability of the technology and portability of the design. However, the drawback of this approach is the anomalous behavior of CMOS transistors at temperatures below 30-40K, known as kink and hysteresis effects and under certain conditions the presence of excess noise. These cryogenic phenomena disturb the normal functionality of commonly used circuits or building blocks like buffer amplifiers and opamps. We were able to overcome these problems and developed a library of digital and analog building blocks based on the modeling of cryogenic behavior, and on adapted design and layout techniques. These techniques have been validated in an automated cryogenic test set-ups developed at IMEC. We will present here in detail the full design of the 18 channel CRE circuit, its interface with the Ge:Ga sensor, and its electrical performance and demonstrate that all major specifications at 4.2 K were met. Future designs and implementations will be equally presented.

Fabrication of direct hybrid far infrared photoconductor arrays, especially for low background astronomy, is particularly challenging due to arrays' relatively large pixel size, susceptibility to stray radiation even from the faintest IR source, and the requirement for low bias levels. In fact, these problems have hindered the development of far IR direct hybrid arrays, which has been the standard practice for the near and mid IR arrays. In this paper, a new and novel design is presented that addresses the complications afflicting the direct hybrid approach and paves the way for the development of large-format far IR arrays. In particular, the readout glow, detector heating, and thermal mismatch between the readout and the detector array are addressed. The use of a capacitive transimpedance amplifier effectively eliminates the detector debiasing.

In today's Astronomy, there is little observational results in the spectral window ranging from far-infrared to submillimeter wavelength. As one of the main reason of this, there is no high performance detector in this spectral region. We started the development of the extrinsic photoconductor for this region utlizing shallow donor levels of gallium arsenide (GaAs) as a host material. GaAs is a good candidate as the material of the photoconductor, according to its small effective mass of conduction electrons, which leads to high performance of the detector. We began this development from the crystal growth of GaAs wafer using the liquid phase epitaxy (LPE). Using this method, we have grown about 60 samples of GaAs epitaxial wafer, and several of these samples showed very low-carrier concentration, which is suitable for the detector. And we also fabricated a proto-type detector from LPE grown GaAs wafer, and measured its response for far-infrared photons with several different conditions. The photons of the wavelength ranging between 100 and 300 micron were detected by the detector, and it was observed the highest efficiency of detection of about 0.07.

We have been developing an architecture for producing large format, two-dimensional arrays of close-packed bolometers, which will enable far-infrared to millimeter wavelength (lambda=100μm-2mm) cameras and spectrometers to obtain images and spectra orders of magnitude faster than present instruments. The low backgrounds achieved in these instruments require very sensitive detectors with NEPs ranging from 10^-17 to 10^-19 W/(Hz^-1/2). Superconducting transition edge sensor bolometers can be close-packed using the Pop-Up Detector (PUD) format, and SQUID multiplexers operating at the detector base temperature can be intimately coupled to them. The array unit cell is 8x32 pixels, using 32-element detector and multiplexer components. We have fabricated an engineering model array with this technology featuring a very compact, modular approach for large format arrays. We report on the production of the 32-element components for the arrays. Planned instruments using this array architecture include the Submillimeter and Far-InfraRed Experiment (SAFIRE) on the SOFIA airborne observatory, the South Pole Imaging Fabry-Perot Interferometer (SPIFI) for the AST/RO observatory, the Millimeter Bolometer Camera for the Atacama Cosmology Telescope (MBC/ACT), and the Redshift "Z" Early Universe Spectrometer (ZEUS).

The SPEED camera is being developed to study the spectral energy distributions of high redshift galaxies, Sunyaev-Zel'dovich effect in X-ray clusters and other cold objects in the universe. Its initial runs will be done on the 10 m Heinrich Hertz Submillimeter Telescope (HHSMT), with later runs using the Large Millimeter Telescope (LMT). SPEED requires a 2x2 pixel cryogenic detector array of Frequency Selective Bolometers (FSB). Each of the pixels will have four frequency bands in the ~150-350 GHz range. Here we describe the development of the detector array of these high efficiency FSBs. The FSB design provides the multi-pixel multi-spectral band capability required for SPEED in a compact, light weight, stackable array. The SPEED FSB bolometers will use proximity effect superconducting transition edge sensors (TES) as their temperature-sensing element permitting significantly higher levels of electronic multiplexing in future applications where larger numbers of detectors may be required.

We present in this paper the front-end design and the results of RF simulations, carried out with Microwave Studio (CST) and HFSS for SHAHIRA (Submillimeter Heterodyne Array for High-speed Radio Astronomy), a 4x4 heterodyne array at 2.5 THz and 4.7 THz. One can then observe 16 spatial positions at 2 frequencies. The design has been chosen to be quasi-optic, because of its simplicity, novelty and multi-pixels applicability. Pixels are made of Niobium Nitride HEB mixers with double-slot antennas, processed on 1 μm thick stress-less Si₃N₄/SiO₂ membrane. The use of the membrane shows numerous advantages: for instance the use of the mixers at higher RF frequencies, a better power coupling efficiency or a solution for avoiding dielectric modes, losses and reflections. This work is supported by ESA and is a collaboration between LERMA, CHALMERS and LAAS. The Camera is expected to find applications, for SOFIA or CIDRE.

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.

Fourier Transform Spectrometers (FTS) are commonly operated in a rapid-scan (RS) mode, in which an interferogram of an astronomical source is obtained as quickly as possible, followed by one of a nearby background position. In an alternate operating mode, known as step-and-integrate (SI), the optical path difference in the interferometer is incremented in discrete steps, and the signal is integrated only when the interferometer mirrors are stationary. This mode requires some other means of modulating the signal, such as chopping the secondary mirror so that the detector alternately views source and background. The noise bandwidth in the SI mode (typically ~1 Hz) is much smaller than in the RS mode (~1 KHz), which in principle can lead to an increase in overall sensitivity. The main problem with the SI mode is that it takes much longer (~30x) to acquire an interferogram. At submillimetre wavelengths, through the use of narrowband optical filters, which are matched to regions of low atmospheric opacity, it is possible to sample the interferogram at less than the interval determined from the DC band limited Nyquist frequency (a condition known as aliasing) and still unambiguously recover the spectral information. We describe in detail the aliased, SI mode of operation of an FTS at the JCMT and present first results of astronomical spectra obtained using this mode. The resulting spectra are compared and contrasted to data obtained in the RS mode.

We present the recent developments and current design and of an imaging Fourier transform spectrometer (IFTS) for use with SCUBA-2, the second generation, wide-field, submillimetre camera currently under development for the James Clerk Maxwell Telescope (JCMT). The spectrometer will offer variable resolution with resolving powers ranging from R ~10 to 5000. The IFTS uses a folded Mach-Zehnder configuration with novel intensity beam dividers and dual input ports for continuous atmospheric cancellation. This system, which is planned for operation in 2006, will provide simultaneous, broadband, intermediate spectral resolution imaging across both the 850 and 450 μm bands. The optics, observing modes, and projected telescope performance of the IFTS are discussed.

A compact sub-millimetre wavelength Nb superconducting tunnel junction receiver (TIRGO) has been installed on the UKIRT facility, Hawaii. The receiver, used in combination with an acousto-optic spectrometer, exhibited excellent noise performance, achieving a best noise equivalent temperature of 280K (DSB) at 808GHz. Despite unfavourable observing conditions, spectral observations of a variety of astronomical sources were made that effectively verified the sensitivity and usefulness of the instrument for astronomical research. The design, construction and performance of the receiver system are described and some of the astronomical data acquired during the observation period briefly presented.

Because of their special properties profiled corrugated waveguide-horn structures are popular as both single-moded and multi-moded bolometer feeds in CMB experiments (e.g. PLANCK, Archeops, QUaD). Although optimised at a spot frequency the horns are usually employed over a relatively wide bandwidth and for single-moded horns the waveguide itself acts as the high pass filter. The horns can be profiled to reduce the horn length and sometimes also a front flared section is added to provide minimum edge taper and spill-over levels (e.g. on PLANCK). In this paper we report on our detailed analysis of the bandwidth properties of such corrugated horns. In the case of multi-moded horns an important issue is how the number of modes varies across the band with the resulting impact on the beam patterns. The so-called "phase centre" location 3 is also an issue. For polarisation selective systems we probe the polarisation purity of the relayed signal across the band and also investigate waveguide details that determine the exact location of band edges. Furthermore any leakage below the expected cut-off will lead to non-idealised cross-polar effects. We have also undertaken a series of laboratory measurements of bandwidth effects in corrugated waveguides to verify the models used.

The measure of the faint polarized signal of the Cosmic Microwave Background (few percent of the CMB Anisotropy) requires instruments with very low contamination from systematic effects, high stability and high sensitivity. The BaR-SPOrt experiment, in sharing with the SPOrt project on ISS, is based on analog correlation receivers with components custom designed to match all of these requirements. Here we present the architecture, the design analysis and the status of the realization of the 32 GHz receiver.

To measure extremely faint signals like Cosmic Microwave Background Polarization (a few percent of CMB anisotropy) it is necessary to use very high sensitivity radiometers. This means to adopt low noise cryogenic front-end and long integration times. This is the case of BaR-SPOrt (Balloon borne Radiometer for Sky Polarization Observations), an experiment designed to measure the CMB polarization at sub-degree angular scales. In the millimeter range, where coherent radiometers (polarimeters) are typically employed, usual mechanical coolers can represent a limit to the final sensitivity due to their base temperature instability. As a matter of fact, in correlation polarimeter, temperature fluctuations of the front-end devices, can both mimic a polarized signal and severely limit instrumental sensitivity. Here we discuss in detail the thermal design of the cryostat housing the instrument with particular attention to the closed loop cryocooler adopted, which is able to guarantee 6W at 77K with a stability better than 0.1 K over several hours.

The noise of radioastronomy receivers is usually kept low by cooling the front-end into cryostats, where the internal and the external environments are interfaced by optical windows. Such dielectric windows can sistematically correlate the incoming unpolarized radiation and decorrelate its polarized component. Here, we present a study on the effects of dielectrics in high sensitivity microwave polarimetry, including a model of the induced spurious polarization, a selection of materials in term of their optical properties as well as measurements of their optical parameters. The interesting result is that isotropic dielectrics can produce spurious polarization both when transmit anisotropic diffuse radiation or are not thermally uniform. Finally, such a model can be used to design a calibrator which generates very low polarized signal.

The Low Frequency Instrument aboard the PLANCK satellite will employ pseudo-correlation radiometers, operating over three broad bands centred at 30, 44 ,and 70 GHz. The radiometer scheme is based on the simultaneous comparison of two input signals, one coming from the sky and the other coming from a reference blackbody at a stable cryogenic temperature (near 4K) as close as possible to the sky temperature (about 2.7K). This choice is made in order to minimize non-white instrumental noise, typically exhibiting a 1/f spectrum. Effects due to the residual offset are minimised with a gain modulation factor applied in software. Fluctuations of the reference signal, due to fluctuation in the cooling chain or to straylight radiation, can also produce a parasitic signal which would mimic a true sky fluctuation. The PLANCK scientific goal of a high precision imaging of the CMB anisotropy requires an accurate characterisation of each part constituting the chain by using tools of modellisation and experimental tests. In this work we describe the concept of the radiometric chain, its functioning and the main sources of systematic errors, showing how, only with a hard modelling effort, it is possible to characterise, reduce and then remove in the data processing those systematic effects that may in principle compromise the quality of the whole instrument response.

This paper describes the design, development, testing and performance at Ball Aerospace of long life, 4-10 K temperature space cryocoolers. For temperatures down to 10 K, Ball has developed long life Stirling cycle cryocoolers. For temperatures to 4 K and below, Ball has developed a hybrid Stirling/J-T (Joule-Thomson) cooler. The hybrid cooler has been verified in test to 3.5 K on a Ball program and a 6 K Development Model is in development on the NASA/JPL ACTDP (Advanced Cryocooler Technology Development Program). The Ball ACTDP cooler Development Model will be tested in 2005. The ACTDP cooler provides simultaneous cooling at 6 K (typically, for either doped Si detectors or as a sub-Kelvin precooler) and 18 K (typically, for optics or shielding) with cooling stages also available at 40 and 180 K (typically, for thermal shields or other components). The ACTDP cooler is under development for the NASA JWST (James Webb Space Telescope), TPF (Terrestrial Planet Finder), and Con-X (Constellation X-Ray) missions. The 4-10 K Coolers are highly leveraged off previous Ball space coolers including multiple life test and flight units.

Upcoming major NASA missions such as the Einstein Inflation Probe and the Single Aperture Far-Infrared Observatory require arrays of detectors with thousands of elements, operating at temperatures near 100 mK and sensitive to wavelengths from ~50 μm to ~3 mm. Such detectors represent a substantial enabling technology for these missions, and must be demonstrated soon in order for them to proceed. In order to make rapid progress on detector development, the cryogenic testing cycle must be made convenient and quick. We have developed a cryogenic detector characterization system capable of testing superconducting detector arrays in formats up to 8x32, read out by SQUID multiplexers. The system relies on the cooling of a two-stage adiabatic demagnetization refrigerator immersed in a liquid helium bath. This approach permits a detector to be cooled from 300 K to 50 mK in about 4 hours, so that a test cycle begun in the morning will be over by the end of the day. The system is modular, with two identical immersible units, so that while one unit is cooling, the second can be reconfigured for the next battery of tests. We describe the design, construction, and predicted performance of this cryogenic detector testing facility.

Far-infrared detector arrays such as the 16x32 superconducting bolometer array for the SAFIRE instrument (flying on the SOFIA airborne observatory) require systems of readout and control electronics to provide translation between a user-driven, digital PC and the cold, analog world of the cryogenic detector. In 2001, the National Institute of Standards and Technology (NIST) developed their Mark III electronics for purposes of control and readout of their 1x32 SQUID Multiplexer chips. We at NASA's Goddard Space Flight Center acquired a Mark III system and subsequently designed upgrades to suit our and our collaborators' purposes. We developed an arbitrary, programmable multiplexing system that allows the user to cycle through rows in a SQUID array in an infinite number of combinations. We provided 'hooks' in the Mark III system to allow readout of signals from outside the Mark III system, such as telescope status information. Finally, we augmented the heart of the system with a new feedback algorithm implementation, flexible diagnostic tools, and informative telemetry.

A millimeter (mm) wave broadband video detecting system using high temperature superconducting (HTS) junction and compact pulse tube cryocooler (PTC) has been studied. The lowest attainable temperature of the PTC is 42K and the operating temperature (T) can be adjusted by changing the pressure difference in the compressor. By measuring the linewidth of the Josephson oscillation as well as the dynamic range of the Josephson detector, it is found that the PTC has no excess noise compared with other kinds of cryostats such as liquid helium cryostats, and is very suitable for the applications in the mm wave detecting system. Furthermore, to improve the sensitivity of the system, the coupling efficiency of the system has been studied in detail. It is found that the coupling efficiency increases with the increase of RN linearly, and is better than 1% for RN of 1.7 Ohm. A sensitivity of about 318V/W has been obtained for the system based on the PTC and a junction with RN=1.7 Ohm and ICRN =1mV.

Cryogenic detectors for astrophysics depend on cryocoolers capable of achieving temperatures below ~ 100 mK. In order to provide continuous cooling at 50 mK for space or laboratory applications, we are designing a miniature adiabatic demagnetization refrigerator (MADR) anchored at a reservoir at 5 K. Continuous cooling is obtained by the use of several paramagnetic pills placed in series with heat switches. All operations are fully electronic and this technology can be adapted fairly easily for a wide range of temperatures and cooling powers. We are focusing on reducing the size and mass of the cooler. For that purpose we have developed and tested magnetoresistive heat switches based on single crystals of tungsten. Several superconducting magnets are required for this cooler and we have designed and manufactured compact magnets. A special focus has been put on the reduction of parasitic magnetic fields in the cold stage, while minimizing the mass of the shields. A prototype continuous MADR, using magnetoresistive heat switches, small paramagnetic pills and compact magnets has been tested. A design of MADR that will provide ~ 5 uW of continuous cooling down to 50 mK is described.

Smooth walled Winston horns have been extensively used as light collectors for bolometric instruments. Used in multimoded operation without a waveguide, the beam shape is top-hat like and a simple equation is sufficient to define its Full Width Half Max. On the other hand, it is well known that corrugated feed horns are more efficient than smooth walled horns and much better well behaved with respect to polarization characteristics and sidelobe rejection. We present in this paper a study of corrugated Winston feed horns which could be used for future Astronomical instruments devoted to Cosmic Microwave Background (CMB) polarization measurements. We show that in this case low cross-polarization can be expected in single moded operation and that they could produce lower sidelobes levels compared to conical or profile shapes.

In this paper we analyse the properties of the Bloch oscillating transistor as a preamplifier in cryogenic devices. We consider here especially the readout of hot electron bolometers (HEBs) based on Normal-Superconductor-Insulator tunnel junctions, but the results also apply more generally. We show that one can get an equivalent noise voltage below 1 nV/&sqrt;Hz with a single BOT. By using N BOTs in a parallel array configuration, a further reduction by factor &sqrt;N may be achieved.

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.

We present a method for simulating the performance of millimetre-wave and submillimetre-wave STJ direct detectors when combined with commercially available op-amp based readout circuits. We employ full nonlinear modelling, together with frequency-domain analysis, to determine the responsivity, and then we use this responsivity, in conjunction with a detailed noise model, to calculate the NEP. By modelling the saturation of these devices, we are also able to calculate the dynamic range. Our method is capable of simulating a wide range of devices and takes into account the RF matching circuits. Using this approach, we have explored the effect of cooling STJs to different temperatures, and the effect of changing the frequency of operation. To achieve the best noise performance, the energy gap should be tailored to suit the operating frequency, and the device should be biased at a low voltage. We have performed detailed simulations to show that by using TaAl devices, and suitably chosen op-amp feedback components, an NEP of 6.0x10^-18 W/√Hz and dynamic range of 80 dB should be possible at 150 GHz: these conclusions draw on results already known from optical photon-counting experiments.

The Astronomical Instrumentation Group at University of Cardiff is already a UK center for submillimetre bolometric detector testing. The next generation of submillimetre astronomical instrumentation will incorporate arrays of transition-edge sensor (TES) bolometers. With the recently expanded facilities and personnel, the University of Cardiff is poised to become a UK centre for TES development and testing. We have undertaken a coordinated programme to develop TES simulation and test capabilities. One aspect of the programme is to address the problem of saturation of TES bolometers at high optical loads. We have developed a "tunable-G" device, which can vary its thermal conductance whilst in operation. For infrastructure, several sub-Kelvin cryogenic testbeds have been specifically designed to suit the requirements of testing submillimetre TES development bolometers. A description of our tunable-G device to solve the optical saturation problem will be given along with a description of the test facilities available at Cardiff.

We describe a new concept for a detector for the submillimeter and far infrared that uses a distributed hot-electron transition edge sensor (TES) to collect the power from a focal-plane-filling slot antenna array. Because superconducting transmission lines are lossy at frequencies greater than about 1 Thz, the sensors must directly tap the antenna, and therefore must match the antenna impedance (≫ 30 ohms). Each pixel contains many TESs that are all wired in parallel as a single distributed TES, which results in a low impedance that can match to a multiplexed SQUID readout. These detectors are inherently polarization sensitive, with very low cross-polarization, but can also be easily configured to sum both polarizations for imaging applications. The single polarization version can have a very wide bandwidth of greater than 10:1 with a quantum e±ciency greater than 50%. The dual polarization version is narrow band, but can have a higher quantum e±ciency. The use of electron-phonon decoupling obviates the need for micro-machining, making the focal plane much easier to fabricate than with absorber-coupled, geometrically isolated pixels. An array of these detectors would be suitable for an imager for the Single Aperture Far Infrared (SAFIR) observatory. We consider two near-term applications of this technology, a 32 £ 32 element imaging polarimeter for SOFIA and a 350¹m camera for the CSO.

We discuss a new type of direct detector, a silicon hot-electron bolometer, for measurements in the far-infrared and submillimeter spectral ranges. High performance bolometers can be made using the electron-phonon conductance in heavily doped silicon to provide thermal isolation from the cryogenic bath. Noise performance is expected to be near thermodynamic limits, allowing background limited performance for many far infrared and submillimeter photometric and spectroscopic applications. We report measurements of device I-V characteristics and terahertz surface impedance.

We present the design of two 22 GHz tunable bandpass filters based on variable capacitors (in Niobium MEMS technology) realized as short sections of superconductive lines with properties similar to microstrips. The air gap between the top electrode (the microbridge) and the bottom electrode of the thin film Niobium (Nb) microstrips can be varied by ~30% through the electrostatic force generated by a DC bias voltage. Electromagnetic simulation of the two filters predicts a tuning range of ~11% and ~14% of the central filter frequency. One goal of this development is to demonstrate the application of Nb microbridges for variable filters at 22 GHz in view of a transfer to several hundreds of GHz. All steps of the low temperature (<150°C) fabrication procedure are compatible with the fabrication of Nb-Al/AlOx-Nb SIS tunnel diodes, used in heterodyne high frequency mixers operated at 4 K. This fabrication procedure sets limits to the dimensions of the microbridges.