CASIMIR, the Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver, is a far-infrared
and submillimeter heterodyne spectrometer, being developed for the Stratospheric Observatory For Infrared Astronomy,
SOFIA. CASIMIR will use newly developed superconducting-insulating-superconducting (SIS) mixers.
Combined with the 2.5 m mirror of SOFIA, these detectors will allow observations with high sensitivity to be
made in the frequency range from 500 GHz up to 1.4 THz. Initially, at least 5 frequency bands in this range
are planned, each with a 4-8 GHz IF passband. Up to 4 frequency bands will be available on each flight and
bands may be swapped readily between flights. The local oscillators for all bands are synthesized and tuner-less,
using solid state multipliers. CASIMIR also uses a novel, commercial, field-programmable gate array (FPGA)
based, fast Fourier transform spectrometer, with extremely high resolution, 22000 (268 kHz at 6 GHz), yielding
a system resolution > 106. CASIMIR is extremely well suited to observe the warm, ≈ 100K, interstellar medium,
particularly hydrides and water lines, in both galactic and extragalactic sources. We present an overview of the
instrument, its capabilities and systems. We also describe recent progress in development of the local oscillators
and present our first astronomical observations obtained with the new type of spectrometer.
CASIMIR, the Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver is a multiband, far infrared
and submillimeter, high resolution, heterodyne spectrometer under development for SOFIA. It is a first generation, PI
class instrument. CASIMIR is designed for detailed, high sensitivity observations of warm (100 K) interstellar gas both
in external galaxies and Galactic sources, including molecular clouds, circumstellar envelopes, and protostellar cores.
Combining the 2.5 m SOFIA mirror with state of the art superconducting mixers, will give CASIMIR unprecedented
sensitivity. Initially, CASIMIR will have two bands, at 1000 and 1250 GHz, and a further three bands, 550, 750, 1400
GHz, will be added soon after. Any four bands will be available on each flight. The availability of multiple bands during
each flight will allow for efficient use of flight time. For example, searches for weak lines from rare species in bright
sources can be carried out on the same flight with observations of abundant species in faint or distant objects.
We summarize the development and the delivery of two SIS mixers for the 1.1-1.25 THz band of the heterodyne
spectrometer of Herschel Observatory (HSO). The quasi-optical SIS mixer has two Nb/AlN/NbTiN junctions with
the area of 0.25 um2. The Josephson critical current density in the junction is 30-50 kA/cm2. The tuning circuit
integrated with SIS junction has the base electrode of Nb and a gold wire layer.
With the new SIS mixers the test receiver maximum Y factor is 1.41. The minimum receiver uncorrected DSB
noise temperature is 450 K. The SIS receiver noise corrected for the loss in the optics is 350-450 K across the
1100-1250 GHz band. The receiver has a uniform sensitivity in the full IF range of 4-8 GHz. The sub-micron
sized SIS junction shape is optimized to ease the suppression of the Josephson current, and the receiver operation
is stable. The measured mixer beam pattern is symmetrical and, in a good agreement with the requirements, has
the f/d =4.25 at the central frequency of the operation band. The minimum DSB SIS receiver noise is close to
6 hv/k, the lowest value achieved thus far in the THz frequencies range.
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.
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.
The Heterodyne Instrument for Far Infrared (HIFI) on ESA's Herschel Space Observatory is comprised of five SIS receiver channels covering 480-1250 GHz and two HEB receiver channels covering 1410-1910 GHz. Two fixed tuned local oscillator sub-bands are derived from a common synthesizer to provide the front-end frequency coverage for each channel. The local oscillator unti 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, and novel material systems in the SIS mixtures. The National Aeronautics and Space Administration's Jet Propulsion Laboratory is managing the development of the highest frequency (1119-1250 GHz) SIS mixers, the highest frequency (1650-1910 GHz) HEB mixers, local oscillators for the three highest frequency receivers as well as W-band power amplifiers, 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. The current state of the art for each of these devices is presented along with a programmatic view of the development effort.
The objective of the European project EMCOR was the development of a heterodyne receiver for the frequency range of 201 to 210 GHz for the measurement of the amounts of various minor constituents of the stratosphere involved in ozone chemistry. In order to be able to measure even very faint spectral lines a superconducting tunnel junction has been chosen as mixer element. Additionally, special care has been taken in developing the calibration unit of the system. Besides the classical hot-cold calibration three different balancing methods can be employed: a beam-switch technique with an atmospheric reference signal, a beam switch technique with a reference signal from a variable reference load or a frequency switch technique. The system has been integrated and is currently under testing. It will be installed at the International Scientific Station Jungfraujoch in he Swiss Alps and operated within the framework of the European Alpine stations of the Network for the Detection of Stratospheric Change.
We present a study of requirements on the sensitivity of mm and sub millimeter wave receivers for application in radio astronomy. The study is based on the experience of the operation of the SIS receivers at the radio telescopes of IRAM and considers also the conditions of operation at the future radioastronomical instruments such as LSA, MMA and LSMA. Using a radio telescope and atmospheric model we consider the effect of the receiver sensitivity on the radio telescope operation in terms of observing time. The observing time at a radio telescope depends on the system noise temperature as strongly as on collecting area, which defines nearly all the cost of modern mm and sub mm instruments. As a compromise between the receiver development effort and the system performance we introduce a criterion of an 'optimum' receiver noise. A strict requirement on the receiver operation is obtained. For an efficient use of the existing and the future radio telescopes, a 10 - 20 K SSB receiver noise in the mm band and 20 - 40 K SSB at sub millimeter wavelength are necessary. Finally the status and the perspectives for SIS receiver development for mm and sum mm radio astronomy are presented.
The integration of many receiver units into a receiver array is a common method of improvement of imaging systems. This approach, well known in the mm band for Schottky mixer arrays, has not so far been developed for Superconductor-Insulator- Superconductor (SIS) junction mixers, which give the best sensitivity in the short mm wave range and in the submm range. We demonstrate for the first time a practical low noise multibeam receiver module using SIS mixer technology. The module comprises three identical SIS mixers integrated with a common local oscillator, coupled through a three branch waveguide directional coupler. The multibeam module has been developed for a focal plane array receiver of the 30 meter radio telescope of the Institut de Radioastronomie Millimetrique (IRAM). Three such modules will be used in a 3 X 3 array operating near 230 GHz frequency. We discuss the requirements on the performance of the multibeam receiver module and compare it with a single beam receiver arrangement. After the presentation of a single mixer receiver operation the performance of the three mixer module is described. The basis for the integration of several SIS mixers with a common local oscillator source is given by the saturation of the SIS receiver noise dependence upon local oscillator power. The 1.3 mm SIS mixer block is built with a reduced height waveguide. The individual SIS junction area is 2.2 micrometer2 with a Josephson critical current density of about 3.6 KA/cm2. The minimum SSB receiver noise temperature at 230 GHz in a single beam receiver is as low as 50 K. In the module a common local oscillator power source is connected to the three mixers through a common three branch directional coupler. The performance of the three mixers is nearly identical across the 200 - 250 GHz band. The minimum DSB receiver noise temperature of 37 K is obtained simultaneously in all three channels around 230 GHz.
We present the noise properties of a mixer with the NbN-MgO-NbN quasiparticle tunnel junctions. Our work is based on experiment in the 120-180 GHz range with the NbN tunnel junctions in the mixer. Mixer printed circuit is totally made of NbN. The mixer operates at 5.4 K temperature unacceptable with Nb junctions. The minimum DSB receiver noise temperature is about 65 K at 162 GHz and approaches the Nb SIS mixer performance in mm band. It has been found that the noise sources in the NbN junctions are comparable to the Nb junctions and that the receiver noise with the NbN SIS mixer may be only few times more than the quantum limit of noise in the frequency range below the gap frequency. Output noise of the SIS mixer has been found constant in a wide frequency band and within an important range of the local oscillator amplitudes.