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TREND – a low noise terahertz receiver user instrument for AST/RO
at the South Pole
Eyal Gerechta, Sigfrid Yngvessonb, John Nicholsonb, Yan Zhuangb, Fernando RodriguezMoralesb, Xin Zhaob, Dazhen Gub, Richard Zannonib, Michael Coulombec, Jason Dickinsonc,
Thomas Goyettec, Jerry Waldmanc, Christopher Groppid, Abigail Heddend, Dathon Golishd,
Christopher Walkerd, Antony Starke, Christopher Martine, and Adair Lanee
a
Department of Astronomy, University of Massachusetts at Amherst, Amherst, MA 01003;
Department of Electrical and Computer Engineering, University of Massachusetts at Amherst,
Amherst, MA 01003; cSubmillimeter Wave Technology Laboratory, University of Massachusetts at
Lowell, Research Foundation, Lowell, MA 01854; dDepartment of Astronomy and Steward
Observatory, University of Arizona, Tucson, AZ 85721; eSmithsonian Astrophysical Observatory,
Cambridge, MA 02138
b
ABSTRACT
Based on the excellent performance of NbN HEB mixer receivers at THz frequencies which we have established in the
laboratory, we are building a Terahertz REceiver with NbN HEB Device (TREND) to be installed on the 1.7 meter
diameter AST/RO submillimeter wave telescope at the Amundsen/Scott South Pole Station. TREND is scheduled for
deployment during the austral summer season of 2002/2003. The frequency range of 1.25 THz to 1.5 THz was chosen in
order to match the good windows for atmospheric transmission and interstellar spectral lines of special interest. The
South Pole Station is the best available site for THz observations due to the very cold and dry atmosphere over this site.
In this paper, we report on the design of this receiver. In particular, we report on HEB mixer device performance, the
quasi-optical coupling design using an elliptical silicon lens and a twin-slot antenna, the laser local oscillator (LO), as
well as the mixer block design and the plans for coupling the TREND receiver to the sky beam and to the laser LO at the
AST/RO telescope site.
Keywords: terahertz mixers, HEB, superconducting devices, quasi-optical receivers, submillimeter, radio telescopes
1. INTRODUCTION
Terahertz astronomy will eventually be pursued with the help of HEB low-noise receivers from platforms in space
(HERSCHEL), or in the upper atmosphere (SOFIA, balloons). However, these facilities are not yet available, and
meanwhile it is important to gain experience of this new technology by installing and employing HEB receivers on
ground-based telescopes at the best available sites. It has only recently been realized that observations above 1 THz are
feasible at such sites. One can also support an argument in favor of continuing the use of such receivers on ground based
telescopes in the future, given the fact that they can be dedicated to specific tasks for longer periods of time compared
with facilities such as SOFIA or HERSCHEL, and that larger diameter telescopes are feasible, such as the proposed 8
meter telescope at the South Pole Station [1]. Presently, the 1.7 meter diameter AST/RO submillimeter wave telescope is
operated at the South Pole by the Smithsonian Astrophysical Observatory [2], and has been successfully used up to the
800 GHz (350 µm) window for several years (see for example [3]).
NbN HEB THz receivers have been under development at the University of Massachusetts for a few years and are now
ready to be used for astronomical observations. TREND (“Terahertz REceiver with NbN Device”) is a low-noise
heterodyne receiver for the 1.25 THz to 1.5 THz frequency range. The receiver takes advantage of the atmospheric
attenuation window in the above frequency range, as well as the availability of AST/RO. We will describe the work done
so far on the TREND project. The system is being assembled and tested at UMass/Amherst, and will be shipped to the
South Pole before the beginning of the next austral summer season. Observations of NII are planned to start during the
austral winter of 2003.
2. SITE CONSIDERATIONS
The Antarctic Plateau, with an altitude of 2847 meters, is unique among observatory sites for unusually low wind speeds,
absence of rain, and an extremely cold and dry atmosphere. The median Precipitable Water Vapor (PWV) value is less
than 0.3 mm during the austral winter season. Available atmospheric models can be used with the measured amount of
PWV to predict the atmospheric transmission in two windows near wavelengths of about 200 µm, occurring from about
1.25 THz to 1.4 THz, and from 1.45 THz to 1.6 THz [4]. Expected median transparency at frequencies corresponding to
important spectral lines is from 5% to 11%, and on unusually good days may reach values 2 or 3 times higher.
Atmospheric transmission measured with an FTS instrument from the South Pole site [5] is shown in Fig. 1. These
measurements show good atmospheric transparency and confirm the above model predictions. It is clear that installing a
low noise THz receiver at the South Pole site is thus well justified.
We have identified three spectral lines, located in the above atmospheric windows, which are of special interest: (i) NII
(singly ionized nitrogen), at 1461.3 GHz (205.4 µm), is the second strongest spectral line overall in a typical galaxy
(only CII at 156 µm is stronger). NII should be ubiquitous in the warm interstellar medium (WIM) of our galaxy; (ii)
H2D+, at 1370.0853 GHz, is an important probe for gas phase chemistry; and (iii) the J=13→12 and J=11→10 lines of
CO at 1267.014 GHz. It is important to observe higher order CO lines and compare these with the well studied
millimeter lines of CO in warmer, denser sources.
The locations of the spectral lines for the above lines relative to the atmospheric transmission spectrum are marked in
Fig. 1. We plan to concentrate on the NII line during the first observing season.
Fig. 1: Atmospheric transmission at the South Pole measured with a Fourier Transform Spectrometer on 26 June 2001 [5]. The
spectral resolution is about 15 GHz.
3. RECEIVER DESIGN
3.1 General considerations
Waveguide-coupled NbN HEB mixers have been used on astronomical telescopes up to 1.04 THz [6] and have proven to
be easy to operate in this environment. For the 200 µm window, we will make use of quasi-optical coupling as in most
laboratory experiments with HEB mixers. The lowest receiver noise temperatures measured so far at about 1.5 THz are
500 K [7] and 650 K [8]. NbN HEB mixers are also very insensitive to changes in bias conditions and LO power and
should be easy to adapt to the observing logistics at AST/RO, where all observations in the Austral winter season are
performed by one or two “winter-over” operators. Whereas multiplier LO sources are expected to be available in the
future, a laser LO was chosen for TREND, since it is a mature technology in the THz regime and will lend itself well to a
future upgrade of the system to incorporate a multi-pixel focal plane array. Laser sources can also be made tunable over
±100 GHz by using Sideband Generator Technology [9].
3.2 Active device
Phonon-cooled HEB mixers are typically fabricated from NbN films on either silicon or MgO substrates. The MgO
substrate has the advantage that the phonon transmission probability is higher than for NbN on silicon. Since the phonon
escape time contributes substantially to the thermal time-constant of NbN bolometers, NbN mixers on MgO have wider
IF bandwidth for a given NbN film thickness (t). There is also some evidence that NbN/MgO devices may yield lower
receiver noise temperature [8]. We therefore plan to use NbN devices on MgO substrates for TREND. The IF
(conversion gain) bandwidth has been measured to be as large as 4.8 GHz for NbN/MgO [10]. It is important to note that
the receiver noise bandwidth is about twice the gain bandwidth. As explained in section 3.4 below, we expect an IF of
1.7 GHz for observations of the NII line; the IF bandwidth thus poses no problem. Future needs for other spectral lines
which yield IF frequencies of up to 5 to 6 GHz can easily be satisfied as well using a different IF amplifier. Present
devices are 1 µm × 4 µm and are fabricated with UV lithography. Smaller devices can be made with e-beam lithography.
Both processes are much simpler than the one required for diffusion-cooled HEBs.
3.3 Quasi-optical coupling to the HEB mixer
In our NbN HEB development work [7], we have made use of a quasi-optical coupling scheme consisting of a 4
millimeter diameter elliptical silicon lens, coupled to a self-complementary toothed log-periodic antenna on a silicon
substrate. The optimum polarization direction is frequency-dependent for log-periodic antennas. We have therefore
chosen a twin-slot antenna for the TREND receiver, see Fig. 2 and Fig. 3. The bandwidth of similar twin-slot antenna
HEB mixers has been shown to be wider than required for matching the entire 200 µm atmospheric window, about 1.25
to 1.6 THz [10]. The dielectric constant of MgO is very similar to that of silicon and it has been shown that devices on
MgO substrates perform well with silicon lenses without modification [8]. Our NbN HEB mixers do not saturate on the
total thermal noise picked up by the antenna. Moreover, choosing a twin-slot antenna with a narrower bandwidth than
the log-periodic antenna makes the receiver even less sensitive to saturation and direct detection effects. We will make
use of a parylene AR coating in order to reduce the receiver noise temperature by about 30%, as described in [12-14].
Fig. 2: Photograph of a NbN mixer device. The device is coupled to the twin-slot antenna to the right in the picture. To the left
is the filter through which the IF is extracted.
Fig. 3: Quasi-optical coupling scheme for the TREND receiver.
The beamwidth of the quasi-optical system is primarily determined by the diameter of the elliptical lens and has been
measured as reported in an earlier paper [15]. The 3 dB beamwidth was determined to be 3.4 degrees at 1.56 THz and
2.15 degrees at 2.24 THz. The beamwidth will be matched to the beam of the AST/RO telescope in the plane used for
installing receivers.
3.4 Local oscillator
The LO source is a model # SIFIR-50 FPL THz gaseous laser system which was designed and built by the
Coherent/DEOS company [16], and was delivered in December, 2001. As other THz gas lasers, it is pumped by a CO2
laser. In the case of the TREND laser, the pump laser is sealed, and is expected to be able to operate at least 10,000 hours
before it needs to be refilled with gas, a feature which facilitates operation at a remote site. The pump laser is RF excited
and thus does not require a high-voltage power supply. Its maximum power output is 50 W on one of the strongest lines.
The CO2 laser is grating tuned through a PZT translator, and is actively frequency locked to the resonance frequency of a
high-Q temperature-stabilized Fabry-Perot resonator. The THz laser uses a thermally compensated design for amplitude
and frequency stability. All of the above components are integrated into a rugged, transportable package, with
dimensions of about 185 cm × 50 cm, and height of about 25 cm. The power supply and the control electronics are
housed in a single compact control unit, from which various features of the laser operation, such as for example whether
the pump laser is frequency locked or not, can be observed. The laser system requires liquid cooling. Fig. 4 shows a
photograph of the TREND laser system.
The THz output power of the laser system is in the range of 100 mW on strong lines. The frequency spectrum has a
width of about 100 kHz. The center laser frequency is stable to within about 1 MHz and is determined by maximizing
the THz laser output power. It is empirically known that the laser frequency will then be within about a MHz from the
center of the rotational spectral line used for the lasing transition. The line frequency is stable to a small fraction of a
MHz. This absolute frequency stability is sufficient for all spectral line use in astronomical observations of lines from
the Interstellar Medium (ISM). Note that the excellent laser frequency stability eliminates the need for the traditional
frequency-lock system, based on a multiplied low-frequency source, in the TREND receiver. The short term amplitude
stability depends on the particular line used, and has been measured to be ±0.5% per minute for similar lasers, with a
long term stability of ±5% per hour. We are presently measuring the stability of the particular line to be used for
observing NII, as described below.
Fig. 4: Photograph of the TREND laser system with the laser cover off. The power and control unit can be seen to the right.
It is usually possible to find a THz laser line which matches a particular line in the ISM, within the IF bandwidth of
typical HEB mixers (about 5 GHz). However, some portions of the THz range may not have a strong laser line available.
In the future, we anticipate developing a sideband generator [9], which will allow tuning of up to ±100 GHz from a
given laser line. This will allow us to choose a strong and amplitude stable laser line, which can be tuned to match any
interstellar line in that frequency range. At the present time, the best known laser line sufficiently close to the required
frequency of 1461.31 GHz for NII is a line produced by CD3OH at 1459.3913 GHz (205.423 µm wavelength), which
yields a convenient IF of 1.7 GHz. We should also point out that spectral tables for THz laser lines are often not
sufficiently accurate to predict the IF for a particular line combination. We found by direct measurement in comparison
to a multiplied frequency synthesizer source that the above CD3OH line was off by 3 GHz from the value reported in
tables. This measurement was reported in an earlier paper [14].
The CD3OH laser line demonstrates some of the constraints on obtaining a laser local oscillator at a specific THz
frequency. It was first operated in a different laser setup at STL with a maximum output power of about 2 mW. This
required quite a large amount of CO2 laser pump power (70 W) and lasing was only obtained when a very small (1 mm
diameter) hole coupler was used, indicating low laser gain for this particular line. The center of the CO2 pump laser line
(10P36) is offset from the center of the CD3OH line to be pumped. Since the offset is larger than the free spectral range
of the particular pump laser used, the laser cannot be operated at the optimum pump frequency. This explains the lower
than normal gain. The Coherent/DEOS pump laser has a larger free spectral range than the one used at STL, but still not
sufficiently large to reach the line center of the CD3OH pump line. Either a 1 mm or a 1.5 mm diameter hole coupler is
optimum. The maximum output power was measured at DEOS to be 6 to 9 mW, but operating at about half the
maximum power results in more stable operation. Operation at UMass/Amherst confirms this.
There are actually two lines in CD3OH which are pumped by the same pump line; the second one is located at about
215.8 µm, with the same polarization as the 205.4 µm line. We distinguish between the two lines by using a silicon
etalon, which has different attenuation for the two lines.
CD3OH has a second line (with a different CO2 pump line) which we also operated at STL and measured to be at
1265.513 GHz. This line matches that of the J = 11→10 transition of CO, with a conveniently low IF of 1.5 GHz. There
is no equally good match for the J = 13 → 12 CO transition. We have not yet settled on the optimum laser line for H2D+.
3.5 Mixer block and biasing
We have designed a new mixer block which is compatible with other mixer blocks used at the AST/RO facility. The
mixer block contains the lens, the substrate on which the NbN device and the antenna were fabricated, and a circuit
board which supplies the DC bias and connects the mixer to the IF amplifier. Fig. 5 and Fig. 6 show the present
configuration of the bias circuit, as well as the mixer block. The bias scheme utilizes a total of five wires plus ground.
Two pairs of wires are joined as shown in Fig. 5. The circuit enables us to monitor the voltage across the HEB device
(Vheb) as well as the current through the device (Isense), by measuring the voltage across the 20 Ω resistor. The bias supply
is fed through Vbias. The feedback of the actual device voltage to the supply realizes a very low Thevenin-equivalent
source resistance (3 Ω) at the device. The bias electronics was built at the University of Arizona, which is also providing
the wiring for a new IRLAB liquid-helium dewar to be used with TREND. The IF amplifier will be a 1-2 GHz balanced
amplifier design, which was originally designed at CalTech and has been used on earlier AST/RO receivers.
Fig. 5: Bias/IF circuit for the TREND receiver.
Fig. 6: Mixer block for the TREND receiver.
3.6 Optical system for coupling the TREND receiver to the AST/RO telescope
The AST/RO telescope has provisions for four different receivers, each occupying a receiver pallet/optical breadboard of
size 75 cm x 75 cm. Fig. 7 shows the planned position for the TREND receiver on one of these pallets. The laser will be
located on a separate optical breadboard in the ceiling, together with a HeNe laser for alignment, a pyroelectric detector
for continues power monitoring, and an attenuator/polarizer (two crossed wire grids). The laser beam is guided to the
receiver by mirrors L1 through L5. The “sky beam” from the telescope is guided through a hole in the ceiling onto a
rooftop mirror system, which is used to select the particular receiver one wants to use for observing. It is then directed to
the TREND receiver through three further mirrors. A thin mylar beam splitter is located in front of the dewar window,
and allows low-loss transmission of the sky beam, while reflecting only a small fraction of the laser LO beam toward the
TREND cryostat window. Most of the laser power is dissipated in an LO “beam dump”. This flexible arrangement
eliminates the need to use a diplexer for the LO injection. The TREND cryostat is mounted on an xy translator system for
alignment with the two beams. Further alignment degrees of freedom are available by turning the elliptical injection
mirrors (the closest mirrors for the respective beams).
Fig. 7: Optical system for the TREND receiver at the AST/RO telescope.
4. FUTURE EXTENSIONS
Given the availability of the laser LO at the AST/RO site, sufficient LO power is available such that future extensions to
THz focal plane arrays are possible. Such arrays will also include MMIC IF amplifiers integrated with the HEB
mixers/antennas, which are being developed under a new contract from the NASA Langley Research Center under the
Cross-Enterprise Technology Development program [17]. We are also developing a capability for NbN film fabrication
in collaboration with NIST/Boulder [18]. This capability will benefit future implementation of NbN HEB mixer
receivers for THz astronomy.
ACKNOWLEDGEMENTS
We gratefully acknowledge support for this project from the NSF program for Advanced Technologies and
Instrumentation (ATI), Division of Astronomical Sciences, NSF award # AST 9987319.
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