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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. REFERENCES [1] A. Stark et al, "Plans for a 10-m Submillimeter Wave Telescope at the South Pole," SPIE Proceedings, Advanced Technology: Millimeter Wave, Radio and Terahertz Telescopes, Vol. 3357, pp. 495-506, 1998. [2] A. Stark, "AST/RO: A Small Submillimeter Telescope at the South Pole", Chapter in the forthcoming book "Small Telescope in the Age of Big Glass", ed. T. D. Oswalt, 2002. [3] C. Walker et al., "Pole Star: An 810 GHz Array Receiver for AST/RO," Proc. 12th Intern. Symp. Space THz Technol., San Diego, CA, Febr. 2001, p. 540. [4] A. P. Lane, "Submillimeter Transmission at South Pole", in Astrophysics From Antarctica, A. S. P. Conf. Ser., eds. G. Novak and R. H. Landsberg, (San Francisco, CA: Astr. Soc. of the Pacific), 141, 289-295. [5] R. A. Chamberlin, "South Pole Submillimeter Sky Opacity and Correlations with Radiosonde Observations", Journal of Geophysical Research: Atmospheres, 106 (D17), p. 20101-20113, 2001. [6] J. Kawamura et al., "Successful Operation of a 1 THz NbN Hot-Electron Bolometer Receiver," 11th Intern. Symp. Space THz Technology, The University of Michigan, Ann Arbor, MI, May 2000, p. 57. [7] E. Gerecht et al, "Development of Focal Plane Arrays Utilizing NbN Hot Electron Bolometric Mixers for the THz Regime," Proc. 11th Intern. Symp. Space THz Technology, The University of Michigan, Ann Arbor, MI, May 2000, p. 57. [8] M. Kroug et al., "NbN Hot Electron Bolometric Mixers for Terahertz Receivers," IEEE Trans. Applied Superconductivity, Vol. 11, 962-965, March 2001. [9] E.R. Mueller et al., "Widely Tunable Laser-Sideband THz Source for Spectroscopy and LO Applications,". Proc. 12th Intern. Symp. Space THz Technol., San Diego, CA, Febr. 2001, p. 504. [10] S. Cherednichenko et al., "IF Bandwidth of Phonon Cooled HEB Mixers Made from NbN Films on MgO Substrates," Proc. 11th Intern. Symp. Space THz Technology, The University of Michigan, Ann Arbor, MI, May 2000, p. 228. [11] D. Loudkov et al., “Broadband Fourier Transform Spectrometer (FTS) Measurements of Spiral and Double-Slot Planar Antennas at THz Frequencies”, Proc. 13th Intern. Symp. Space THz Technol., Harvard University, Cambridge, MA, March. 2002, to be published. [12] M. Ji et al., "Study of Parylene as Anti-Reflection Coating for Silicon Optics at THz Frequencies," Proc. 11th Intern. Symp. Space THz Technology, The University of Michigan, Ann Arbor, MI, May 2000, p. 407. [13] M. Ji, "Lens Coupled Printed Antenna Characterization," M.Sc. thesis, University of Massachusetts at Amherst, Department of Electrical and Computer Engineering, May 2001. [14] A. Gatesman et al., “An Anti-Reflection Coating For Silicon Optics At Thz Frequencies,” IEEE Microwave and Guided Wave Letters, Vol. 10, pp 264-266, July 2000. [15] K. S. Yngvesson et al., “Terahertz Receiver with NbN Device (TREND) – A Low-Noise Receiver User Instrument for AST/RO at the South Pole,” Proc. 12th Intern. Symp. Space THz Technol., San Diego, CA, Febr. 2001, p. 262. [16] Coherent/DEOS, Bloomfield, 1280. Blue Hills Ave, Bloomfield, CT 06002. [17] NASA Contract # NAS1-01058. [18] E. Gerecht et al., “NbN Film Development for Phonon-Cooled HEB Devices,” Proc. 13th Intern. Symp. Space THz Technol., Harvard University, Cambridge, MA, March. 2002, to be published.