We present a design for a multipixel, multiband (100 GHz, 200 GHz and 400 GHz) submillimeter instrument: SAMBA (Superconducting Antenna-coupled, Multi-frequency, Bolometric Array). SAMBA uses slot antenna coupled bolometers and microstrip filters. The concept allows for a much more compact, multiband imager compared to a comparable feedhorn-coupled bolometric system. SAMBA incorporates an array of slot antennas, superconducting transmission lines, a wide band multiplexer and superconducting transition edge bolometers. The transition-edge film measures the millimeter-wave power deposited in the resistor that terminates the transmission line

Bock, James J.

Goldin, Alexey

Hunt, Cynthia

Lange, Andrew E.

LeDuc, Henry

Vayonakis, Anastasios

Zmuidzinas, Jonas

English

Alexey Goldin

James J. Bock

Cynthia Hunt

Andrew E. Lange

Henry LeDuc

Anastasios Vayonakis

Jonas Zmuidzinas

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SAMBA: Superconducting antenna-coupled, multi-frequency, bolometric array

Caltech Authors - Main

SAMBA: Superconducting Antenna-coupled,
Multi-frequency, Bolometric Array
Alexey Goldin*, James J. Bock*, Cynthia Huntf, Andrew E. Lange1",
Henry LeDuc*, Anastasios Vayonakis1^ and Jonas Zmuidzinas1^
*JPL, 4800 Oak Grove drive, Pasadena, CA
* California Institute of Technology, Pasadena, CA
Abstract. We present a design for a multipixel, multiband (100 GHz, 200 GHz and 400 GHz) sub-
millimeter instrument: SAMBA (Superconducting Antenna-coupled, Multi-frequency, Bolometric
Array). SAMBA uses slot antenna coupled bolometers and microstrip filters. The concept allows
for a much more compact, multiband imager compared to a comparable feedhorn-coupled bolo-
metric system. SAMBA incorporates an array of slot antennas, superconducting transmission lines,
a wide band multiplexer and superconducting transition edge bolometers. The transition-edge film
measures the millimeter-wave power deposited in the resistor that terminates the transmission line.
INTRODUCTION
The SAMBA architecture promises a lot of advantages compared to existing systems.
Unlike phase sensitive detection techniques, direct detectors can be photon-noise limited
under extremely low-backgrounds, even those of dispersive spectroscopy with a cooled
aperture. Antenna-coupled bolometers provide additional systems advantages such as
greatly reduced thermally active area, volume, heat capacity and thermal conductivity,
and improved rejection of stray light and out-of-band radiation. The large sub-K feed-
horn optics and metal-mesh filters currently required by missions such as Hershel and
FIRST which comprise > 95% of the sub-K focal plane mass and volume would be
eliminated by the new focal plane architectures.
To realize these advantages, we need to refine and combine the following existing
technologies: wideband superconducting stripline filters, long superconducting trans-
mission lines, and a large (for this band) area synthesized phased array using slot anten-
nae.
FILTERS
To design a frequency multiplexor, we needed fairly wideband filters with high out-of-
band input impedance. Using the prototype from [1], we designed a lumped elements
prototype 30% wide Chebyshev three pole 0.5 db filter (figure 1). Then capacitors
and LC elements were approximated by radial stubs and a combination of coplanar
waveguide with microstrip and analyzed with Supermix[2] — a C++ library, developed
CP605, Low Temperature Detectors, edited by F. S. Porter et al.
© 2002 American Institute of Physics 0-7354-0049-0/02/$ 19.00
251
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0.25 pF == 0.25 pF=p   
FIGURE 1. Filter schematics. Center frequency is 100 GHz, transmission line impedance - 5 Q. On
the right: 350GHz band filter manufactured at Micro Devices Laboratory (MDL) at JPL.
V
FIGURE 2. On the left: simulated transmission of three filters with bandpasses centered at 150,250 and
350 GHz. These are not the same bands as required for antenna in figure 3. Because of small (compared to
wavelength) size of filter elements, the next resonance of 150 GHz filter does not start until well beyond
the range of highest frequency filter, where it can be easily suppressed by lowpass filter if necessary. On
the right impedances of all 3 filters are shown. Out of band impedances are high as needed for multiplexor.
in Caltech primarily for simulation of SIS mixers. To finalize design, we used numerical
simulation with well-known Sonnet package.
FOCAL PLANE ANTENNA
The SAMBA architecture is using a passive phased array antenna. A familiar analog
of the SAMBA is a phased array of transmitting antennas. The amplitude and phase
of the electric field at each antenna control the pointing and far-field beam pattern of
such a phased array. Rather than actively controlling the electric field of each antenna,
SAMBA operates by passively adding the electric fields of each antenna in a phase-
coherent manner to synthesize diffraction-limited beams at multiple frequencies.
The fundamental element of such an array is the long slot antenna cut into a niobium
ground plane. Our numerical studies of such an array demonstrate fairly good broad-
band performance. The focal plane structure consists of slots separated by d, and every
slot tapped with a step d. One of the restrictions for d is the following: d < Xm/n/2, where
kfnin is the shortest wavelength of interest in silicon (to avoid scattering radiation into
sidelobes and surface waves). Another requirement is to maintain real part of antenna
252
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FIGURE 3. SAMBA integrated focal plane consisting of a densely packed array of slot antennas in a Nb
ground plane deposited on silicon. Microstrip taps cross the slot antennae and are coherently summed, first
along columns (right top inset), then along rows to a single transmission line per Airy disk at 400 GHz,
tapered to impedance of 5 Q. The line passes through a bank of filters. Low frequency lines are coherently
summed again, forming virtual diffraction size limited low frequency detectors. Then microstrip line goes
through a filter bank and terminates at normal film resistor, placed on SiN insulated island. The resistor
temperature is read out by TES thermistor, located at the same island. On the right: design of a 100 GHz
filter. Both groundplane and microstrip layer are 3000 Aniobium, with penetration depth assumed to be
about 700 A. Dielectric between two layers of niobium - 2000 A of silicon oxide. The whole structure is
photolitographed on silicon wafer.
impedance high enough to assure good coupling from transmission line to antenna. In
our case this second requirement was more restrictive, setting d to no more then 100 /jm.
The total size of the array is dictated by the size of Airy pattern at lowest frequency.
In our case, for F/3.3 optical system, the total array size is 11000 ^ m by 11000 jum.
We have done extensive EM simulations of the antenna array using our own software
which generalizes method described in [3]. The program is written in Numerical Python
and scales well up to arrays of 16 by 16 antennas on 450 MHz Pentium III desktop
computer. We also wrote another program to simulate infinite periodic array of slots.
The output of these programs is antenna impedance and voltage distributions along the
slot, which can be used to calculate beammap. Both of these programs give very similar
results for impedance of the antenna and beamshape.
LOSSES
The major sources of inefficiency are losses in transmission lines (see poster M03
by Anastasios Vayonakis), dielectric losses (unwanted radiation into lower hemisphere
and reflection at dielectric interfaces) and losses due to impedance mismatch between
antenna and transmission line. Transmission line losses will be highest (probably at least
253
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1 o
100 200 300 400
Frequency (GHz)
frequency =405.845 GHz
-0.4 -0.2 0 0.2 0.4
sin(0) cosfo)
FIGURE 4. On the left: Impedance of an antenna array shown on figure 3. Calculations were done using
generalization of method described in [3]. On the right: Beammap of one of 400 GHz pixels. Beammaps
of 100 GHz and 200 GHz channels have the same shape.
ffo-AA/r
- Dielectric
• Impedance matching
100 200 300 400 500 600
Frequency (GHz)
FIGURE 5. Losses due to coupling of real impedance transmission line to antenna array and due to
reflections on dielectric interfaces superposed with filters bandpasses. In a case of infinite silicon slab
dashed line would be a horizontal equal to n/(n+ 1) = 0.77, where n is the silicon refraction index.
30-40%). To reduce dielectric interface reflections, "layered cake" structure with graded
refraction coefficient was used. Silicon wafer with thickness 172 jum from the direction
of incoming radiation is covered with quartz 216 /mi thick and 405 /urn teflon layer.
ACKNOWLEDGEMENTS.
This research was supported by NASA grant NAG5-10317. Alexey Goldin is supported
by NRC Associateship stipendary award.
REFERENCES
1. Blinchikoff, H. J., and Zverev, A. I., Filtering in the time and frequency domains, Wiley, New York,
1976
2. Ward, J., et al, "Supermix: a Flexible Software Library for High Frequency Circuit Simulation, In-
cluding SIS Mixers and Superconducting Elements", in Proceedings, Tenth International Symposium
on Space Terahertz Technology, 1999, http://www.submm.caltech.edu/supermix
3. Eleftheriades, G. V., and Rebeiz, G. M., IRMMW (1993)
254
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Filtering in the time and frequency domains,

Supermix: a Flexible Software Library for High Frequency Circuit Simulation, Including SIS Mixers and Superconducting Elements&quot;,

to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

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SAMBA: Superconducting antenna-coupled, multi-frequency, bolometric array

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