Horn Coupled Multichroic Polarimeters for the Atacama Cosmology Telescope Polarization Experiment

0.45 MB·

by  Rahul Datta

JournalofLowTemperaturePhysicsmanuscriptNo.
(willbeinsertedbytheeditor)
R.Datta,1 · J.Hubmayr,2 · C.Munson,1 · J.
Austermann,3 · J.Beall,2 · D.Becker,2 · H.M.
Cho,2 · N.Halverson,3 · G.Hilton,2 · K.Irwin,2 ·
D.Li,2 · J.McMahon,1 · L.Newburgh,4 · J.
Nibarger,5 · M.Niemack,2,6 · B.Schmitt,7 · H.
Smith,1 · S.Staggs4 · J.VanLanen,2 · E.Wollack,8
Horn Coupled Multichroic Polarimeters
4
for the Atacama Cosmology Telescope
1
0
Polarization Experiment
2

n
a 07.20.2013
J

0
3 Abstract Multichroic polarization sensitive detectors enable increased sensitiv-
ityandspectralcoverageforobservationsoftheCosmicMicrowaveBackground
]
M (CMB). An array optimized for dual frequency detectors can provide 1.7 times
gain in sensitivity compared to a single frequency array. We present the design
I andmeasurementsofhorncoupledmultichroicpolarimetersencompassingthe90
.
h and 150 GHz frequency bands and discuss our plans to field an array of these
p detectorsaspartoftheACTPolproject.
-
o Keywords cosmic microwavebackground, superconducting detectors, feedhorn,TES,
r polarimeter,millimeter-wave,siliconlenses,antireflectioncoating
t
s
a
[

1 Introduction
1
v
Measurementofthefaintpolarizedsignalsofthecosmicmicrowavebackground
9
2 will provide a new window into the physics of inflation1, a measurement of the
0 neutrinomasssum2,andarichassortmentofotherastrophysicalandcosmologi-
8 calresults.Detectingthesesignalsrequiresinstrumentswithhighsensitivityand
. excellentcontroloversystematicerrors. Multichroicdetectorswithsensitivityto
1
0
4 1DepartmentofPhysics,UniversityofMichiganAnnArbor,MI48109
1 E-mail:[email protected]
2NationalInstituteofStandardsandTechnology,325Broadway,Boulder,CO80305
:
v 3DepartmentofAstrophysicalSciences,UniversityofColorado,Boulder,CO80309
i 4PrincetonUniversityDepartmentofPhysics,JadwinHall,P.O.Box708,Princeton,NJ08544
X
5Boulder Micro-Fabrication Facility, National Institute of Standards and Technology, 325
r Broadway,MS817.03,Boulder,CO80305
a
6CornellUniversityPhysicsDepartment,109ClarkHall,Ithaca,NewYork14853
7University of Pennsylvania Department of Physics and Astronomy, 209 South 33rd St.,
Philadelphia,PA19104
8NASAGoddardSpaceFlightCenter,8800GreenbeltRoad,Greenbelt,MD20771
2
Fig.1 Left:designofasinglehorncoupledmultichroicpolarimeterwithlabelsonthemajor
components.Center:Aphotographofacross-sectionofabroad-bandring-loadedcorrugated
feed horn fabricated by gold plating a stack of etched silicon platelets. Right: A photograph
ofaprototype90/150multichroicdetectorwiththemajorcomponentslabeled.Adescription
of these components is in the text. For clarity, the path light follows to reach the bolometer
correspondingtoYpolarizationinthe150GHzbandhasbeenhighlighted.(colorfigureonline)
multiple spectral bands within a single focal plane element offer an avenue to
achieve these goals. We present the status of our work on horn coupled multi-
chroicpolarimetersandourplanstodeployanarrayofthesedetectorsontheAt-
acamaCosmologyTelescope(ACT)aspartoftheACTPolproject3.TheACTPol
instrument is optimized to measure CMB polarization, CMB lensing, and other
secondaryanisotropies.
In§2wepresentthedesignofourhorncoupledpolarimeterswhichoperatein
both the 90 and 150 GHz bands. Measurements of these detectorsare compared
tosimulationsin§3.In§4wedescribethearrayoptimizationandimplementation
onACTPol.Thisincludesadiscussionofourapproachtobroad-bandARcoating
the silicon lenses used in the ACTPol optical design. We conclude in §5 with
a discussion of the future applicability of this technology and the schedule for
deploymentonACTPol.
2 MultichroicPixelDesign
Figure1showsarenderingofasinglemultichroicpolarimeterwithsensitivityto
the90and150GHzCMBbandsandphotographsofaprototypehornanddetector
chip. The beam pattern is defined by a broad-band ring-loaded corrugated feed
horn.Theincominglightisconvertedintoelectricalsignalsusingsuperconducting
circuitryonaplanardetectorwaferthatseparatestheincominglightaccordingto
polarization and frequency band. The signals are detected using transition edge
sensor(TES)bolometersandreadoutusingSQUIDscoupledtotheTESs.
Thisring-loaded4horndesignissimilartoastandardcorrugatedfeedbutwith
theadditionoffivering-loadedsectionsthatformabroad-bandimpedancematch-
ingtransitionbetweentheroundinputwaveguideandthecorrugatedoutputguide
thatdefinesthebeampattern.Thishorn wasoptimizedtoproduce around beam
with low cross-polarization over a band from 70 to 175 GHz, but has excellent
3
0 95 GHz ACTPol 120 GHz 165 GHz measured
E-pol Aperature simulation
−10 H-pol Stop
B) X-pol
er (d−20
w
o−30
P
−40
−50 0 50 −50 0 50 −50 0 50
Angle (deg) Angle (deg) Angle (deg)
Fig.2 Measurementsandsimulationsofthebeampatternoftheringloadedcorrugatedfeed
horn.ThesemeasurementsoftheE-,H-,andX-polbeamsweremadeusingaroomtemperature
beammappingsystem.TheanglesatwhichthebeamshittheACTPolaperturestoparegreyed
out.(colorfigureonline)
performance up to 225 GHz. Prototypes (including the one that has been cut in
half and photographed in Figure 1) have been fabricated by gold-plating stacks
of micro-machinedsiliconwafers. This fabrication method is nearly identicalto
whatwasdoneforthepreviousgenerationofsinglefrequencyNISThorncoupled
polarimeters5andisthemostcosteffectivewaytoproducethermalcoefficientof
expansionmatchedringloadedfeedsforthemillimeterband.
After passing through the horn, incoming radiation enters a detector holder
that supports a planar detector chip in front of a waveguide back short and uses
a waveguide choke to prevent leakage of fields from the waveguide. The detec-
tor chip (See Figure 1, right) uses a broad-band orthomode transducer6 (OMT)
to couple the incoming light from the waveguide onto high impedance coplanar
waveguide(CPW)lines.TheOMTseparatestheincominglightaccordingtolin-
ear polarization. The Y-polarized light is split onto the two vertically oriented
OMTprobesandpropagatethroughidenticalelectricalpathsthathavebeenhigh-
lighted in the figure. Along each path, a broad-band CPW to micro-strip (MS)
transitioncomprised of 7 alternatingsections of CPW and MS is used to transi-
tion the radiation onto MS lines. Next, diplexers comprised of two separate five
pole resonant stub band-pass filters separate the radiation into 75-110 and 125-
170GHzpass-bands.Thesignalsfrom oppositeprobes withinasinglesub-band
are then combinedonto a single MS line using the difference output of a hybrid
tee7.Signalsappearingatthesumoutputofthehybridareroutedtoatermination
resistoranddiscarded.
Thesedetectorsoperateovera2.25:1ratiobandwidthoverwhichroundwaveg-
uide is multimoded. However, the TE11 mode (which has desirable polarization
properties)couplestooppositefinsoftheOMTwitha180◦ phaseshiftwhilethe
higherordermodeswhichcoupleefficientlytotheOMT probes havea0◦ phase
shift. This fact allows the hybrid tee to isolate the TE11 signal at the difference
port and reject the unwanted modes at the sum port. This ensures single moded
performanceoverour2.25:1bandwidth.Fortestingpurposes,theprototypepixel
(shown in Figure 1) included additional bolometers connected to the hybrid tee
sum port. The architecture described above offers excellent control over beam
systematicsofcorrugatedfeeds,afrequencyindependentpolarizationaxisdefined
bytheorientationoftheplanarOMT,andametalskywardaperturetominimize
electrostaticbuildupthatisusefulforfuturespaceapplications.
4
90/150 GHz Multichroic Passband
2.5
Fig. 3 FTS measurements and simula-
e tionsofdetectorpass-bands.Forthe90and
ns2.0 150GHzchannels,measurements ofboth
o
p linear polarizations as well as the termi-
es1.5 natedsumportwerecarriedout.Theagree-
R
d ment between the sum port (even mode)
e measurementsandsimulations(graylines)
z1.0
ali show that these detectors are properly re-
m jecting unwanted waveguide modes. The
or0.5 agreementbetweenthemeasuredandsim-
N
ulatedpass-bandforthenominalXandY
0.0 polarizedpixelsisgood,buthasa2%shift
60 100 140 180 inthebandedges.(colorfigureonline)
Frequency (GHz)
3 MultichroicPixelPerformance
Extensivemeasurementshavebeencarriedoutonprototypepixels.Figure2shows
measurements of the horn beam pattern made using a warm beam mapping fa-
cility. The horn was mounted on a three-axis (Az/El/phi) rotating stage and the
measurements were carried out using a tunable microwave source and a diode
detector. These measurements show better than 1% agreement with theoretical
predictionsbasedonHFSS8 simulations,anddemonstrateroundbeamswithlow
cross-polarization.Calculationsofthetemperaturetopolarizationleakageinduced
bythesebeamsdemonstratethatthesepixelsachievetemperaturetopolarization
leakagesubdominanttotheCMBB-modesforr=0.01.Figure3showsmeasure-
ments and simulations of the detector pass-bands. These results show excellent
agreementwithsimulationsforthe90and150GHzpassbandsbothforthenom-
inal polarizationsensitivebolometers and the extra testbolometers connectedto
thesumportsofthehybridtee.Thisagreementverifiesthefunctionofthediplexer
andconfirmsthatunwantedmodesarebeingcorrectlyrejectedbythehybridtee.
Measurementsoftheopticalefficienciesofthesehorncoupleddetectors,carried
outbyplacing300Kand77Kloadsinfrontofacryostat,showend-to-endopti-
calefficienciesof65% and50% respectivelyforthe90 andthe150 GHz bands.
Weestimatethattheopticalefficiencyofthedetectorsareroughly10%higheraf-
teraccountingforthelossesinthenon-AR-coatedwindowsandfilterstackinthe
measurementset-up.Theseefficienciesareconsistentwiththepredictedlossesfor
thevariousdetectorcomponentsthatarelistedinTable1basedontand =0.005
for the SiO and tand = 0.003 for the SiN 9 dielectric layers. The agreement
2 x
betweensimulationsandmeasurementsofthebeam,pass-band,andopticaleffi-
Table1 Lossesinpost-OMTmicrowavecomponents
Item 90GHzloss 150GHzloss
CPW→MStransition 2.5% 4.1%
bandpassfilter 10.0% 13.5%
hybridtee 3.5% 3.3%
microstripline 7.8% 13.1%
totalefficiency 76% 66%
5
ciencydemonstratethefunctionof theprototype detectors.Wearenow working
onincorporatingthispixeldesignintoadetectorarrayforACTPol.
4 ArrayOptimizationandImplementationonACTPol
Maximizing the sensitivityof a fixed diameterdetectorarray for a given optical
system requires choosing the optimal horn aperture size. If the aperture is too
small,diffraction decreasesthe optical coupling to the telescopereducing sensi-
tivity.Iftheapertureistoolarge,thenfewdetectorsfitintothedetectorarrayand
sensitivity is also reduced. This tradeoff is evaluated quantitatively in Figure 4
whichpresentsthemappingspeedasafunctionoffeedapertureforasixinchdi-
ameterdetectorarraywiththeACTPoloptics.Thiscalculationtakesintoaccount
adetailedopticalmodelincludingtransmissionefficiencyandemissionfrom the
stop,filters,optics,andatmosphere.Thecalculationisperformedforavarietyof
CMB bands each with 1.4:1 ratio bandwidth. The peak of a single curve corre-
spondstotheoptimalhornapertureforasinglefrequencyarray.Foramultichroic
array one must choose a horn aperture that maximizes the sensitivity of two or
morechannelssimultaneously.Forthecaseoftwoneighboringbands(e.g.90and
150) it is possible to achieve90% of sensitivityof an optimized90 GHz and an
optimized150Ghzsinglefrequencyarraywithonemultichroicarray.
TheoriginalplanforACTPolwastofieldtwo150GHzarraysandathird90
GHzarrayusingthereadoutsystemfromACTthathad1024channelsperarray.
Thus for the ACTPol multichroic array optimization we were restricted by the
numberofavailablechannelswhichlimitedusto256feedhornswith4detectors
per pixel. This drove us to a larger aperture (7 mm) than the true optimum for
a multichroic array. This leads to nearly optimal mapping speed at 90 GHz, but
reducedsensitivityat150GHz.RepeatingthecalculationsshowninFigure4in-
cludingthedetectorlossesdescribedin§3showsthatthisarraywillachieve90%
themappingspeedofa90GHzarrayand60%themappingspeedofa150GHz
single frequencyarray. This comparison assumes lowerlossesfor thesingle fre-
quencyarraysowingtotheshorterMSlengthsanduseoffreespacefiltersinstead
of the stub filters. With this array we gain a factor of 1.35 in sensitivity, which
motivatesthedecisiontoswitchfromthebaselineplanforthe90GHzonlyarray.
Moreover,thisonlyresultsina4% inflationofthe 90GHz mapnoise measured
inm K.Infutureapplicationsofthistechnologyweplantoincreasethenumberof
readoutchannelsandoptimizetheopticaldesigntooptimallyfieldmultichroicar-
Fig.4 Asimulationofthemappingspeedasa
functionofhornapertureforasixinchdetector
arraywith1.4:1ratiobandwidthdetectorswith
thebandcentersaslabeled.Foramultichroicar-
raywithtwowidebandsitispossibletoachieve
sensitivityequivalentto1.7singlefrequencyar-
rays. Adding additional frequency bands leads
todiminishingreturnsinsensitivityastheseare
sub-optimally coupled to the telescope. (color
figureonline)
6
Three Layer Wideband AR coating
−15
target passband target passband
−20 TE @ 30º incidence Fig. 5 Left: A sim-
B)−25 TM = TE @ 0º incidence ulation of a wide
d
nce (−30 bmaentdamwiadtitehratlharneteirelflayece-r
ecta−35 TE @ 15º incidence tioncoatingforsilicon.
efl−40 TM @ 15º incidence Right: A silicon pro-
R
−45 TM @ 30º incidence totype of this coating.
(colorfigureonline)
−50
80 100 120 140 160
Frequency (GHz)
rays.Withthisoptimizationasinglemultichroicarraywouldbeequivalentto1.7
timesasinglefrequencydetectorarrayintermsofsensitivitywhilealsoboosting
spectralcoverage.
Implementing multichroic arrays requires optical designs with the required
widebandwidth.ThelimitingelementforACTPolopticsistheantireflection(AR)
coatingforthethreesiliconlensesthatmakeupthecryogenicre-imagingoptics.
WehavedevelopedanewapproachtoARcoatingcryogenicsiliconlensesbycut-
tingsub-wavelengthfeaturesintothe surfacesusingsilicondicingsawblades10.
Figure 5 shows the simulated performance and a photograph of a mechanical
prototype. This coating will meet the requirements for fielding this multichroic
array on ACTPol. A similar approach, consisting of conical holes drilled into
UHMWPE,willbeusedforARcoatingthereceiverdewarwindow.
5 ConclusionsandFutureWork
We have presented the design of our horn coupled multichroic polarimeters and
have shown data thatverifiesthe performance of prototypes. We are working on
thedevelopmentofamultichroicarraywith255feedhornsand1020bolometers
thatwillbedeployedonACTPolinearly2014.Thisarrayisprojectedtoachieve
thesensitivityof1.35singlefrequencyarrays.Futurearrayswithfullyoptimized
instruments can achieve performance equivalent to 1.7 single frequency arrays.
Wehavemadeprototypesofthisdesignforhigherfrequencychannels(180to360
GHz) which will soon be tested.We are also developing a new design based on
quad-ridge waveguide.Thisnewdesignwillenablewiderbandwidth(3.5:1ratio
bandwidth appears feasible) while substantially shortening the lengths of wires,
eliminatingthe need for a hybrid tee and thereby reducing loss especiallyat the
highfrequencyendofthepixeldesign.
Acknowledgements ThisworkwassupportedbyNASAthroughawardNNX13AE56Gandthe
NASASpaceTechnologyResearchFellowshipgrantNNX12AM32HandbytheU.S.National
ScienceFoundationthroughawardsAST-0965625andPHY-1214379.
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