Deployment of TREND -A Low-Noise Receiver User Instrument at 1.25 THz to 1.5 THz

for AST/RO at the South Pole

Eyal Gerechta, Sigfrid Yngvessona, John Nicholsona, Yan Zhuanga, Fernando RodriguezMoralesa, Xin Zhaoa, Dazhen Gua, Richard Zannonia, Michael Coulombeb, Jason

Dickinsonb, Thomas Goyetteb, Bill Gorveattb, Jerry Waldmanb, Pourya Khosropanahc,Christopher Groppid, Abigail Heddend, Dathon Golishd, Christopher Walkerd, Jacob Kooie,

Richard Chamberline, Antony Starkf, Christopher Martinf, Robert Stupakf, NicholasTothillf, and Adair Lanef

aUniversity of Massachusetts at AmherstbSubmillimeter Wave Technology Laboratory, University of Massachusetts at Lowell

cChalmers University of Technology, Gothenberg, SwedendDepartment of Astronomy and Steward Observatory, University of Arizona

eCalifornia Institute of TechnologyfSmithsonian Astrophysical Observatory

AB STRACT - We have de vel oped and con structed a low noise re ceiver user in stru ment based

on HEB tech nol ogy, TREND (Terahertz RE ceiver with NbN HEB De vice). The plan was to in stall

TREND on the 1.7 me ter di am e ter AST/RO submillimeter wave tele scope at the Amundsen/Scott

South Pole Sta tion dur ing the aus tral sum mer sea son of 2002/2003. The fre quency range of 1.25 THz

to 1.5 THz was cho sen in or der to match the best win dows for at mo spheric trans mis sion and in ter stel -

lar spec tral lines of spe cial in ter est. The South Pole Sta tion is the best avail able site for ground-based

THz ob ser va tions due to the very cold and dry atmosphere over this site.

The TREND team is now able to re port that this re ceiver has been in stalled on sched ule and met

our goals for its per for mance. TREND is thus ready to per form as tro nom i cal ob ser va tions in the up -

com ing aus tral win ter sea son as soon as the weather be comes suit able for THz work. The first spec -

tral lines which will be ob served are the CO J = 11®10 line at 1.27 THz and the 1.46 THz line of NII.

TREND is an NbN Hot Elec tron Bolometer (HEB) type re ceiver, and the dou ble side band noise tem -

per a ture at 1.27 THz has been mea sured on the tele scope to be 1,200 K. The lo cal os cil la tor is a CO2

la ser pumped, am pli tude sta bi lized CD3OH gas la ser. The TREND re ceiver will pi o neer ob ser va tions

from a ground-based tele scope at fre quen cies well above 1 THz. This is also the first time that a re -

ceiver can po ten tially per form an ex ten sive study of the ubiquitous NII ion, first noted by COBE.

I. INTRODUCTION

A num ber of sig nif i cant tech no log i cal re search ef forts aimed at the de vel op ment of terahertz

low-noise het ero dyne in stru ments are un der way in lab o ra to ries around the world. In stru ments that

will be op er a tional in a few years in clude the far in fra red space tele scope (HERSCHEL) and other

plat forms in the up per at mo sphere (SO FIA, bal loons). Low-noise re ceiv ers based on HEB de vices

de ployed on ground-based tele scopes at the best avail able sites are be com ing op er a tional now, how -

ever. It has only re cently been re al ized that ob ser va tions above 1 THz are fea si ble at such sites.

Ground based tele scopes can be ded i cated to spe cific tasks for lon ger pe ri ods of time com pared with

fa cil i ties in space. Fur ther more, larger di am e ter tele scopes, such as the 8 me ter one un der con struc -

tion at the South Pole, will be su pe rior to air and space borne dishes in terms of an gu lar res o lu tion.

Pres ently, the 1.7 me ter di am e ter AST/RO submillimeter wave tele scope is op er ated at the South

Pole by the Smith so nian As tro phys i cal Ob ser va tory, and has suc cess fully per formed ob ser va tions

up to the 800 GHz (350 mm) window for several years.

NbN HEB THz re ceiv ers have been un der de vel op ment at the Uni ver sity of Mas sa chu setts for a

few years and are now ready to be used for as tro nom i cal ob ser va tions. TREND (“Terahertz RE ceiver

with NbN De vice”) is a low-noise het ero dyne re ceiver for the 1.25 THz to 1.5 THz fre quency range.

The re ceiver takes ad van tage of the at mo spheric trans mis sion win dows in the above fre quency

range, as well as the avail abil ity of AST/RO.

II. SITE CONSIDERATIONS

The Ant arc tic Pla teau, with an al ti tude of 2847 me ters, is unique among ob ser va tory sites for

un usu ally low wind speeds, ab sence of rain, and an ex tremely cold and dry at mo sphere. The me dian

Precipitable Wa ter Va por (PWV) value is less than 0.3 mm dur ing the aus tral win ter sea son. Avail -

able at mo spheric mod els can be used with the mea sured amount of PWV to pre dict the at mo spheric

trans mis sion in two win dows near wave lengths of about 200 mm, oc cur ring from about 1.25 THz to

1.4 THz, and from 1.45 THz to 1.6 THz. Ex pected me dian trans par ency at fre quen cies cor re spond ing

to im por tant spec tral lines is from 5% to 11%, and on un usu ally good days may reach val ues 2 or 3

times higher. At mo spheric trans mis sion mea sured with an FTS in stru ment from the South Pole site

[2] is shown in FIG. 1. These mea sure ments show good at mo spheric trans par ency and con firm the

above model pre dic tions. It is clear that in stall ing a low noise terahertz re ceiver at the South Pole site

is thus well jus ti fied.

We have iden ti fied two spec tral lines, lo cated in the above at mo spheric win dows, which are of

spe cial in ter est. The first is the NII (sin gly ion ized ni tro gen) line, at 1461.3 GHz (205.4 mm), which is

the sec ond stron gest spec tral line over all in a typ i cal gal axy (only CII at 156 mm is stron ger). NII

should be ubiq ui tous in the warm in ter stel lar me dium (WIM) of our gal axy. The other line is the

J=11®10 line of CO at 1267.014 GHz. It is im por tant to ob serve higher or der CO lines and com pare

these with the well stud ied mil li me ter lines of CO in warmer, denser sources. The lo ca tions of the

above lines rel a tive to the at mo spheric trans mis sion spec trum are marked in FIG. 1.

III. RECEIVER DESIGN

We have cho sen the quasi-op ti cal cou pling de sign for our phonon-cooled HEB (PHEB) mix ers.

Typ i cal state-of-the-art DSB re ceiver noise tem per a tures for PHEB re ceiv ers mea sured at about 1.5

THz are in the range of 500-1000 K [1,3]. NbN HEB mix ers are also very in sen si tive to changes in

bias con di tions and LO power and should be easy to adapt to the ob serv ing lo gis tics at AST/RO,

where all ob ser va tions in the austral win ter sea son are per formed by one or two “win ter-over” op er a -

tors. Whereas mul ti plier LO sources are ex pected to be avail able in the fu ture, a la ser LO was cho sen

for TREND, since it is a ma ture tech nol ogy in the terahertz re gime and will lend it self well to a fu ture

up grade of the sys tem to in cor po rate a multi-pixel fo cal plane ar ray.

Ac tive De vice

Phonon-cooled HEB mix ers are typ i cally fab ri cated from NbN films on ei ther sil i con or MgO

sub strates. The MgO substrate po ten tially re sults in a wider IF band width than for NbN on sil i con.

The IF band width re quire ment for TREND (1 – 2 GHz) can be sat is fied by us ing sil i con substrates,

FIG. 1. At mo spheric trans mis sion at the South Pole mea sured with an FTS [2].

Frequency [GHz]

400 600 800 1000 1200 1400 1600 1800

Tra

nsm

issi

on

0.0

0.2

0.4

0.6

0.8

1.0

NIIn

oi ssi ms

nar

T

CO 11®10

how ever, and our de vices for TREND were fab ri cated on this sub strate. We have de vel oped de vices

using (1) UV li thog ra phy and (2) e-beam writing. De vices fab ri cated with UV li thog ra phy at

UMass/Amherst are 1 mm (length) × 4 mm (width) whereas a set of smaller de vices, writ ten with

e-beam at Chalmers Uni ver sity of Tech nol ogy, are 0.4 mm × 4 mm. The 1mm long de vices have

some what higher re sis tance (200 – 400 W) and re cent de vices have shown noise tem per a tures from

1,500 K to 2,000 K. The shorter e-beam de vices are better matched to the an tenna and have yielded

noise temperatures below 1,000 K as discussed below.

Quasi-Op ti cal Cou pling

In our NbN HEB de vel op ment work, we have made use of a quasi-op ti cal cou pling scheme con -

sist ing of a 4 mm di am e ter el lip ti cal sil i con lens, cou pled to a self-com ple men tary toothed log-pe ri -

odic an tenna on a sil i con sub strate. The op ti mum po lar iza tion di rec tion is fre quency-de pend ent for

log-pe ri odic an ten nas. We have there fore cho sen a twin-slot an tenna de signed for a cen ter fre quency

of 1.3 THz for the TREND re ceiver, as shown in FIG.2. This an tenna is lin early po lar ized, per pen dic -

u lar to the slots. The band width of sim i lar twin-slot an tenna HEB mix ers has been shown to be wider

than re quired for match ing the en tire 200 mm at mo spheric win dow, about 1.25 to 1.6 THz. Our NbN

HEB mix ers show no sat u ra tion due to ther mal noise, and also no di rect de tec tion ef fects.

The beam width of the quasi-op ti cal sys tem is pri mar ily de ter mined by the di am e ter of the el lip -

ti cal lens and has been mea sured for an ear lier ver sion as re ported in [4]. The 3 dB beam width of the

TREND lens/an tenna com bi na tion was de ter mined to be 3.6 de grees roughly in the cen ter of the band

(1.4 THz) by us ing ring-shaped cut-outs in the hot load while do ing Y-fac tor mea sure ments. The

FIG.2. (a) A quasi-op ti cal de sign il lus tra tion; (b) A pho to graph of the twin-slot an tenna.The PHEB de vice in the cen ter is too small to be seen.

28 mm = 0.15 l0

(b)(a)

beam width var ies lin early with the wave length, and the fo cus ing op tics for the LO and sky beams on

the AST/RO tele scope were de signed to match the mea sured beam width of the TREND sys tem in the

cen ter of the band, see be low for fur ther de tails on the optics.

La ser Lo cal Os cil la tor

The LO source is a model # SIFIR-50 FPL terahertz gas eous la ser sys tem that was de signed and

built by the Co her ent/DEOS com pany [5]. As other terahertz gas la sers, it is pumped by a CO2 la ser.

In the case of the TREND la ser, the pump la ser is sealed, and is ex pected to be able to op er ate at least

10,000 hours be fore it needs to be re filled with gas, a fea ture which fa cil i tates op er a tion at a re mote

site. The pump la ser is RF ex cited and thus does not re quire a high-volt age power sup ply. Its max i -

mum power out put is 50 W on one of the stron gest lines. The CO2 la ser is grat ing tuned through a PZT

trans la tor, and is ac tively fre quency locked to one of the res o nance fre quen cies of a high-Q tem per a -

ture-sta bi lized Fabry-Perot res o na tor. The terahertz la ser uses a ther mally com pen sated de sign for

am pli tude and fre quency stability. Its cav ity length can be ad justed ei ther by a mi crom e ter or a PZT

trans la tor. All of the above components are in te grated into a rug ged, trans port able pack age, with di -

men sions of about 185 cm × 50 cm, and height of about 25 cm. The la ser sys tem re quires liq uid cool -

ing. FIG. 3 shows a pho to graph of the TREND laser system while being tested at AST/RO.

FIG.3. Pho to graph of the TREND la ser sys tem with the la ser cover off.

It is usu ally pos si ble to find a terahertz la ser line which matches a par tic u lar line in the ISM,

within the IF band width of typ i cal HEB mix ers (about 5 GHz). How ever, some por tions of the

terahertz range may not have a strong la ser line avail able. At the pres ent time, the best known la ser

line suf fi ciently close to the re quired fre quency of 1461.31 GHz for NII is a line pro duced by CD3OH

at 1459.3913 GHz (205.423 mm wave length), which yields a con ve nient IF of 1.7 GHz. The out put

power on this line in sta ble op er a tion is about 1 mW, which is suf fi cient since we found that less than

10 mW of la ser power at the dewar win dow was needed to pump the mixer to its op ti mum point. We

use a 6 mm thick beam split ter, which re flects about 1 % of the power. The CD3OH la ser line dem on -

strates some of the con straints on ob tain ing a la ser lo cal os cil la tor at a spe cific terahertz fre quency.

The cen ter of the CO2 pump la ser line (10P36) is off set from the cen ter of the CD3OH line to be

pumped. Since the off set is larger than the free spec tral range of the par tic u lar pump la ser used, the la -

ser can not be op er ated at the op ti mum pump fre quency. This re sults in lower than typ i cal la ser gain

and thus less out put power. There are ac tu ally two lines in CD3OH which are pumped by the same

pump line; the sec ond one is lo cated at about 215.8 mm, with the same po lar iza tion as the 205.4 mm

line. We dis tin guish be tween the two lines by us ing a sil i con etalon, which has dif fer ent at ten u a tion

for the two lines. CD3OH has an ad di tional line (with a dif fer ent CO2 pump line) mea sured to be at

1265.513 GHz. This line matches that of the J = 11®10 tran si tion of CO, with a con ve niently low IF

of 1.5 GHz.

The TREND La ser power is ac tively sta bi lized to ±0.01 dB (±0.25 %) over many min utes by a

feed back sys tem which senses the DC bias cur rent of the HEB de vice di rectly, and ap plies this sig nal

through an am pli fier/fil ter cir cuit to the FIR la ser PZT trans la tor, thus keep ing the op er at ing point

very sta ble. The time-con stant of this cir cuit is about 10 sec onds. Ad di tional long term sta bi li za tion

is achieved by con trol ling the con tin u ous flow of the gas in the FIR laser us ing a gas man i fold.

Mixer Block and Bi as ing

We have de signed a new mixer block that is com pat i ble with other mixer blocks used at the

AST/RO fa cil ity. The mixer block con tains the lens, the sub strate on which the NbN de vice and the

an tenna were fab ri cated, and a cir cuit board that sup plies the DC bias and con nects the mixer through

a ca pac i tor to the IF am pli fier. FIG.4 shows the pres ent con fig u ra tion of the bias cir cuit, as well as

pho to graphs of the mixer block. The bias scheme uti lizes a to tal of five wires plus ground (three wires

plus ground in side the dewar). Chip re sis tors of about 2 kW are placed in se ries with all leads, while

chip ca pac i tors in side the mixer block shunt tran sients to ground. Fur thermore, the DC ground is

sep a rate from the mixer block/IF ground. This bias scheme pro tects the de vices and min i mizes line

and other pickup. The bias elec tron ics was built at the Uni ver sity of Ar i zona. The IF am pli fier is a

1-2 GHz bal anced am pli fier de sign, which was orig i nally de signed at CalTech and has been used on

ear lier AST/RO re ceiv ers. The noise tem per a ture con tri bu tion of the IF chain is about 5 K.

IV. OPTICAL SYSTEM FOR TREND AT AST/RO

The AST/RO tele scope has pro vi sions for four dif fer ent re ceiv ers, each oc cu py ing a re ceiver

pal let/op ti cal bread board of size 75 cm × 75 cm. FIG.5 shows the po si tion of the TREND re ceiver on

one of these pal lets. The la ser is lo cated on a sep a rate op ti cal bread board in the ceil ing, to gether with

a HeNe la ser for align ment, a pyroelectric de tec tor for power mon i tor ing, and an attenuator/po lar izer

(two crossed wire grids). The la ser beam is guided to the re ceiver by mir rors L1 through L5. The “sky

beam” from the tele scope is guided through a hole in the ceil ing onto a roof top mir ror sys tem, which

is used to se lect the par tic u lar re ceiver one wants to use for ob serv ing. It is then di rected to the

TREND re ceiver through three fur ther mir rors. A 6 mm thick my lar beam split ter is lo cated in front of

the dewar win dow that al lows low-loss trans mis sion of the sky beam, while re flect ing only a small

frac tion of the la ser LO beam to ward the TREND cryostat win dow. Most of the la ser power is dis si -

pated in an LO “beam dump”. This flex i ble ar range ment elim i nates the need for a di plex er for the LO

in jec tion. The TREND cryostat is mounted on an x-y trans la tor sys tem for align ment with the two

beams. Fur ther align ment de grees of free dom are avail able by turn ing the el lip ti cal in jec tion mir rors

(L5 and R3, the clos est mir rors for the re spec tive beams). We have con structed two op ti cal paths for

the LO beam to com pen sate for the different polarization of the LO beam at the two frequencies of

operation (see FIG.5).

FIG.4. Pho to graphs of the mixer block and a draw ing of the bias cir cuitry.

V. RESULTS AND CONCLUSION

Four mixer blocks with PHEB de vices were con structed and trans ported to AST/RO. The mixer

blocks are readily ex change able and pro vide re dun dancy in case a de vice fails dur ing the win ter sea -

son. The best noise tem per a ture re sults mea sured at UMass with one of the smaller de vices are shown

in FIG.6. These mea sure ments show that the re ceiver cov ers the re quired tun able band width with

about a 30 % vari a tion in re ceiver noise tem per a ture across the band. The min i mum of the noise tem -

per a ture agrees with the de sign fre quency of the twin-slot an tenna, cen tered at 1.3 THz, and the band -

width is what is ex pected for this an tenna. The best dou ble side band to tal re ceiver noise tem per a ture

FIG.5. Op ti cal sys tem for the TREND re ceiver at the AST/RO tele scope. (a) 3-D sim u la tions of the re ceiver room; (b) TREND dewar with LO and Sky beams; (c) LO beam con fig u ra -

tion for the 205 mm line; (d) LO beam con fig u ra tion for the 237 mm line.

(c)

(a) (b)

(d)

mea sured on the tele scope was 1200 K at the 237 mm line. Other mea sure ments in the lab o ra tory prior

to ship ping the TREND sys tem showed that the noise tem per a ture was es sen tially un changed for a 1

dB vari a tion in LO power, when the bias volt age was low (about 0.6 mV). The to tal noise out put

power var ied by 2.5 dB for the same change in LO power. Such vari a tions do not oc cur on the tele -

scope sys tem since the LO power is ac tively sta bi lized in a way that main tains a con stant bias cur -

rent. We also mea sured short term fluc tu a tions of the out put power and de rived the Allen vari ance

from such data. Due to space lim i ta tions, we will de fer the pub li ca tion of this data to a fu ture pa per.

In con clu sion, we have shown that a com plex, la ser-pumped, HEB re ceiver sys tem can be trans -

planted from the lab o ra tory to a fairly re mote site and dem on strate sim i lar per for mance to that mea -

sured in the lab o ra tory. The sys tem will now be used for ac tual ob ser va tions as the best terahertz

weather con di tions at the South Pole usually oc cur dur ing the months of June through Au gust.

FIG.6. Double side band receiver noise tem per a tures at dif fer ent LO fre quen cies mea suredon one of the PHEB de vices fab ri cated by e-beam. The IF was 1.3 GHz. A mea sured point

at the IF for the CO 11®10 line is also plot ted. The noise tem per a ture point for the NII linewas found by in ter po la tion.

ACKNOWLEDGMENTS

We grate fully ac knowl edge sup port for this pro ject from the NSF pro gram for Ad vanced Tech -

nol o gies and In stru men ta tion (ATI), Di vi sion of As tro nom i cal Sci ences, NSF award # AST

9987319.

REFERENCES

1. E. Gerecht, C.F. Musante, Y. Zhuang, K.S. Yngvesson, T. Goyette, J. Dickinson, J. Waldman,

P.A. Yagoubov, G.N. Gol’tsman, B.M. Voronov, and E.M. Gershenzon, “NbN Hot Elec tron

Bolometric Mix ers - A New Tech nol ogy for Low Noise THz Re ceiv ers,” IEEE Trans. Mi cro -

wave The ory Tech., vol. 47, pp. 2519-2527, Dec. 1999.

2. Chamberlin R A, Mar tin B, Mar tin C, Stark A A, “South Pole Submillimeter Fou rier Trans form

Spec trom e ter [4855-83]”, in Mil li me ter and Submillimeter De tec tors, Pro ceed ings of SPIE

(Vol ume 4855), Kona, Ha waii, 2002),

3. See for in stance Mer kel, H., Khosropanah, P, Cherednichenko, S, and Kollberg, E., “Com par i son

of Noise Per for mance of NbTiN and NbN Hot Elec tron Bolometer Het ero dyne Mix ers at THz

Fre quen cies”, this sym po sium.

4. Yngvesson et al., “Terahertz Re ceiver With NbN HEB De vice (TREND) - A Low-Noise Re ceiver

User In stru ment for AST/RO at The South Pole”, 12th In tern. Symp. Space THz Tech nol ogy,

San Diego, CA, Febr. 2001, p. 262.

5. Co her ent/DEOS, 1280 Blue Hills Ave, Bloomfield, CT 06002.