Realization of multiplying type interferometerwith 2-elements 0.3K bolometer detectores

Yuan Luol , Makoto Hattori', Izumi S. Ohtal, Yuji Chinonel, Junichi Takahashi3,Yoshihiro Hamaji4, Hiroshi Matsuo5 and Nario Kuno6

'Astronomical Institude, Tohoku University, 6-3 Aoba Aramaki, Aoba-ku, Sendai, Miyagi, 980-8578, Japan2Purple Mountain Observatory, Chinese Academy of Science, 2 West Beijing Road, Nanjing, Jiangsu, 210008,China

3PROGRESS TECHNOLOGIES, Inc., 6-5-1 Nishi-Shinjyuku, Shinjyuku-ku, Tokyo, 163-1304, Japan4Toshiba I.S.Corp.,1-1-43 Shibaura, Minato-ku, Tokyo, 105-0023, Japan

5Advanced Technology Center, National Astronomical Observatory of Japan, 1-21-2, Ohsawa, Mitaka, Tokyo, 181-8588,Japan6Nobeyama Radio Observatory, National Astronomical Observatory of Japan, 462-2, Nobeyama Minamimaki,

Minamisaku, Nagano,384-1305,Japan

Abstract- We have developed two-elements 0.3K highsensitive bolometer detectors system optimized to Multi-Fourier Transform interferometer (MuFT) to realize themultiplying type bolometric astronomical interferometerin millimeter and submillimeter wave bands. A wiregridpolarizer is set in the detector system. The signal led to thebolometer detectors system is then divided into two parts.One of the two bolometer elements receives the reflectedsignal while the other receives the transmitted signal. Sincephases of mutual coherence functions in two signals areshifted if ,the difference of reflected and transmittedsignals reproduces full mutual coherence function and caneasily remove the baseline fluctuations. This method is forthe first time succeeded in moon observation at NobeyamaRadio Observatory, Japan, 2007.

I. INTRODUCTION

A bolometric interferometer is able to use a direct detectoras a focal plane detector for an aperture synthesis typeinterferometer. It could be one of the next generationinstruments for millimeter and submillimeter and far infraredwave band since it shares the advantages of both directdetectors and interferometer. We have been developing aMichelson type bolometric interferometer applying theaperture synthesis technique to the Martin & Puplett typeFourier transform spectrometer which is named as Multi-Fourier Transform interferometer (abbreviated as MuFT). Thissystem makes possible the imaging and spectroscopy in abroadband. Currently, all the proposed bolometric inter-ferometers, except MuFT, are adding type interferometer, bywhich the removal of baseline fluctuations is troublesome. Toresolve this problem, multiplying type interferometer has beenused for usual radio interferometer.We set a multiplying type interferometer astronomical

observation system based on MuFT with the 2-elements 0.3Khigh sensitive bolometer detectors in Nobeyama RadioObservatory, Japan. In this paper, we report results ofastronomical testing observations by this system.

II. MULTIPLYING TYPE ASTRONOMICAL INTERFEROMETEROBSERVATION SYSTEM

A. SpecificationsThe astonomical testing observations using the multiplying

type astronomical interferometer observation system set in inNobeyama Radio Observatory, Japan, have been in progress.This system uses a heliostat, which is an equatorial mountingsystem, as a tracking celestial system. It tracks an object byrotating the primary mirror along the polar axis. The beamfrom the targeted object is reflected along the polar axistoward the sencondary mirror. The sencondary mirror reflectsdown the beam along the vertical axis. The divison of wavefront of this beam is performed by two plane mirrors mountedunder the sencondary mirror of the heliostat. The two beamsare guided to the Fourier Interference part, and thenrecombined in this part and finally guided to the detector.Table I summarized specifications ofthis observation system.

TABLE ITHE PARAMETERS OF THE RECEIVING SYSTEM

Tracking Heliostat 0 =70(cm)Effective apertures 5(cm)x2Baseline length 14(cm)-35(cm)(center distance)Spatial resolution 49'-20' l150GHzSpectral resolution About 5(GHz)Dtector system 2-elements 0.3K NTD Ge bolometer detectors

B. Multi-Fourier Transform InterferometerThe Multi-Fourier Transform interferometer (MuFT) is a

millimeter and submillimeter Michelson-type bolometricinterferometer based on a Martin-Puplett type FourierTransform spectrometer. A wire grid polarizer (WG) is usedas a beam splitter in this system. So the wavelengthdependence of the reflectivity of a WG is small and is suitablefor a wideband measurement system. By setting the two inputWGs appropriately, one can measure the two-dimensionalintensity distribution of four Stokes parameters. The signalobtained by our system is a Fourier transformation of spectraand intensity distributions of four Stokes parameters (multiple

978-1-4244-1886-2/08/$25.00 ©2008 IEEE. GSMM2008 Proceeding

components) in the sky. Therefore we refer to this system as amulti-Fourier Transform interferometer.

Figure 1 shows the structure of MuFT with detector. Itconstitutes of Light Concentrating part(LiC), FourierInterference part(FI) and Detection & Sampling part(DeS).LiC part first performs a division of wave front of the incidentsource signal, and then guide the signal to the interference part.Fl part is the instrument in which the divided light beams arerecombined after modulating the one of the light path lengthand the heart of the MuFT system. The method of beammixing is the Michelson type. The instrument was designed byapplication of a Martin-Puplett-type Fourier Transform (MP-FT) to aperture synthesis. It has two entrance windows toreceive two light beams guided from the LiC. WGs are used asbeam splitters and beam combiners. The direction of WGs 1and 2 must be determined as follows: Consider light rays thatenter WGs 1 and 2 along the vertical axis. These rays arereflected by the WGs and travel toward rooftop mirrors alonga horizontal axis and are then reflected by these mirrors andtravel back toward WGs 1 and 2. Consider the projection ofWGs 1 and 2 onto the planes normal to these light rays. Thewires of these WGs must be titled 450 from the vertical axison these planes. The wires of the beam-splitter (BS) WG isdirected vertically as shown in Figure 8, to combine the twobeams taken by WGs l and 2 at the BS. The combined beam,either reflected or transmitted by the BS, is led to WG A or B45° from the vertical axis on the projected plane normal tothe transmission axis. Two direct detectors, bolometerdetectors, are mounted on the DeS unit in the system. One oftwo bolometer detectors detects transmitted signals throughthe output WG while the other one detects reflected signals.The difference in reflected and transmitted signals reproducesthe full mutual coherence function, as the phase difference inthe interferograms between two signals is -z And also,unwanted DC components are removed by this process. Thisis the essential point to realize multiplying type bolometerinterferometer.

Figure 1. The structure ofMuFT with detector

C. Bolometer Receiving SystemThe detector system is consists of two bolometer elements.

A wiregrid is set in the cryostat to separate the input signal. Asis shown in Figure 2, the transmitted signal is led to onebolometer so called chl and the reflected signal is led to theother so called ch2. The operation temperature can reach about0.3K by pulling liquid 4He and decompression to about 1.5Kand then releasing and absorbing 3He in the refrigeratorinstalled in the cryostat. Several types of optical filters arechosen and installed in the cryostat to cut the noise inunwanted frequency range. They are metelmesh low pass filter,polyethelene, black polyethelene filter, zitex, glass beads filterand yoshinaga filter. Including the wire grid, the transmittanceof the system optics is 14% and the accepted frequency rangeis 75GHz-170GHz. The property of a bolometer detector isexpressed by some physical parameter: time constant,sensitivity (Se), noise equivalent power (NEP), the quantumefficiency, etc. The time constant (cutoff frequency) isestimated by fitting with different frequency andcorresponding output voltage.

Figure 2. Top view of bolometer detector system

Table II summarized the parameters of receiver system.

TABLE IITHE PARAMETERS OF THE RECEIVING SYSTEM

Detector NTD Ge bolometerElements 2Band width(GHz) 75-170Operation Temperature(K) 0.3Cryogenic method Liquid 4He and 3HeCutoff frequency(Hz)(time constant) -16Se(V/W) 2x107

NEP( W/ Hz) 1.3x10-16

Transmittance ofthe system optics 14%Quantum Efficiency of detector 600%

III. RESULTS

We observed the moon using the multiplying typeinterferometer observation system on 3rdMarch, 2007. Figure3 is the interferogram of the moon. The horizontal axesshowes the sample number. In our system, the scan speed v ofthe moving mirror is 8 mm/s and for one scan, the movingdistance is 64 mm. In one scan, it takes 12600 sample points.

The vertical axes shows the amplitude. The black line showsthe output of bolometer chI and the blue line shows the outputof bolometer ch2. The output of bolometer chl minus that ofbolometer ch2 is the red line which is the interferogram of themoon.

The integration time for one scan is 8 seconds. Theamplitude and phase of the spectrum for each bolometer, chl1and ch2, are shown in Figure 4. The black line shows chI, theblue line shows ch2 and the red line shows (chil-ch2). This isthe first success of realization of multiplying type bolometricinterferometer!

500

E 0

-500

-00no I0 2.x0 .xG5BO 138( 13J XJ0 .xO .x0

sample number

Figure 3. The Interferogram of the moon (2007.03 .03)

IV. DiscuissoN

From Figure 4 we can see that there is 1/f baselinefluctuation for the amplitude for each In the spectrum (chl-ch2), the baseline fluctuation is removed. The signal to noiseratio is improved dramatically.This is the merit of multiplyingtype interferometer.

REFERENCES

[1] I.S.Ohta, M.Hattori and Hiroshi Matsuo, "Development ofmulti-Fouriertransform interferometerfundamentar', Appl.Opt, vo145, pp.2576-2585,Apr. 2006

[2] I.S.Ohta, M.Hattori and Hiroshi Matsuo, "Development ofa multi-Fourier transformInterferometer: imaging experiments in millimeter andsubmillimeter wave bands", Appl.Opt, vol46,pp.2881-2892, May. 2007

[3] I.S.Ohta et at., "The first astronomical mm and submm observations withMulti-Fourier Transform interferometerz", IRMMW-THz,2006

[4] I. S.Ohta et at: Development ofAstronomical Interferometer applyingFourier transform spectroscopy to aperture synthesis system in mm andsub-mm band, Doctor thesis, Tohoku University, 2003

[5] J.Takahashi, Development ofhigh sensitive millimeter wave bolometerdetectorfor Multi-Fourier Interferometer, Master thesis, TohokuUniversity, 2006

[6] Y.Luo,Development of2-elements 0.3K high sensitive bolometerdetectorsfor realization ofthe multiplying type bolometric astronomicalinterferometer, TohokuUniversity, 2007

Figure 4. Amplitude and Phase of Spectrum of the moon

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