basic detection techniques front-end detectors for the submm andrey baryshev/wolfgang wild lecture...

Basic Detection TechniquesBasic Detection Techniques

Front-end Detectors for the SubmmFront-end Detectors for the Submm

Andrey Baryshev/Wolfgang Wild Andrey Baryshev/Wolfgang Wild

Lecture on 21 Sep 2006Lecture on 21 Sep 2006

Basic Detection Techniques – Submm receivers 2

Contents overviewContents overview

• Submm / THz regimeSubmm / THz regime• Definition and significanceDefinition and significance• Science examplesScience examples

• Submm detection: direct + heterodyneSubmm detection: direct + heterodyne• Heterodyne receiver systemsHeterodyne receiver systems

• Signal chain, block diagramSignal chain, block diagram• Heterodyne principleHeterodyne principle• Noise temperature and sensitivityNoise temperature and sensitivity• Heterodyne frontendHeterodyne frontend

• MixersMixers• Local oscillatorsLocal oscillators• IF amplifiersIF amplifiers

• Spectrometers: Filterbank, AOS, Autocorrelator, FFTSpectrometers: Filterbank, AOS, Autocorrelator, FFT• Overview submm astronomy facilitiesOverview submm astronomy facilities• Examples of heterodyne receiver systemsExamples of heterodyne receiver systems

• ALMA 650 GHzALMA 650 GHz• HIFI space instrumentHIFI space instrument

• Direct detection systemsDirect detection systems• Signal chain, block diagramSignal chain, block diagram• Types of direct detectors and operating principlesTypes of direct detectors and operating principles• Noise equivalent power (NEP)Noise equivalent power (NEP)• Examples of a direct detection systemExamples of a direct detection system

• Quasi opticsQuasi optics

Practical work at SRONMeasurement of sensitivityof heterodyne and direct detection system.


Submillimeter/THz Wavelength Regime ISubmillimeter/THz Wavelength Regime I

• λλ ~ 0.1 … 1 mm ~ 0.1 … 1 mm

• Photon energy corresponds 2-20 K in temperature scalePhoton energy corresponds 2-20 K in temperature scale(hF =kT) (hF =kT)

• Between infrared/optical and radio wavesBetween infrared/optical and radio waves

• Submm technology is relatively new (~ 20 years)Submm technology is relatively new (~ 20 years)(Compare to optical technology: ~ 400 years)(Compare to optical technology: ~ 400 years)

• Submm astronomy is crucial for understanding star and Submm astronomy is crucial for understanding star and planet formationplanet formation

• Range of 0.1… 0.3 mm is one of the last unexplored Range of 0.1… 0.3 mm is one of the last unexplored regimes in astronomyregimes in astronomy


Submillimeter Wavelength Regime IISubmillimeter Wavelength Regime II

• Technically challenging and interestingTechnically challenging and interesting

Challenging: small Challenging: small λλ means high precision means high precision fabricationfabrication

Interesting: Combination of optical and Interesting: Combination of optical and electronic techniqueselectronic techniques

• Submm astronomy and technology are very Submm astronomy and technology are very dynamic fieldsdynamic fields


Advantages of THz radiationDefinition

Frequency range 0.5 - 6 THz

Emerging field (largely unexplored)

Unique properties

Many spectral features in THz region

See through many materials

Sensitive to water

Presently used for astronomy, Earth observation

1296 1296.5 1297 1297.5 1298 1298.5 1299 1299.5

0.2

0.4

0.6

0.8

1

1.2

Frequency (GHz)

Sig

nal (

a.u

.)

Image of a galaxy

Water, gas

Spectrum of ethanol and water

THz imageTHz radar image

Image made by A. Baryshev

Image made by A.Baryshev


Why submillimeter ?Why submillimeter ?Sub-/Millimeter vs. optical astronomy Sub-/Millimeter vs. optical astronomy

ItemItem Sub-/MillimeterSub-/Millimeter Optical / IROptical / IR

WavelengthWavelength

FrequencyFrequency0.1 mm to 3 mm0.1 mm to 3 mm

100 GHz to 3 THz100 GHz to 3 THz0.4 to 30 0.4 to 30 μμmm

10 to 600 THz10 to 600 THz

TargetsTargets Cold medium Cold medium

(10-100K)(10-100K)

Molecular cloudsMolecular clouds

Extended structuresExtended structures

Hot medium Hot medium

(a few 1000K)(a few 1000K)

StarsStars

Point sourcesPoint sources

Sub-/Millimeter astronomy studies the Cold Universe.And most of the sky is dark and cold …


Radiation at (sub)mm wavelengthsRadiation at (sub)mm wavelengths

Continuum:

cold dust at 10-100 K

(black body of 30K peaks at

0.1 mm)

Lines: pure rotational transitions of molecules

Sub-/mm radiation probes cold molecular clouds of gas and dust

Energy levels of CO and CS


The Earth atmosphere at submm wavelenghtsThe Earth atmosphere at submm wavelenghts

• The Earth atmosphere is only partially transparent for The Earth atmosphere is only partially transparent for submillimeter wave radiationsubmillimeter wave radiation

• Several atmospheric “windows” existSeveral atmospheric “windows” exist

• Water vapor and oxygen cause strong absorptionWater vapor and oxygen cause strong absorption

dry, high observatory sitesdry, high observatory sites

airplane, balloon and space platformsairplane, balloon and space platforms


Atmospheric transmission at 5000m altitudeAtmospheric transmission at 5000m altitude

pwv = precipitable water vapour, i.e. the column height of condensed water vapour


Submillimeter astronomy – star formationSubmillimeter astronomy – star formation

• New stars form in molecular cloudsNew stars form in molecular clouds

• These clouds are best observed in the infrared and submm These clouds are best observed in the infrared and submm regime since they are cold and have high optical extinction regime since they are cold and have high optical extinction

• Star and planet formation is associated with a rich interstellar Star and planet formation is associated with a rich interstellar chemistry chemistry many lines observable in IR/submm/mm many lines observable in IR/submm/mm

JCMT Spectral SurveySpectral Survey IRAS16293- 2422IRAS16293- 2422


Cazaux et al. 2003


Optical vs. Submm/Far-InfraredOptical vs. Submm/Far-Infrared

Orion Trapezium Region at Optical

Wavelengths Highlighted Region at IR


Molecular gas in M31Molecular gas in M31

CO line emission traces molecular gas.

This is where new stars form.

Nieten et al. 2005


Dust and CO at z=6.4 !Dust and CO at z=6.4 !

IRAM 30m MAMBOBertoldi et al. 2003

Sloan survey:optical image

Contours: dust

=> Heavy elements formed shortly after Big Bang

Z=6.4


Bertoldi et al. 2003

Design of a Scientific Instrument 06 June 2006 19

Two Main Detection Schemes for Sub-/mm Radiation

• Incoherent detection direct detectors (bolometer)

• total power detection• no phase information used on single antenna• low spectral resolution

• Coherent detection heterodyne receiver

• frequency down conversion• high spectral resolution• phase information single antenna and interferometer

Heterodyne technique and receivers will be treated here.


Heterodyne Signal ChainHeterodyne Signal Chain

• Convert incoming radiation into electronic signal (IF) for further processing

• Spectral information is preserved (spectral resolution Δf/f determined by backend)

• Heterodyne detection achieves spectral resolution > 106

Heterodyne Instrument

Spectrometer/Correlator Data acquisition

“Front End” “Backend”

IntermediateFrequency (IF)

optical

electrical


Principle of Heterodyne Mixing

//0 fIF

LORFIF

Heterodyne principle = mixing of two frequencies (signal + local oscillator) to produce (sum and) difference signal (intermediate frequency = IF)

Mixing needs non-linear element (e.g. diode, SIS junction) = mixer

RF

Lowersideband

(LSB)

Uppersideband

(USB)

freq

f IF = | f LO - f RF |

Double sideband mixer: both sidebands converted to same IF

Single sideband mixer: Only one sideband converted to IF

Sideband separating mixer:two sidebands converted to different IF outputs


Combine strong LO signal VLO= cos(LOt) (e.g. 996 GHz)+ A weak RF signal VS= cos(St+) (e.g. 1002 GHz)

Gives total power absorbed P ~ VS VLO cos((S - LO)t + )+….

Amplitude and phase information conserved in IF signal

Detect radiation at frequencies where no amplifiers are available

IF signal

F requ en cy (G H z)

Pow

er

1 0 04

S ig na l S pec trum

F requ en cy (G H z)P

ower

IF S p ec trum

10 00 4 8

Heterodyne Mixing

996

LocalOscillator

Mixing needs strong non-linear detector charcteristic


Block Diagram of a Heterodyne ReceiverBlock Diagram of a Heterodyne Receiver

to correlatoror spectrometer

Astronomical RF signal(e.g. 650 GHz)

Optics

Mixer

Localoscillator

IF amp(s)

LO ref in

4 K

IF signal out(e.g. 4 GHz)

LO signal(e.g. 646 GHz)

Components: Optics• Mixer• Local Oscillator (LO)• Calibration source

• IF amplifier(s)• Dewar and cryogenics• Bias electronics• Spectrometer(s)

Calsource


A Heterodyne Receiver


A heterodyne receiver for spaceA heterodyne receiver for space

7 LO Beams

TelescopeBeam

~ 50 cm

HIFI = Heterodyne Instrumentfor the Far-Infrared

Will fly on the Herschel Space Observatory in 2008


HIFI Signal PathHIFI Signal Path

mixer

optics

WBS

IF

LOU

LSU HRS

ICU

IFspectrometers

InstrumentControl Unit

LocalOscillator

Unit

Local OscillatorSource Unit

FocalPlaneUnit

Telescope

To Astronomer


Main components of a heterodyne front-endMain components of a heterodyne front-end

• Optics Optics last part of this college last part of this college

• Submillimeter wave mixerSubmillimeter wave mixer SIS = Superconductor-Insulator-Superconductor SIS = Superconductor-Insulator-Superconductor HEB = Hot-Electron-BolometerHEB = Hot-Electron-Bolometer (Schottky = Semiconductor-metal contact diode)(Schottky = Semiconductor-metal contact diode)

• Local OscillatorLocal Oscillator Multiplier chainMultiplier chain Quantum-Cascade-Laser (QCL)Quantum-Cascade-Laser (QCL)

• Intermediate frequency (IF) amplifiersIntermediate frequency (IF) amplifiers


Sensitivity and Noise TemperatureSensitivity and Noise Temperature

• In radio and submm astronomy, the signal unit “Temperature” In radio and submm astronomy, the signal unit “Temperature” is used.is used.

• This is really a signal power W = k T This is really a signal power W = k T ΔνΔν (k Boltzman constant) (k Boltzman constant)

• Usually the signal power is much smaller than the noise power Usually the signal power is much smaller than the noise power (“noise temperature”) of the receiving system.(“noise temperature”) of the receiving system.

• The noise temperature of a system is defined as the physical The noise temperature of a system is defined as the physical temperature of a resistor producing the same noise power.temperature of a resistor producing the same noise power.

• Difference measurements are used to detect the signal, e.g.Difference measurements are used to detect the signal, e.g.

(sky + signal source) minus (sky)(sky + signal source) minus (sky)


The “ideal” submillimeter wave receiverThe “ideal” submillimeter wave receiver

Converts all incoming radiation into an electric signalConverts all incoming radiation into an electric signal

no photons “lost”no photons “lost”

has no own noise contributionhas no own noise contribution

However: Heisenberg’s uncertainty principle (However: Heisenberg’s uncertainty principle (ΔΔE x E x ΔΔt ≥ h/2t ≥ h/2ππ) ) makes such a noiseless mixer impossible.makes such a noiseless mixer impossible.

Why ? – A heterodyne mixer measures signal amplitude Why ? – A heterodyne mixer measures signal amplitude andand phase. This corresponds to number of photons and time in the phase. This corresponds to number of photons and time in the photon picture which – according to the uncertainty principle – photon picture which – according to the uncertainty principle – cannot be measured simultaneously with infinite precision. This cannot be measured simultaneously with infinite precision. This uncertainty results in a minimum noise of a heterodyne mixer, uncertainty results in a minimum noise of a heterodyne mixer, the “quantum limit”.the “quantum limit”.

Current best mixers are ~few times worse than the quantum limit.Current best mixers are ~few times worse than the quantum limit.


Sensitivity of a receiving systemSensitivity of a receiving system

The answer is the

Radiometer formula (Sensitivity): Tmin = c1 Tsys / (t )1/2

Received noise power from an antenna / receiver system:

Noise power Wsys = WA + Wrx = k Tsys = k (TA + Trx )

Tsys = TA + Trx

receiver noise temperature

antenna temperature (signal, atmosphere, antenna losses)

system temperature integration time

system bandwidth

Question: What is the smallest detectable signal ?


Noise Contributions from Receiver ComponentsNoise Contributions from Receiver Components

Receiver as a series of linear two-ports:

T1, G1 T2, G2 T3, G3 Tn, Gn

Todetector

Trx = T1 + T2 / G1 + T3 / (G1 G2 ) + … + Tn / ( G1 G2 …. Gn )

Receiver noise temperature determined by first few elements

Cooled optics for high frequencies

Optics Mixer 1st IF amplifier

Question: What is the noise contribution from different receiver components ?

T: noise tempG: Gain


HIFI signal chainHIFI signal chain

HIFI Dual IF System - one polarisationN. D. Whyborn, 021016

N.B. There is an identical arrangement for the other polarisation.

min. level:

IF gain:

max. level:

-128 dBm/MHz

-118 dBm/MHz

HRS-V

WBS-V

-108 dBm/MHz

-98 dBm/MHz

-95 dBm/MHz

-85 dBm/MHz

-100 dBm/MHz

-90 dBm/MHz

-3 dB -2 dB-5 dB25 dB-1 dB +21 dB -8 dB

6H

6L

5

4

3

2

1

2.4

- 4.

8 G

Hz

IF4

- 8

GH

z IF

10.4 GHz

2.4 - 4.8GHz

8 - 5.6GHz

min. level:

max. level:

-128 dBm/MHz

-118 dBm/MHz

-103 dBm/MHz

-93 dBm/MHz

-79 dBm/MHz

-69 dBm/MHz

-100 dBm/MHz

-90 dBm/MHz

mixer &isolator

IF up-converter spectrometerscryoharness

290 K (SVM)

warmharness

leveltrimming

15 K level4 K

level

IF-1amplifier

2 K level

IF-2assembly

IF gain: -16 dB -3 dB -2 dB-3 dB29 dB-1 dB +30 dB -6 dB

(+31 dB) (-10 dB)


Sub-/millimeter OpticsSub-/millimeter Optics

Main function: coupling of the antenna signal into mixer

Used components:

• Lenses (e.g. PTFE, quartz)• Mirrors (plane and focusing)• Feed horn• Grids (polarization separation)• quarter / half-wave plates• Martin-Puplett Interferometers

Gaussian optics used in sub-/mm regime (separate lecture)


Cryogenic submillimeter mixersCryogenic submillimeter mixers

SIS = Superconductor-Insulator-Superconductor SIS = Superconductor-Insulator-Superconductor

- used in mm and submm from ~70 GHz to ~1200 GHz- used in mm and submm from ~70 GHz to ~1200 GHz

- very good performance- very good performance

- theory well understood- theory well understood

- submm detector of choice at ground-based and space - submm detector of choice at ground-based and space

telescopestelescopes

HEB = Hot-Electron-BolometerHEB = Hot-Electron-Bolometer

- used above ~1200 GHz into THz regime- used above ~1200 GHz into THz regime

- performance better than SIS above 1200 GHz- performance better than SIS above 1200 GHz

- theory not well understood- theory not well understood

- active research on-going- active research on-going


The SIS mixerThe SIS mixer

The SIS mixer (Superconductor-Insulator-Superconductor) The SIS mixer (Superconductor-Insulator-Superconductor) element is a sandwich structure with a very thin insulator.element is a sandwich structure with a very thin insulator.

Superconductor-Insulator-Superconductor (SIS) Tunnel Junctions

SEM view of junction top electrode (1x1 µm²)

Cross section of a typical Niobium SIS tunnel junction

• insulator thickness <= 1nm : tunneling

S SI


Bandgap structure of an SIS mixerBandgap structure of an SIS mixer

Energy gap in density of states no current below Vbias = 2/e low shot noise

root singularity in density of states: large current flow at VGap

extremely sharp nonlinearity

0 2 4 60

50

100

150

200

Isg

Rsg

= 2mV/Isg

RN = dI / dV

VGap

Cu

rren

t [

A]

Bias Voltage [mV]

Superconductor 1at V ~ VGap

Superconductor 2grounded

Ins.

„Semiconductor“ model for SIS„Quasiparticle Excitations“ ~ Electrons

(Cooper pair tunneling effects not shown !)


SIS mixer principle = photon assisted tunnelingSIS mixer principle = photon assisted tunneling

F + eU

h F

Photon assisted tunneling (Dayem&Martin)series of steps at V = UGap – nh/e

Frequency limit for mixing at h = 4 (1400 GHz for Nb)

LO power: PLO ~ (h/e)²/RN (800 GHz, 20 Ohms: 0.5µW)


Some formulasSome formulas


300, 400, 800 GHz photon steps300, 400, 800 GHz photon steps

0 1 2 3 4 5 6 70.0

0.1

0.2

0.3

0.4

Volt age mV

Cur

rent

mA


Different RF powerDifferent RF power

0 1 2 3 4 50.00

0.05

0.10

0.15

0.20

0.25

0.30

Volt age mV

Cur

rent

mA

Load line


Typical SIS mixer responceTypical SIS mixer responce


SIS mixer implementationSIS mixer implementation

Task: Couple the astronomical signal to the (very small, ~1 Task: Couple the astronomical signal to the (very small, ~1 μμmm22) ) tunnel junction. Two ways are used:tunnel junction. Two ways are used:

• Feedhorn and waveguide (waveguide mixer)Feedhorn and waveguide (waveguide mixer)

oror• A lens and antenna structure (quasi-optical mixer)A lens and antenna structure (quasi-optical mixer)


Example of a waveguide SIS mixer (540-700 GHz)Example of a waveguide SIS mixer (540-700 GHz)

Feed horn MagnetJunctionholder

Lens

10 mm


Precision machiningPrecision machining

Backshort cavity Mixer backpiece Terahertz mixer

Humanhair

0.1 mm

With SIS chipand tunnel junction


HIFI mixers 800-960 GHz and 960-1120 GHzHIFI mixers 800-960 GHz and 960-1120 GHz

These mixers fly now on the Herschel Space Observatory


HIFI mixer designHIFI mixer design

magnet

Device mount with backshort, substrate channel and alignment spring

Magnet pole shoes

IF-board

Corrugated horn

ESD protection, bias and LF filtering

Pressure unit

Re-alignmentspring

Cover for bias/ESD PCB


Example of a quasioptical mixer structureExample of a quasioptical mixer structure

Mixer chip LensAntennastructure

SIS junction Stripline

0,25

mm

10 mm


Quasi-optical mixer implementationQuasi-optical mixer implementation

Silicon lens IF board

Quasi-optical mixer for the Space instrument HIFI

Chalmers Technical UniversityGothenburg, Sweden

1.5 THz

Main challenges: - chip alignment on lens- optical properties, beam direction


Hot electron bolometer (HEB) principleHot electron bolometer (HEB) principle

Thin superconducting film

Square law power detector

thermal time constant t = C/GC: thermal capacitanceG: thermal conductivity

Mixer operation: can detect beat frequency between LO and signal

has to be very fast (ps) for few GHz IF(needed for spectroscopy)


Hot electron bolometer (HEB) mixerHot electron bolometer (HEB) mixer

e

e e

ph

ph

S

Substrate

radiation

eL

ee

ee ee

phph

phph

S

Substrate

radiation

eeL

• Radiation heats electrons R• Cooling either by phonons or out-diffusion• Direct or heterodyne detection

Principle of operation

1 m x 0.15 m (W x L)

Hot Electron Bolometer

Limitations• IF bandwidth, sensitivity


Typical I-V cirvesTypical I-V cirves

0 0 .5 1 1 .5 2 2 .5B ias V o ltage (m V )

-0 .03

-0 .02

-0 .01

0

0 .01

0 .02

0 .03

)A

m( tnerruC sai

B

I -V curves

380.

370.

360.

350.

340.

330.

320.

310.

300.

290.

280.

270.

260.

250.

240.

230.

220.

210.

200.

190.

180.

Pumping power


Submm mixer noise temperaturesSubmm mixer noise temperatures

HIFI spaceinstrument

Jan 2006

• Mixer noise increases with frequency (increased losses)

basic detection techniques front-end detectors for the submm andrey baryshev/wolfgang wild lecture...

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