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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.
Basic Detection Techniques – Submm receivers 3
Basic Detection Techniques – Submm receivers 4
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
Basic Detection Techniques – Submm receivers 5
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
Basic Detection Techniques – Submm receivers 6
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
Basic Detection Techniques – Submm receivers 7
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 …
Basic Detection Techniques – Submm receivers 8
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
Basic Detection Techniques – Submm receivers 9
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
Basic Detection Techniques – Submm receivers 10
Atmospheric transmission at 5000m altitudeAtmospheric transmission at 5000m altitude
pwv = precipitable water vapour, i.e. the column height of condensed water vapour
Basic Detection Techniques – Submm receivers 11
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
Basic Detection Techniques – Submm receivers 12
Cazaux et al. 2003
Basic Detection Techniques – Submm receivers 13
Basic Detection Techniques – Submm receivers 14
Optical vs. Submm/Far-InfraredOptical vs. Submm/Far-Infrared
Orion Trapezium Region at Optical
Wavelengths Highlighted Region at IR
Basic Detection Techniques – Submm receivers 15
Basic Detection Techniques – Submm receivers 16
Molecular gas in M31Molecular gas in M31
CO line emission traces molecular gas.
This is where new stars form.
Nieten et al. 2005
Basic Detection Techniques – Submm receivers 17
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
Basic Detection Techniques – Submm receivers 18
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.
Design of a Scientific Instrument 06 June 2006 20
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
Design of a Scientific Instrument 06 June 2006 21
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
Design of a Scientific Instrument 06 June 2006 22
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
Design of a Scientific Instrument 06 June 2006 23
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
Design of a Scientific Instrument 06 June 2006 24
A Heterodyne Receiver
Design of a Scientific Instrument 06 June 2006 25
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
Design of a Scientific Instrument 06 June 2006 26
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
Basic Detection Techniques – Submm receivers 27
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
Basic Detection Techniques – Submm receivers 28
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)
Basic Detection Techniques – Submm receivers 29
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.
Basic Detection Techniques – Submm receivers 30
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 ?
Basic Detection Techniques – Submm receivers 31
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
Basic Detection Techniques – Submm receivers 32
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)
Basic Detection Techniques – Submm receivers 33
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)
Basic Detection Techniques – Submm receivers 34
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
Basic Detection Techniques – Submm receivers 35
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
Basic Detection Techniques – Submm receivers 36
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 !)
Basic Detection Techniques – Submm receivers 37
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)
Basic Detection Techniques – Submm receivers 38
Some formulasSome formulas
Basic Detection Techniques – Submm receivers 39
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
Basic Detection Techniques – Submm receivers 40
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
Basic Detection Techniques – Submm receivers 41
Typical SIS mixer responceTypical SIS mixer responce
Basic Detection Techniques – Submm receivers 42
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)
Basic Detection Techniques – Submm receivers 43
Example of a waveguide SIS mixer (540-700 GHz)Example of a waveguide SIS mixer (540-700 GHz)
Feed horn MagnetJunctionholder
Lens
10 mm
Basic Detection Techniques – Submm receivers 44
Precision machiningPrecision machining
Backshort cavity Mixer backpiece Terahertz mixer
Humanhair
0.1 mm
With SIS chipand tunnel junction
Basic Detection Techniques – Submm receivers 45
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
Basic Detection Techniques – Submm receivers 46
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
Basic Detection Techniques – Submm receivers 47
Example of a quasioptical mixer structureExample of a quasioptical mixer structure
Mixer chip LensAntennastructure
SIS junction Stripline
0,25
mm
10 mm
Basic Detection Techniques – Submm receivers 48
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
Basic Detection Techniques – Submm receivers 49
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)
Basic Detection Techniques – Submm receivers 50
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
Basic Detection Techniques – Submm receivers 51
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
Basic Detection Techniques – Submm receivers 52
Submm mixer noise temperaturesSubmm mixer noise temperatures
HIFI spaceinstrument
Jan 2006
• Mixer noise increases with frequency (increased losses)