Transcript SRON presentation - University of Groningen

Basic Detection Techniques
Front-end Detectors for the Submm
Wolfgang Wild
Lecture on 21 Sep 2006
Contents overview
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Submm / THz regime
• Definition and significance
• Science examples
Submm detection: direct + heterodyne
Signal chain, block diagram
Heterodyne principle
Noise temperature and sensitivity
Heterodyne frontend
• Mixers
• Local oscillators
• IF amplifiers
Spectrometers: Filterbank, AOS, Autocorrelator, FFT
Overview submm astronomy facilities
Examples of heterodyne receiver systems
• ALMA 650 GHz
• HIFI space instrument
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Submillimeter Wavelength Regime I
•
λ ~ 0.1 … 1 mm
•
Between infrared/optical and radio waves
•
Submm technology is relatively new (~ 20 years)
(Compare to optical technology: ~ 300 years)
•
Submm astronomy is crucial for understanding
star and planet formation
•
Range of 0.1… 0.3 mm is one of the last
unexplored regimes in astronomy
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Submillimeter Wavelength Regime II
•
Technically challenging and interesting
 Challenging:
fabrication
small λ means high precision
 Interesting:
Combination of optical and
electronic techniques
•
Submm astronomy and technology are very
dynamic fields
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Why submillimeter ?
Sub-/Millimeter vs. optical astronomy
Item
Sub-/Millimeter
Optical / IR
Wavelength
Frequency
0.1 mm to 3 mm
100 GHz to 3 THz
0.4 to 30 μm
10 to 600 THz
Cold medium
(10-100K)
Molecular clouds
Extended structures
Hot medium
(a few 1000K)
Stars
Point sources
Targets
Sub-/Millimeter astronomy studies the Cold Universe.
And most of the sky is dark and cold …
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Radiation 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
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The Earth atmosphere at submm wavelenghts
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The Earth atmosphere is only partially transparent for
submillimeter wave radiation
•
Several atmospheric “windows” exist
•
Water vapor and oxygen cause strong absorption
 dry, high observatory sites
 airplane, balloon and space platforms
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Atmospheric transmission at 5000m altitude
pwv = precipitable water vapour, i.e. the column height
of condensed water vapour
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Submillimeter astronomy – star formation
•
New stars form in molecular clouds
•
These clouds are best observed in the infrared and submm
regime since they are cold and have high optical extinction
•
Star and planet formation is associated with a rich interstellar
chemistry  many lines observable in IR/submm/mm
JCMT Spectral Survey IRAS16293- 2422
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Cazaux et al. 2003
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Optical vs. Submm/Far-Infrared
Orion Trapezium Region at Optical Wavelengths
Highlighted Region at IR
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Molecular gas in M31
CO line emission
traces molecular
gas.
This is where new
stars form.
Nieten et al. 2005
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Dust and CO at z=6.4 !
Sloan survey:
optical image
Z=6.4
Contours: dust
=> Heavy elements
formed shortly after
Big Bang
IRAM 30m MAMBO
Bertoldi et al. 2003
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Bertoldi et al. 2003
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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
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Heterodyne Signal Chain
electrical
Intermediate
Frequency (IF)
Heterodyne
Instrument
optical
“Front End”
Spectrometer/
Correlator
Data acquisition
“Backend”
• 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
Design of a Scientific Instrument
06 June 2006
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Principle of Heterodyne Mixing
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
f IF = | f LO - f RF |
Double sideband mixer:
both sidebands converted to same IF
IF
0
fIF
RF
LO
Single sideband mixer:
Only one sideband converted to IF
RF
//
freq
Lower
sideband
(LSB)
Upper
sideband
(USB)
Sideband separating mixer:
two sidebands converted to different
IF outputs
Design of a Scientific Instrument
06 June 2006
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Heterodyne Mixing
Combine strong LO signal
+
A weak RF signal
VLO= cos(LOt) (e.g. 996 GHz)
Gives total power absorbed
P ~ VS VLO cos((S - LO)t + )+….
VS= cos(St+) (e.g. 1002 GHz)
Amplitude and phase information conserved in IF signal
Detect radiation at frequencies where no amplifiers are available
Power
Power
996
IF Spectrum
Signal Spectrum
Local
Oscillator
IF signal
1000
1004
Frequency (GHz)
4
8
Frequency (GHz)
Mixing needs strong non-linear detector charcteristic
Design of a Scientific Instrument
06 June 2006
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Block Diagram of a Heterodyne Receiver
LO ref in
LO signal
(e.g. 646 GHz)
IF signal out
(e.g. 4 GHz)
Local
oscillator
Cal
source
Astronomical RF signal
(e.g. 650 GHz)
Components:
to correlator
or spectrometer
Optics

4K
Mixer
IF amp(s)
Optics
• Mixer
• Local Oscillator (LO)
• Calibration source
• IF amplifier(s)
• Dewar and cryogenics
• Bias electronics
• Spectrometer(s)
Design of a Scientific Instrument
06 June 2006
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A Heterodyne Receiver
Design of a Scientific Instrument
06 June 2006
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A heterodyne receiver for space
Telescope
Beam
HIFI = Heterodyne Instrument
for the Far-Infrared
Will fly on the Herschel
Space Observatory in 2008
7 LO Beams
Design of a Scientific Instrument
~ 50 cm
06 June 2006
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HIFI Signal Path
Local
Oscillator
Unit
Telescope
LOU
optics
Focal
Plane
Unit
mixer
IF
LSU
Local Oscillator
Source Unit
HRS
WBS
ICU
IF
spectrometers
Instrument
Control Unit
To Astronomer
Design of a Scientific Instrument
06 June 2006
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Main components of a heterodyne front-end
•
Optics  own lecture on “quasi-”optics
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Submillimeter wave mixer
 SIS = Superconductor-Insulator-Superconductor
 HEB = Hot-Electron-Bolometer
 (Schottky = Semiconductor-metal contact diode)
•
Local Oscillator
 Multiplier chain
 Quantum-Cascade-Laser (QCL)
•
Intermediate frequency (IF) amplifiers
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Sensitivity and Noise Temperature
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In radio and submm astronomy, the signal unit “Temperature” is
used.
•
This is really a signal power W = k T Δν (k Boltzman constant)
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Usually the signal power is much smaller than the noise power
(“noise temperature”) of the receiving system.
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The noise temperature of a system is defined as the physical
temperature of a resistor producing the same noise power.
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Difference measurements are used to detect the signal, e.g.
(sky + signal source) minus (sky)
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The “ideal” submillimeter wave receiver
Converts all incoming radiation into an electric signal
 no photons “lost”
 has no own noise contribution
However: Heisenberg’s uncertainty principle (ΔE x Δt ≥ h/2π)
makes such a noiseless mixer impossible.
Why ? – A heterodyne mixer measures signal amplitude and
phase. This corresponds to number of photons and time in the
photon picture which – according to the uncertainty principle –
cannot be measured simultaneously with infinite precision. This
uncertainty results in a minimum noise of a heterodyne mixer,
the “quantum limit”.
Current best mixers are ~few times worse than the quantum limit.
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Sensitivity of a receiving system
Question: What is the smallest detectable signal ?
The answer is the
Radiometer formula (Sensitivity):
Tmin = c1 Tsys / (t )1/2
system bandwidth
system temperature
integration time
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)
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Noise Contributions from Receiver Components
Question:
What is the noise contribution from different
receiver components ?
Receiver as a series of linear two-ports:
Optics
T 1 , G1
Trx
=
T1
1st IF amplifier
Mixer
T2, G2
+
T2 / G1
T3, G3
+
T: noise temp
G: Gain
Tn, Gn
To
detector
T3 / (G1 G2 ) + … + Tn / ( G1 G2 …. Gn )
 Receiver noise temperature determined by first few elements
 Cooled optics for high frequencies
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HIFI signal chain
HIFI Dual IF System - one polarisation
N. D. Whyborn, 021016
2 K level
4K
level
2.4 - 4.8 GHz IF
mixer &
isolator
15 K level
IF-1
amplifier
level
trimming
290 K (SVM)
IF-2
assembly
cryoharness
warm
harness
IF up-converter
spectrometers
max. level:
-118 dBm/MHz
-93 dBm/MHz
-69 dBm/MHz
-90 dBm/MHz
min. level:
-128 dBm/MHz
-103 dBm/MHz
-79 dBm/MHz
-100 dBm/MHz
IF gain:
-1 dB
29 dB
-3 dB
+30 dB
-6 dB
-16 dB
-3 dB
-2 dB
10.4 GHz
6H
8 - 5.6
GHz
6L
2.4 - 4.8
GHz
HRS-V
5
4 - 8 GHz IF
4
3
WBS-V
2
1
(+31 dB)
IF gain:
-1 dB
25 dB
-5 dB
(-10 dB)
+21 dB
-8 dB
-3 dB
-2 dB
max. level:
-118 dBm/MHz
-98 dBm/MHz
-85 dBm/MHz
-90 dBm/MHz
min. level:
-128 dBm/MHz
-108 dBm/MHz
-95 dBm/MHz
-100 dBm/MHz
N.B. There is an identical arrangement for the other polarisation.
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Sub-/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)
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Cryogenic submillimeter mixers
SIS = Superconductor-Insulator-Superconductor
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used in mm and submm from ~70 GHz to ~1200 GHz
very good performance
theory well understood
submm detector of choice at ground-based and space
telescopes
HEB = Hot-Electron-Bolometer
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used above ~1200 GHz into THz regime
performance better than SIS above 1200 GHz
theory not well understood
active research on-going
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The SIS mixer
The SIS mixer (Superconductor-Insulator-Superconductor)
element is a sandwich structure with a very thin insulator.
Superconductor-Insulator-Superconductor (SIS) Tunnel Junctions
S
I
S
Cross section of a typical
Niobium SIS tunnel junction
• insulator thickness <= 1nm : tunneling
SEM view of junction top electrode
(1x1 µm²)
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Bandgap 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
„Semiconductor“ model for SIS
„Quasiparticle Excitations“ ~ Electrons
Superconductor 1
at V ~ VGap
Ins.
Superconductor 2
grounded
Current [A]
200
150
RN = dI / dV
100
Rsg= 2mV/Isg
50
Isg
0
0
2
VGap
4
6
Bias Voltage [mV]
(Cooper pair tunneling effects not shown !)
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SIS 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)
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SIS mixer implementation
Task: Couple the astronomical signal to the (very small, ~1 μm2)
tunnel junction. Two ways are used:
•
Feedhorn and waveguide (waveguide mixer)
or
•
A lens and antenna structure (quasi-optical mixer)
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Example of a waveguide SIS mixer (540-700 GHz)
10 mm
Lens
Feed horn
Magnet
Junction
holder
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Precision machining
0.1 mm
Human
hair
Backshort cavity
Mixer backpiece
Terahertz mixer
With SIS chip
and tunnel junction
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HIFI mixers 800-960 GHz and 960-1120 GHz
These mixers will fly on the
Herschel Space Observatory
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HIFI mixer design
magnet
Pressure unit
IF-board
Re-alignment
spring
Magnet pole shoes
Device mount with
backshort, substrate
channel and
alignment spring
ESD protection, bias and LF filtering
Cover for bias/ESD PCB
Corrugated horn
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Example of a quasioptical mixer structure
10 mm
Antenna
structure
SIS junction
Stripline
Mixer chip
Lens
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Quasi-optical mixer implementation
Quasi-optical mixer for the
Space instrument HIFI
Chalmers Technical University
Gothenburg, Sweden
1.5 THz
Silicon lens
Main challenges:
IF board
- chip alignment on lens
- optical properties, beam direction
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Hot electron bolometer (HEB) principle
Thin superconducting film
Square law power detector
thermal time constant t = C/G
C: thermal capacitance
G: 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)
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Hot electron bolometer (HEB) mixer
Principle of operation
• Radiation heats electrons  R
• Cooling either by phonons or out-diffusion
• Direct or heterodyne detection
1 m x 0.15 m (W x L)
Limitations
• IF bandwidth, sensitivity
radiation
S
e
e
e
ph
e
L
ph
Substrate
Hot Electron Bolometer
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Submm mixer noise temperatures
HIFI space
instrument
Jan 2006
• Mixer noise increases with frequency (increased losses)
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