Basic Detection Techniques Front-end Detectors for the Submm

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Basic Detection TechniquesFront-end Detectors
for the Submm

• Andrey Baryshev
• Lecture on 21 Sept 2006

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Outline

• Stability measurements of practical receiver
  system, Allan variance plot, calibration
  intervals
• Direct detectors (principle)
• Photo-detectors
• Bolometers
• Other types (pyro-detectors, Golay cell)
• Noise in direct detectors
• NEP -- noise equivalent power
• Photon noise
• Electronics noise
• Low noise detectors in submm THz region
• Transition edge sensors
• Kinetic inductance detectors
• SIS junction as direct detector
• Practical measurement of NEP

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Practical receiver at a telescope/or any lab
Front-end
Calibrator
Receiver
Back-end
Tsys, Gsys
Thot Tcold
Tsky(t)
Source spectrum
a
1 - bin
Telescope
100 K
f
Signal amplitude 1 K on top of 100 K
background Spectrometer bin bandwidth 1 MHz,
Tsys 100 K
What to do to detect with accuracy 5 s (S/N5)?
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Integrate (wait)! How long?
Uncertainty dT
Tsky
dT
Radiometer equation
t B
Tsky2
NOTE Tsky TbkgTsys 200 K,
t
dT2 B
201 K for the line! Not Tsys!
dT 1 K / 5 (S/N) 0.25 K
Ideally after continuous integration of 0.64 s
accuracy is achieved!
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Why radiometer equation?
Fundamental noise is photons -gt statistics is
white noise uniform spectral density
Fourier -gt transform
White photon noise statistics results in
radiometer equation
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Real life receivers
Ideal
Real
System instability Standing waves, drift, 1/f
noise, ambient temperature, Atmosphere, many
more
How often do we need to calibrate (loose time)?
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Allan variance
Measurement sequence with minimum integration
time tmin
s1,s2 sn sN
1
s(t)2
lt(yn1-yn)2gt where yn is the average of subset
of sn over integration time t
2
s1 s2 s3 s4 s5 s6 s7 s8 s9 s10 s11 s12 s13 s14 s15 s16
y1 y1 y2 y2 y3 y3 y4 y4 y5 y5 y6 y6 y7 y7 y8 y8
y1 y1 y1 y1 y2 y2 y2 y2 y3 y3 y3 y3 y4 y4 y4 y4
2t
4t

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Allan time
real
ideal
Maximum integration time between recalibrations
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Direct detector principles

• Direct detector gives signal proportional to the
  power of incoming radiation or amount of photons.
• Usually detector pixel is much simpler than
  heterodyne counterpart, so large arrays are
  possible
• Photo detector (electronic)
• Bolometric principle (Thermal detectors)
• Coherent detectors (diode)
• Other principles

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Parameters of direct detectors

• Quantum efficiency
• Noise
• Linearity
• Dynamic range
• Number and size of pixels
• Time response
• Spectral response
• Spectral bandwidth

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NEP
NEP is input power at the input of the detector
to produce SNR1
One can add the contributions of different noise
sources in square fascion as in the formula
ebove for optics noise contribution
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Photon noise and Johnson noise
Detector is limited by statistics of incoming
photons
2hc(1/t)1/2
NEP
? ?1/2
Detector is limited by Johnson noise (thermal
fluctuations)
2hc(kT)1/2
NEP
??qGR1/2
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Black body facts
Uncertainty in photon numbers
Photon occupation numbers
Photon NEP
F 4 p R2 L
L e (2 h f3)/(c/n)2 /(Exp(hf/(kT))-1)
M sT4
Stefan-Boltzmann law
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Photo detectors

• Arriving photon generate/modify free charge
  carriers distribution
• Classical semiconductor (utilizing band gap)
• It has a lower frequency limit hF gt Egap
• Typical semiconductor work in IR region
• By applying stress to the crystal, it is possible
  to decrease Egap Like in stressed germanium
• SIS junction
• No low frequency limit (effective band gap
  modified by bias point)
• High frequency limit due to gap structure
• Kinetic inductance detectors
• Photons break Cupper pairs
• It has low frequency limit

E
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Example
Detectors PACS instrument on Herschel, Stressed
germanium
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KID arrays for AstronomyPrinciple of Kinetic
Inductance Detection

• Pair breaking detector
• Superconductor LKIN at TltTc/3
• LKIN Nqp power absorbed
• Use LKIN to measure absorbed power
• KID
• a SC material in resonance circuit
• read out at F0 4 GHz
• resonance feature is function of Nqp
• signal in S21 or R and ?

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KID arraysKID radiation coupling
Antenna

• Antenna in focus of Si lens
• Herschell band 5 6
• Radiation from sky FRF gtgt2?/h
• -gt increases Nqp
• -gt change in S21 or R and ?
• F0 ltlt FRF
• antenna ltlt resonator
• F0 ltlt 2?/h
• No qp creation due to readout

Most sensitive area
CPW ¼ ? Resonator
L 5 mm _at_ 6 GHz
Si Lens
Al ground plane
Radiation
Coupler
CPW Through line
substrate
Central conductor
2
1
Readout signal GHz
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KID arraysPrinciple of KID arrays

• Resonances _at_ F0
• F0 set by geometry (length)
• Intrinsic FDM

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KID arrays for astronomyGeneral idea for the FP

• Optical Interface
• flies eye array of Si lenses, size 20?F?/2.
• 90.6 packing efficiency in hexoganal
• Array
• Detectors printed on back Si lens array
• Readout
• 4 SMA coax connectors
• 2 full chains -gt redundancy
• 5000 pixel

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KID focal plane for NIKA 400 pixel test array
for 2 mm
antenna
KID
Through line
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Pair breaking detector fundamental sensitivity
limit
DOS
quasiparticles
e-p coupling
quasiparticle lifetime
Pmax/NEPgt10.000
1 sec
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Measuring Dark NEP

• Measure bare resonators
• Measure all ingredienst of NEP
• Quasiparticle lifetime ?qp
• noise Sx
• Quasiparticle response dx/dNqp
• For R and ?

Cryostat
Shorted end
Quadrature mixer
analyses
Synthesizer
Open end, coupler
Re
Superconductor
ADC
IQ
1
2

Im
LNA
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SIS photon detector
High sensitive in far-IR sub-mm region
Bias voltage, V
SIS junction
q elementary charge hplank constant ?frequency
Isg subgap current ? quantum efficiency
Superconductor
Insulator
Superconductor
photon
E
Density of states
?
qV
Current status 10-16 10-17 W/vHz
EF
Our goal
_at_ 600 GHz
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Comparison with theoretical value (1)
Tinkham (1975)
D(E)density of states, F(E) Fermi function,
? gap energy
Nb/Al-AlN/Nb junction
4.2 K
1.6 K
Current A
Theoretical curves
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Bias voltage V
4/8/2014
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Transition edge sensor principle
Thin superconducting film as thermometer Square
law power detector thermal time constant t
C/GC thermal capacitanceG thermal
conductivity
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Procedure of an NEP measurement

• Determine the signal power
• It is given by Planck formula
• Need temperature of calibrator black-bodies
• Frequency coverage of the detector (measured by
  FTS)
• Knowledge of solid angle of antenna beam pattern
• Determine the responsively
• Measure response from hot/cold radiators
• Calibrate detector output in input power units
• Determine the background noise
• Block connect the detector beam to as little
  background possible
• Measure time trace and using responsively and
  integration time express it in NEP Wt/Hz1/2