Amplitude and Phase Detection

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Amplitude and Phase Detection

• J-C Lehureau
• Thales Research and Technology

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Frequency of electronic components
Ge Si GaAs WDM
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Heterodyne Detection
Radio signal Optical interference
X
X²
DS
sp²-s-p²
Y²
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Coherent-incoherent interference
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Power detection
N photons
p photon
s
sint ? N ? p sshot ?N stot ? sint2sshot2
?N (1 p)
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Power detection
stot ?N (1 p) is a quantum limited
detector if shot noise gtgtadditive
noise Example of microbolometer NETD50 mK on
2500µm² NEP .75 nW NEW 30 pJ 1010
photons The pump must be nearly 1 Joule per
pixel!! Example of CMOS addressed
photodiode reading noise 1000 electrons N gtgt
1 million photons 0.01 pJ
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Amplitude and phase detection
signal
pump
XOR
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Phase detection image reconstruction
Laser source
t
speckle field
wavefront sensor
Propagation model
Field measurements
Synthesis
Phase calibration
Image

Signal processing
Physical measurement
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Short range experimentation

• Digital holography enables wavefront estimation
  on a great number of pixels
• 12 bits 512x512 CCD frames
• Synthesis by means of object rotation
• overlapping holograms increases resolution
• aberration correction by calculus

Z50m
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Thermal source
A mode is defined by geometrical extent and
time-bandwidth the population of a mode is p
1/(exp(hn/kT - 1)
p1 at peak
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An example of phase detectionincoherent
holography
FFT
Quadrator
Integrator
CCD
Color filter
At T3300K, l900nm, Pphoton1 For each shot
S/N -20dB After 10000 shots S/N 20dB
laser
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Faint correlation
signal
Let us normalize the pump to unity each output
gives a signal s1s2?p cos f where p is the
population of a mode f is a random phase The
correlation of the two outputs S s1s2 p is
to be compared to the unity noise One needs
Ngtgt1/p² samples to overcome the noise
pump
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An astronomic example
2 telescopes make an observation at 100
light-years The resolution of each is 1µRd i.e.
10 a.u./pixel l is chosen such that p1
(l5lpeak) Cross section of the star is 1/1 000
000 of a pixel Light from a planet is 1/1 000 000
000 of the star At 1000 billion
samples/second the star generates correlation
within 1s the planet can be seen within 10000
days
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Image formation by a rotating linear antenna
The Ngtgt1/p² rules generates a dramatic loss in
information capacity this can be compensated by
generating more correlation function than the
number of detectors
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Maximum entropy of low S/N
The capacity of a channel is increased by erasing
samples below a threshold
decoded erreur capacity
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Conclusions

• Modern infrared detectors (InP, QWIP) have a
  bandwidth which
• allows the analysis of a significant part of
  optical spectrum.
• Optical heterodyne power detection has the same
  theoretical
• limit as quantum limited detector but will find
  application only
• where detailed analysis of optical spectrum is
  needed.
• Heterodyne detector is cheap multichannel
  structure is possible
• Phase and amplitude detection generates a non
  material link
• between remote telescopes. The number of
  correlation function
• varies as the square of detecting sites.
• Computer correlation allow image formation with
  a posteriori
• phase correction.
• There is a need for information link/storage in
  the peta/exa byte
• range