Coherent Detection with Arrays of PhotonCounting Detectors

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Coherent Detection with Arrays of Photon-Counting
Detectors

• Jonathan B. Ashcom, Sumanth Kaushik, Richard M.
  Heinrichs
• 14th CLRC, 12 July, 2007

This work is sponsored by the Department of the
Air Force under AF Contract FA8721-05-C-0002.
Opinions, interpretations, recommendations and
conclusions are those of the authors and are not
necessarily endorsed by the United States
Government.
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Outline
Experimental demonstration of coherent
(heterodyne) detection with an array of
single-photon detectors

• Laser radar with Geiger-mode APDs
• Dual-mode coherent/direct imaging laser radar
• Coherent detection with Geiger-mode APD arrays
• Results of experimental demonstration
• Application of theoretical model to system
  analysis

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3D Laser Radar at Lincoln Laboratory

• Laboratory demonstrations to fielded airborne
  systems
• ALIRT High coverage rate mapper
• Jigsaw Multiple looks for foliage penetration
• Direct detection mode round trip time of pulse
  gives range to target
• Array of detectors provide imaging capability
  (Angle angle range)
• Geiger-mode avalanche photodiode arrays (GM-APDs)
  are enabling technology

Jigsaw image of helicopter
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Example 3-D Imagery Through Obscurants
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Geiger-Mode Avalanche PhotodiodePhoton-to-Digita
l Conversion
Geiger-mode operation biases diode above
breakdown, generating charge avalanche upon
generation of a single photoelectron Yields
single-photon- counting sensitivity Current
generated is enough to trigger CMOS timing
logic Result is digital time of photon arrival
with no amplifier noise
Single pixel
photon
APD
Digital timing circuit
Digitally encodedphoton arrival time
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APD/CMOS Photon Timing Arrays
Bridge-Bonded Si APDs

• Silicon APDs for ? lt 0.9 ?m
• 20-50 detection efficiency
• 10-kHz dark count rate
• Bridge-bonded to CMOS timing
• InGaAsP/InP APDs for ? 1 ?m and 1.5 ?m
• 20-50 detection efficiency
• 10-kHz dark count rate with TE-cooling (1 ?m )
• Bump-bonded to CMOS timing
• CMOS timing circuit
• 250 - 500-ps timing resolution
• 500 MHz and 1-GHz clocks

100-mm
Bump-Bonded InGaAsP APDs
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Geiger-Mode APD Arrays Bonded to CMOS Timing
Circuitry
32 x 128 Array
32x32 Array Silicon (Visible), InGaAs (1-?m)

• Technology advantages
• Extreme sensitivity (single photon)
• Fine range resolution (lt 10 cm)
• Fully integrated - digital output of range image
• Scalable to large array sizes

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Coherent Laser Radar with GM-APDs

• Photon-counting detector arrays can be used for
  coherent detection with the addition of a local
  oscillator
• Advantages of using GM-APDs for coherent
  detection
• Enables dual-mode laser radar
• Same laser, same detector for efficient direct or
  coherent detection
• Low LO power required
• Arrays already operate near shot-noise limit
• Scalability to large array size
• ROIC only produces digitally-encoded photon
  arrival times

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Interference at Low Photon Flux
Spatial Two slit experiment
Temporal Heterodyne detection
Laser
1
f
Detector
Laser
2
f ?f
Intensity
Intensity
High photon flux
High photon flux
1/ ?f
distance x
time
Counts
Counts
Low photon flux
Low photon flux
distance x
time
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Ubiquitous Ladar System Concept
Amplitude Modulator
Amplifier/ Isolator Stages
Master Oscillator
Waveform Synthesizer
LO Shutter
APD Arrays
Amplitude Modulator

• LO shutter enables system to switch between
  direct/coherent detection
• CW master oscillator and amplitude modulators
  create arbitrary waveform
• Two arrays detect horizontal and vertical
  polarizations of return

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Traditional Coherent Detection
Laser source
Target motion is encoded into phase of return
signal relative to reference local oscillator
AOM
Frequency reference

• Weak return signal is mixed at detector with
  local oscillator (LO) frequency-shifted by
• Detector generates current proportional to power
  averaged over many optical cycles (
  )

m, mixing efficiency
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Traditional Coherent Detection CNR

• Detected signal is current at , and noise
  is shot noise from all noise sources

If LO power is increased above all other noise
sources, CNR becomes limited only by shot noise
of return signal
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Coherent Detection with Geiger-Mode APDs CNR
In absence of other noise sources, LO power
matched to return power yields half the shot
noise limit

• Despite potential 3-dB drop in CNR vs.
  traditional coherent detection, GM APD arrays
  provide unique advantages
• Single detector for coherent and direct detection
  simplifies dual-mode radar systems
• LO power savings Traditional coherent detection
  requires milliwatts of LO power per detector
• Clear path to scale-up to very large arrays

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GM-APD Detection of Signal and LO Photons

• Once one APD has registered the arrival of a
  photon (or dark count), it is inactive until it
  can be reset
• To increase dynamic range, groups of pixels are
  used as one coherent detector (macropixel). Each
  macropixel can represent one image element
  (angle-angle pair)

Luu and Jiang (Appl. Optics 45, 3798) developed
expression for CNR in presence of saturation (NP
total number of photon buckets available)
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Experimental Demonstration

• As a proof of principle, simplified laboratory
  demonstration was assembled
• 1-mm cw laser is split into two paths
• One arm is frequency-shifted by 40 MHz
• Beams attenuated to pW of detected power and
  recombined on APD array, producing a beat note

Detector intensity image
Reference Photodiode
Variable attenuator
40-MHz Acousto-optic modulator
Attenuated signal projected onto array in dark box
1.06 mm microchip laser ( 10 kHz linewidth)
Focal plane package
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Coherent Detection with Geiger-Mode APDs Data
Processing
Laser source
AOM
Frequency reference
Arrival times
Signal Processing
Histogram of arrival times
Power spectral density

• Collection of GM APDs acts as a single coherent
  detector
• Histogram of arrival times gives intensity vs
  time
• FFT or digitally mix with i.f. to obtain
  frequency/phase
• Time resolution of clock (2 GHz) determines
  bandwidth

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Example Power Spectral Density
With LO and signal blocked, PSD is due to dark
counts only
LO on detector increases noise power due to shot
noise
Signal (a.u.)
Signal (a.u.)
15 LO photons
LO and signal blocked
LO and signal blocked
Frequency (MHz)
Frequency (MHz)
Signal-only PSD is similar to LO-only, as powers
are matched
Mixing signal and LO results in beat frequency at
40 MHz
LO and signal
Signal (a.u.)
Signal (a.u.)
15 Signal photons
LO and signal blocked
LO and signal blocked
Frequency (MHz)
Frequency (MHz)
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Carrier-to-Noise Ratio Results

• Measuring the CNR from power spectra at many
  different power levels allows characterization of
  performance and dynamic range of detector

Array Filled
CNR
Coherent CNR (dB)
Number of return photoelectrons
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CNR Results Comparison to Theory

• Very good agreement with saturating-pixel model
  with independently-measured DCR, mixing
  efficiency, and pixel number
• Validation of theory allows performance analysis
  with various system parameters
• Pixels per macropixel, LO power, DCR

Array Filled
Saturating-pixel theory, uniform illumination
Detector intensity image
Coherent CNR (dB)
NP
Saturating-pixel theory, nonuniform illumination
DCR 175 kHz/pixel m 0.4
Number of return photoelectrons
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Effect of Increasing Local Oscillator

• Increasing LO power recovers 3 dB penalty in
  unsaturated regime
• Optimal LO power varies with return power, and
  can be obtained from saturating-pixel model

10 Return photons
1024-pixel array
Shot noise limit (mNR)
50 Return photons
CNR/Shot noise limit
200 Return photons
CNR (dB)
PLOPR
400 Return photons
PLO optimized
3 dB
Number of return photons
Number of local oscillator photons
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Summary

• Coherent detection has been demonstrated in the
  photon-counting regime using an array of
  Geiger-mode APDs
• Agreement between experimental results and
  saturating-pixel theory support our current
  understanding of the process
• Initial lab results provide confidence for
  development of multi-mode coherent/direct
  detection laser radar systems