An Optical Receiver for Interplanetary Communications Jeremy Bailey

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An Optical Receiver for Interplanetary
CommunicationsJeremy Bailey
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JPL Concept for Mars Link
(Ortiz et. al. 2000, TMO Prog. Rep. 42-142)
EARTH
MARS
2 AU
10m Telescope
Si APD detector
Laser transmitter Q switched NdYAG Laser
(1064nm) 1W average power 10cm telescope
256-PPM modulation 30 kb/s 646 photons/pulse
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Problems with JPL Scheme

• 30kb/s is comparable to current radio systems
• Sensitivity falls far short of quantum limit
• Problems with analogue APD detection.
• Large detector size required to match telescope.
• Background from daylight sky and Mars.
• 10m telescopes are expensive - and need several
  of them.
• Operating when Mars is near the Sun impossible.

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Detection Schemes

• Analogue Direct Detection with APDs -
• Measure the output current of the diode with a
  low noise preamplifier.
• Standard technique in fibre optic communication.
• Limited by thermal noise and excess noise.
• Best performance - 40 photons/bit in ESA SILEX
  system - more typically 200 photons per/bit for
  Si detectors, gt1000 photons/bit for InGaAs
  detectors.

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Coherent Detection

• Mix input signal with local oscillator and detect
  at the beat frequency.
• Quantum limited performance - e.g. 4.5
  photons/bit demonstrated in ESA experiments.
• Not suitable for large ground based telescopes.
• Good for space to space systems.

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Photon Counting Detectors

• Essentially noise free provided signal is well
  above dark count.
• Quantum limited performance (0.4 photons/bit
  demonstrated in 256-PPM)
• Katz,1982, TDA Prog. Rep. 42-70
• Dead time (50ns) prevents high speed operation.
• Cant detect a narrow pulse.
• Max count rate 107 photons/sec

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Solution - Multiple Photon Counters
1 photon counter
dead time
n photon counters
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Multi-Telescope Telescope
10m effective aperture, made from 25 individual
telescopes mounted together. Each telescope feeds
a photon counter via optical fibres. Individual
telescopes are f/5, matched to fibres.
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Advantages

• Overall telescope is compact.
• No need to make a large mirror.
• No problems matching detector size.
• No need for active control systems.
• Long (f/5) tube assemblies can be baffled to
  allow operation within 10 degrees of Sun.
• Redundancy.
• Scalable to any size you want.

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Daylight Operation

• Essential for system to be able to operate in
  daylight.
• To follow Mars throughout its orbit.
• Background from Mars itself will be at daylight
  levels.
• Need narrow band filter with width 106
• Tunable to follow orbital motion of spacecraft
• Can be achieved by Fabry-Perot and probably other
  technologies.

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Performance

• 532nm 1W transmitter at 2AU (3 x 108km)
• 20cm transmission telescope - 0.55 arc sec beam
• 800km beam width at Earth
• Received power in 10m telescope 1.6 x 1010W
• 4.2 x 108 photons/sec
• With 20 efficiency and 5 photons/bit this will
  support communication at 16 Mb/s (uncoded).
• Receiver with 25 detectors can handle 50 Mb/s
  (with more detectors could go up to 500Mb/s)

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Transmitter
RS Encoder
Convolutional Encoder
Polarization Modulated beam
Laser
532nm Nd YAG Laser
Data Input
Electro-optic cell switched between / l/4
Transmitter Telescope
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Receiver Data Processing
Photon arrival times
Photon pulses
Time Series reconstruction
Demodulation
Fibres From telescope
Convolutional decoding
RS Decoding
APD Modules
Photon Timing
Computer System
Data Output
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Conclusions

• An optical communication reciever built around
  multiple APD photon counters can provide
• Quantum limited operation.
• Data rates 100-1000 times greater than JPLs
  concept (and existing radio systems).
• Simplification of many large-telescope design
  issues.
• Scalability to any size telescope.
• Data rates up to 500Mb/s.