Designing Free-Space Inter-Satellite Laser Communications Systems

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Designing Free-Space Inter-Satellite Laser
Communications Systems

• Davis H. Hartman

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Next-generation systems bandwidth demands are
unprecedented and still growing

• Bent pipe
• Data transfer
• On-Board signal processing
• Analog / digital
• LEO/GEO/Lunar
• Higher data rates by virtue of tighter beams
• Lower SWaP

• Laser Com
• 6,000 km at 8 Gb/s (or more)
• 1.06 microns (near IR)
• Fully space qualified(member of a vital few)

• Spacecraft Interconnects
• Data aggregation
• Distributed Switching
• Interconnections

Size, weight, and power rule in space
Photonics can interconnect high speed data
efficiently
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LaserCom is out there..
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Why Lasercom?

• Pros
• Tight beam confinement ? High power density ?
  Higher data rates / Longer links
• More Gbps per Watts consumed
• Scalable Data Rates (WDM)
• Deep-space capable

• Cons
• Tight beam confinement ? very challenging
  pointing, acquisition and tracking
• Very much CAPEX - intensive
• Complex systems, extreme vibration sensitivity
• Commercial markets yet to emerge

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Terrestrial Based Networking
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Moon Based Networking
Earth Mars - 50 to 500 M km
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Elements of the Link

• Light generation (E-O) and amplification
• Frequency tuning / stabilization
• Modulation
• Pointing / tracking
• Propagation
• Acquisition
• Demodulation
• Detection / O-E conversion

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Link equation, link budget, link margin

• Received signal is estimated from
• Prec ? Pt Gt Lt LS LR LabsLfadeLAO LP Ltrk Gr Lr
  Limpl

Transmission terms
Receiver terms
Medium terms
Control terms

• Medium terms are unique to air-space link (except
  for range loss)
• Control terms depend on stability of both air
  space assets

• Required signal is a more complex function
• Preq f (Noise terms, Implementation loss,
  Target BER)

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Definition of Terms

• Prec is the received power (W)
• Pt is the laser power (W)
• Gt is the transmitter gain
• Lt is the transmitter loss (transmitter optics
  imperfection)
• LP is the pointing loss (transmit platform
  pointing control noise)
• LR is the range loss (1/r2 dependency)
• LS is the Strehl loss due to induced wave front
  aberrations
• Labs is the loss due to atmospheric attenuation
• Lfade is the loss due to atmosphere-induced
  scintillation
• LAO is the loss due to propagation through the
  aircraft boundary layer
• Gr is the receiver gain
• Lr is the receiver loss (receiver optics
  imperfection)
• Ltrk is the loss due to tracking errors (receive
  platform jitter)

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Source Wavelengths
l Materials Features
0.85 mm AlGaAs/GaAs laser diodes High power launch difficult SOAs under development Modulator damage threshold (more energy per photon) Commercial DataCom reuse
1.06 mm NdYAG NPRO Yterbium doped fiber amplifiers Most stable laser in existence Wavelength Division Multiplexing (WDM) limited
1.55 mm band InGaAsP/InP lasers EDFA Telecomm industry (DWDM) reuse
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Non-Planar Resonating Oscillator (NPRO)

• The front face of the crystal has a dielectric
  coating, serving as the output coupler and also a
  partially polarizing element, facilitating
  unidirectional oscillation.
• The blue beam is the pump beam, normally
  generated with a laser diode.
• Frequency stability 300 kHz for gt 100 sec

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Modulation
At 10 Gb/s, there are 30,000 wavelengths traversed
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BPSK Modulation
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Pointing with diffraction-limited optics
If dtx 20 cm (8 in) and l 1 micron, then qdiv
12 micro-radians
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Propagation Range Loss
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Coherent Receiver Tracking and Signal Generation

• Spatial acquisition
• Frequency acquisition
• Tracking
• Demodulation

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Operating Near the Quantum Limit
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Pointing, Acquisition and Tracking
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Tracking Mode
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Platform Vibration Isolation
Micro-vibration envelope at the LCTs mounting
interface (x-axis in Hz, y-axis in g 2 /Hz,
right-hand plot), or ltq2gt (pointing uncertainty,
left-hand plot)
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Receive Gain
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SAMPLE LCT SPECS

• Full duplex coherent optical homodyne system
  using BPSK modulation
• LCT features
• Mass lt 30 kg
• Power dissipation lt 130 W
• Data Rate 8 GB/s (LEOLEO or LEO-MEO)
• BER lt10-10
• Aperture 13.5 cm
• LEO-LEO, LEO-MEO and MEO-MEO- applications.
• In LEO-MEO and MEO-MEO- applications, tracking
  capable across a full hemisphere
• LCT mounting footprint 500 x 500 mm platform
  with four mounting studs and ICD
• Laser delivers up to 1.5 Watts power in present
  embodiment up to 7 Watts under development
• Beaconless PAT system
• Receiver sensitivity within 8 dB of the quantum
  limit (7.8 photons per bit BPSK Homodyne)
• Doppler compensation 700 MHz/sec verified by
  test with qualified components
• Miniaturized, mechanically stable optical paths
  for spatial acquisition, frequency acquisition
  and phase locking, tracking and communication 20
  x 20 x 10 mm3
• GEO-GEO or GEO-LEO,
• 500 Mb/s across 72,000 km with 123.5 cm aperture
  and 7 Watts launched power

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Experiment Objectives
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Preliminary Data
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Inter-Island Test Summary