Transcript Davis Hartman`s Presentation

Designing Free-Space
Inter-Satellite
Laser Communications Systems
Davis H. Hartman
Next-generation systems bandwidth
demands are unprecedented and
still growing
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Bent pipe
Data transfer
On-Board signal processing
Analog / digital
LEO/GEO/Lunar
Higher data rates by virtue of
tighter beams
• Lower SWaP
General Dynamics AIS
Laser Com:
 6,000 km at 8 Gb/s (or more)
 1.06 microns (near IR)
 Fully space qualified
(member of a vital few)
Payload interconnects
and data aggregation
Spacecraft Interconnects:
 Data aggregation
 Distributed Switching
 Interconnections
Size, weight, and power rule
in space…
Photonics can interconnect
high speed data
efficiently;
Photonics in Space
Laser Communications
Terminal
LaserCom is out there…..
Why Lasercom?
Pros:
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Tight beam confinement  High power density  Higher data rates / Longer links
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More Gbps per Watts consumed
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Scalable Data Rates (WDM)
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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
Terrestrial Based Networking
Moon Based Networking
Earth – Mars - 50 to 500 M km
Elements of the Link
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Light generation (E-O) and amplification
Frequency tuning / stabilization
Modulation
Pointing / tracking
Propagation
Acquisition
Demodulation
Detection / O-E conversion
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
Medium
terms
Control
terms
Receiver
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)
Prec
Preq = Margin
Definition of Terms
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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)
°
90 hybrid,
OPLL
FOR
control
Aperture,
FOV , Focal
plane control
PAT, bus
vibration
mitigation
Beam forming,
power control,
thermal control
Laser oscillator,
OPA, pump,
thermal control
Source Wavelengths
l
0.85 mm
Materials
AlGaAs/GaAs laser diodes
Features
• High power launch difficult
• SOA‘s 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
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 > 100 sec
• Space qualified CW Nd:YAG
laser for homodyne BPSK
modulation with KHz
frequency stability
• High reliability (.9998>10Yr.)
space qualified pump module
for Nd:YAG laser (open
housing, without fiber below)
Modulation
At 10 Gb/s, there are 30,000 wavelengths traversed
BPSK Modulation
Mach-Zehnder
Pointing with diffraction-limited optics
If dtx ~ 20 cm (8 in) and l ~ 1 micron,
then qdiv ~ 12 micro-radians
Airy Disc
W  4p Sr
 
(r)
I(r)  J 1
r

 π 2
Ω  2π 1  cos θ F   θ F Sr

2  4

2
G
4 π 16  d t 
 2  
Ω θF  λ 
2
θ  2.44
div
l
d tx
Propagation: Range Loss
Coherent Receiver: Tracking and Signal Generation
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Spatial acquisition
Frequency acquisition
Tracking
Demodulation
Operating Near the Quantum Limit
Pointing, Acquisition and Tracking
Step 1: Static Pointing
Static line of sight (LOS)
needed to begin
acquisition is ~ ±0.1
degrees
Step 2: Coarse Acquisition
(A) Spacecraft 1 begins spiral-search
over a ±0.1° uncertainty region,
locates target and begins narrowing
search diameter
(B) Spacecraft 2 begins its spiral
search over ±0.1°, locates target, and
begins narrowing uncertainty cone
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Step 3: Fine Acquisition
Spacecrafts 1 and 2 narrow
their uncertainty region to ~
250 micro-radians, through
iterative spiral search
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Step 4: Tracking
Spacecrafts 1 and 2
track and narrow
uncertainty to ~15
micro-radians
Step 5: Comms
Spacecrafts 1 and 2
LCTs phase and
frequency lock,
transition to
communications mode
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(B)
(A)
Static LOS
Uncertainty
~ 0.1°
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Two minutes required
Thirty seconds
required
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Tracking Mode
θ  10 m  radians
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Platform Vibration Isolation
Micro-vibration envelope at the LCT’s mounting interface
(x-axis in Hz, y-axis in g 2 /Hz, right-hand plot), or <q2> (pointing uncertainty,
left-hand plot)
Receive Gain
Inter-satellite link……
Pointing (TX)
and tracking
(RX) ….
data sync, LO
power, AGC
losses, etc.
- 8 dB
Homodyne
DPSK receiver
theoretical
MDS
SAMPLE LCT SPECS
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Full duplex coherent optical homodyne system using BPSK modulation
LCT features
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Mass: < 30 kg
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Power dissipation: < 130 W
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Data Rate: 8 GB/s (LEO–LEO or LEO-MEO)
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BER <10-10
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Aperture: 13.5 cm
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LEO-LEO, LEO-MEO and MEO-MEO- applications.
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In LEO-MEO and MEO-MEO- applications, tracking capable across a full hemisphere
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LCT mounting footprint: 500 x 500 mm platform with four mounting studs and ICD
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Laser delivers up to 1.5 Watts power in present embodiment; up to 7 Watts under development
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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,
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500 Mb/s across 72,000 km with 123.5 cm aperture and 7 Watts launched power
Experiment Objectives
Preliminary
Data
5.6 Gb/s
Inter-Island Test Summary