Upload
bailey-newman
View
242
Download
9
Tags:
Embed Size (px)
Citation preview
Designing Free-Space Inter-Satellite
Laser Communications Systems
Davis H. Hartman
Next-generation systems bandwidth demands are unprecedented and still growing
Photonics in SpaceGeneral Dynamics AIS
Payload interconnectsand data aggregation
Laser Communications Terminal
Laser Com: 6,000 km at 8 Gb/s (or
more) 1.06 microns (near IR) Fully space qualified
(member of a vital few)
Size, weight, and power rule in space…
Photonics can interconnect high speed data efficiently;
• Bent pipe• Data transfer• On-Board signal processing• Analog / digital• LEO/GEO/Lunar• Higher data rates by virtue of
tighter beams• Lower SWaP
Spacecraft Interconnects:
Data aggregation Distributed
Switching Interconnections
LaserCom is out there…..
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
Terrestrial Based Networking
Moon Based Networking
Earth – Mars - 50 to 500 M km
Elements of the Link
• Light generation (E-O) and amplification• Frequency tuning / stabilization• Modulation• Pointing / tracking• Propagation• Acquisition• Demodulation• Detection / O-E conversion
• Received signal is estimated from:
Prec Pt Gt Lt LS LR LabsLfadeLAO LP Ltrk Gr Lr Limpl
Transmission terms
Receiver terms
Medium terms
• Required signal is a more complex function:
Preq = f (Noise terms, Implementation loss, Target BER)
Preq
Prec = Margin
Control terms
• Medium terms are unique to air-space link (except for range loss)• Control terms depend on stability of both air & space assets
Link equation, link budget, link margin
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)
FOR control
Aperture, FOV , Focal plane control
90° hybrid, OPLL
Laser oscillator, OPA, pump, thermal control
Beam forming, power control, thermal control
PAT, bus vibration mitigation
Source Wavelengths
Materials Features
0.85 mAlGaAs/GaAs laser diodes
• High power launch difficult• SOA‘s under development• Modulator damage threshold (more
energy per photon)• Commercial DataCom reuse
1.06 mNdYAG NPRO
Yterbium doped fiber amplifiers
• Most stable laser in existence• Wavelength Division Multiplexing
(WDM) limited
1.55 m bandInGaAsP/InP lasers
EDFATelecomm 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)
At 10 Gb/s, there are 30,000 wavelengths traversed
Modulation
BPSK Modulation
Mach-Zehnder
Pointing with diffraction-limited optics
d2.44θ
txdiv
r
(r)J 2I(r) 1
DiscAiry
If dtx ~ 20 cm (8 in) and ~ 1 micron,
then div~ 12 micro-radians
Sr
Sr4θπ
2θcos12πΩ
2FF
λd
θ
16
Ω
π4G t
2
2F
Propagation: Range Loss
Coherent Receiver: Tracking and Signal Generation
• Spatial acquisition• Frequency acquisition• Tracking• Demodulation
Operating Near the Quantum Limit
Pointing, Acquisition and TrackingStep 4: TrackingSpacecrafts 1 and 2 track and narrow uncertainty to ~15 micro-radians
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
Step 3: Fine AcquisitionSpacecrafts 1 and 2 narrow their uncertainty region to ~ 250 micro-radians, through iterative spiral search
Step 5: CommsSpacecrafts 1 and 2 LCTs phase and frequency lock, transition to communications mode
Two minutes required Thirty seconds required
A
SS
Static LOS Uncertainty
~ 0.1°
A
S
(A)
S
A
(B)
A
S S
A
S
A
Step 1: Static PointingStatic line of sight (LOS) needed to begin acquisition is ~ ±0.1 degrees
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
-10.00 -8.00 -6.00 -4.00 -2.00 0.00 2.00 4.00 6.00 8.00 10.00
( - radians)
Rel
ativ
e Po
wer
(db)
2 sigma pointing
precision
requirement
Tracking loss penalty
Tracking Mode
radiansθ div 10
Micro-vibration envelope at the LCT’s mounting interface(x-axis in Hz, y-axis in g 2 /Hz, right-hand plot), or <2> (pointing uncertainty, left-hand plot)
Platform Vibration Isolation
Receive Gain
Inter-satellite link……
Homodyne DPSK receiver
theoretical MDS
data sync, LO power, AGC losses, etc.
- 8 dB
Pointing (TX) and tracking
(RX) ….
Pointing (TX) and tracking
(RX) ….
SAMPLE LCT SPECS• Full duplex coherent optical homodyne system using BPSK modulation• LCT features
– Mass: < 30 kg
– Power dissipation: < 130 W
– Data Rate: 8 GB/s (LEO–LEO or LEO-MEO)
– BER <10-10
– 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
Experiment Objectives
PreliminaryData
5.6 Gb/s
Inter-Island Test Summary