1. Designing Free-Space Inter-Satellite Laser Communications Systems
Davis H. Hartman

2. Payload interconnects
Photonics in Space
General Dynamics AIS
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 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)
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;

3. LaserCom is out there…..

4. 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

5. Terrestrial Based Networking

6. Moon Based Networking
Earth – Mars to 500 M km

7. Elements of the Link Light generation (E-O) and amplification
Frequency tuning / stabilization
Modulation
Pointing / tracking
Propagation
Acquisition
Demodulation
Detection / O-E conversion

8. 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)
Preq
Prec
= Margin

9. 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)

10. 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

11. Source Wavelengths l Materials Features 0.85 mm
AlGaAs/GaAs laser diodes
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

12. 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

13. 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)

14. Modulation
At 10 Gb/s, there are 30,000 wavelengths traversed

15. BPSK Modulation
Mach-Zehnder

16. Pointing with diffraction-limited optics
If dtx ~ 20 cm (8 in) and l ~ 1 micron,
then qdiv ~ 12 micro-radians
W = 4p Sr

17. Propagation: Range Loss

18. Coherent Receiver: Tracking and Signal Generation
Spatial acquisition
Frequency acquisition
Tracking
Demodulation

19. Operating Near the Quantum Limit

20. Pointing, Acquisition and Tracking

21. Tracking Mode

22. 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)

24. Receive Gain

27. Inter-satellite link……
Homodyne DPSK receiver theoretical MDS
data sync, LO power, AGC losses, etc.
- 8 dB
Pointing (TX) and tracking (RX) ….

28. 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 cm aperture and 7 Watts launched power

31. Experiment Objectives

34. Preliminary
Data

35. 5.6 Gb/s

36. Inter-Island Test Summary