Designing Free-Space Inter-Satellite Laser Communications Systems

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

2. Photonics in Space General Dynamics AIS Laser Communications Terminal 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) • 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 - 50 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. Prec =Margin Preq Link equation, link budget, link margin • Received signal is estimated from: Prec Pt Gt Lt LS LR LabsLfadeLAO LP LtrkGr 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)

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. 90°hybrid, OPLL Aperture, FOV , Focal plane control FOR control PAT, bus vibration mitigation Beam forming, power control, thermal control Laser oscillator, OPA, pump, thermal control

11. Source Wavelengths

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

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 123.5 cm aperture and 7 Watts launched power

31. Experiment Objectives

34. Preliminary Data

35. 5.6 Gb/s

36. Inter-Island Test Summary