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

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Presentation on theme: "Designing Free-Space Inter-Satellite Laser Communications Systems Davis H. Hartman."— Presentation transcript:

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

2 Next-generation systems bandwidth demands are unprecedented and still growing Photonics in Space General Dynamics AIS Payload interconnects and 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

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 Received signal is estimated from: P rec P t G t L t L S L R L abs L fade L AO L P L trk G r L r L impl Transmission terms Receiver terms Medium terms Required signal is a more complex function: P req = f ( Noise terms, Implementation loss, Target BER ) P req P rec = 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

9 Definition of Terms P rec is the received power (W) P t is the laser power (W) G t is the transmitter gain L t is the transmitter loss (transmitter optics imperfection) L P is the pointing loss (transmit platform pointing control noise) L R is the range loss (1/r 2 dependency) L S is the Strehl loss due to induced wave front aberrations L abs is the loss due to atmospheric attenuation L fade is the loss due to atmosphere-induced scintillation L AO is the loss due to propagation through the aircraft boundary layer G r is the receiver gain L r is the receiver loss (receiver optics imperfection) L trk 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 MaterialsFeatures 0.85 m AlGaAs/GaAs laser diodes High power launch difficult SOAs under development Modulator damage threshold (more energy per photon) Commercial DataCom reuse 1.06 m NdYAG NPRO Yterbium doped fiber amplifiers Most stable laser in existence Wavelength Division Multiplexing (WDM) limited 1.55 m 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 At 10 Gb/s, there are 30,000 wavelengths traversed Modulation

15 BPSK Modulation Mach-Zehnder

16 Pointing with diffraction-limited optics If d tx ~ 20 cm (8 in) and ~ 1 micron, then div ~ 12 micro-radians 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 Micro-vibration envelope at the LCTs mounting interface (x-axis in Hz, y-axis in g 2 /Hz, right-hand plot), or (pointing uncertainty, left-hand plot) Platform Vibration Isolation


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 < –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 mm 3 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

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