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

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;

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

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

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 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

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 W = 4p Sr

Propagation: Range Loss

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

Operating Near the Quantum Limit

Pointing, Acquisition and Tracking

Tracking Mode

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…… Homodyne DPSK receiver theoretical MDS data sync, LO power, AGC losses, etc. - 8 dB 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

Preliminary Data

5.6 Gb/s

Inter-Island Test Summary