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