Lunar Optical Data Links

Gregory Konesky

SGK Nanostructures, Inc.

Rutgers Symposium on Lunar Settlements

3-8 June 2007

Rutgers University

Advantages of an Optical Data Link

High Bandwidth due to Intrinsically High Carrier Frequency

Reduced Component Size as compared toElectronic Counterparts

Ability to Concentrate Power in Narrow Beams

Very High Gain with relatively Small Apertures

Reduction in Transmitted Power Requirements

Two Types of Lunar Optical Links

Pan-Lunar:

Between two points on the Moon

(or orbiting relay satellite)

Moon-to-Earth:

Direct transmission to the Earth’s surface

Satellite reception in Earth orbit

followed by retransmission to the surface

Lunar Environment Considerations

Absence of significant atmosphere

-Path absorption losses minimal

-Spreading Loss dominant loss mechanism

-No Beam Wander, Scintillation, etc.

- No Weather (Clouds, Rain, Fog)

Other Environmental Considerations

Top Four Sources of EnvironmentalConcern on the Moon

1. Dust

2. Dust

3. Dust

4. Everything Else

1. DustElectrostatically attaches to surfaces

2. DustAtomically sharp, abrasive

3. DustWide range of particle distribution size

4. Everything ElseRadiation and Solar Flares, Temperature Swings

Micrometeorites (and not so “micro”)

Lunar Line-of-Sight Range Limitation

Absent topological relief limitations,

curvature of the Lunar surface is limiting

For a given height, the Lunar Horizon

Will be approx. ¼ distance of Earth equivalent

Minimal Diffraction Effects (shadows)

at Optical wavelengths

Link Budget Block DiagramMoon-to-Earth Optical Data Link

Transmitter Power, 1 W @ 830 nm 0 dBW Transmitter Antenna Gain, 1 m Dia. 131.6 dBi Transmitter Optical Losses - 6.0 dB Space Propagation Losses -315.3 dB Losses in Vacuum 0 dB Spatial Pointing Losses - 1.0 dB Receiver Antenna Gain, 1 m Dia. 131.6 dBi Receiver Optical Losses - 6.0 dB Spatial Tracking Splitter Losses - 1.0 dB Receiver Sensitivity 84.0 dBW Link Margin 17.9 dB

Assume: 100 Mbps, 10-6 BER

Link Budget Calculation

Aperture Gain 10 cm 111.6 dBi 0.5 m 125.5 dBi 1.0 m 131.6 dBi 1.5 m 135.1 dBi

Various Aperture Gains Expressed in dBi

Laser Power dB Value 100 mW - 10 dBW 500 mW - 3 dBW 1.0 W 0 dBW 2.0 W 3 dBW

Various Laser Powers Expressed in dBW

Moon - to - Earth Distancesand Associated Propagation Losses

Minimum: 364,800 km(Propagation Loss = - 314.8 dB)

Nominal: 384,00 km(Propagation Loss = - 315.3 dB)

Maximum: 403,200(Propagation Loss = - 315.7 dB)

Associated Beam Diameters

Assume:1 meter Aperture on the Moon

830 nm Wavelength

Diffraction-Limited Beam Diameter Near Earth303 meters (364,800 km)335 meters (403,200 km)

Earth Satellite vs Earth Surface-Based Reception

Satellite Optical Reception

Absence of Atmospheric Effects Absorption; Turbulence; Link Availability

Higher Cost Construction; Launch; Inaccessibility

Higher Tracking Slew RatesShort Reception Window; Higher Hand-Off Rates

Greater Point-Ahead Accuracy

Need to Re-Broadcast Down to Earth

Enhanced Satellite Detection by Increasing Beam Divergence

Link Margin is Maximized by Narrowing Beam Divergenceonce Lock-In Occurs

Satellite vs Earth-Based Reception- continued

Earth-Based Optical Reception

Long Reception Window per Station

Atmospheric EffectsAbsorption; Turbulence; Availability; Pulse Spreading

CostRelatively Low Cost; Erecting; Maintaining; Repairing

Permits Building More Receiversas a Hedge Against Weather

Increase Beam Divergence to Enhance Detection Probabilityby Earth-Based Receivers

Narrowing Beam Divergence once Tracking Lock-In Occursto Maximize Link Margin

Issues with Earth-based Reception

Atmospheric Absorption – Link Margin

Forward Scattering – Bandwidth

Turbulence – Adaptive Optics

Hudson,1969

Example of Multipath Forward Scattering

(Gagliardi and Karp, 1995)

Pulse Stretching due to Multipath Scattering(Gagliardi and Karp, 1995)

Issues Related to Atmospheric Turbulence

Naturally Occurring Temperature Variationsgive rise to Wind Eddies

Cause Index of Refraction Changeson the order of 10-6 and are Cumulative

Beam Wander – Eddies Larger than Beam DiameterBeam Spreading – Eddies Smaller than Beam Diameter

Departure from Wavefront Planarity reduces Bandwidth

Example of relatively Clean Optical Pulse Trainand associated Detector Signal

Example of a Distorted Optical Pulse Trainand associated Pulse-Stretched Detector Signal

Adaptive Optics Basics

Cancels Wavefront Distortion through Phase Conjugation

Measures Wavefront Distortion through:Wavefront SensorArtificial Guide Star

Corrects Distortion through:Tilt Mirror

Segmented Mirror

Typical Adaptive Optical System Block Diagram

Shack – Hartmann Wavefront Sensing Technique(Tyson, 1998)

Examples of Tilt Mirrors(Tyson, 1998)

Example of a 19 Segment Deformable Mirror(Hardy, 1998)

Example of a 512 Segment Deformable Mirror(Tyson, 1998)

Example of a 941Segment Deformable Mirror(Tyson, 1998)

Mt. Diablo to LLNL Optical Data Link Test w/wo Adaptive Optics28 km slant range and 2.5 Gbit/sec

(LFW May 2005)

Conclusions

A Moon-to-Earth Optical Data Link can be establishedwith only modest Transmitter Power and Apertures

Satellite-based reception provides high availability dueto absence of Atmospheric Effects, but at a high cost

Conclusions - continued

Earth-based reception prone to Atmospheric Distortions,Reduced Availability, but is a lower cost approach

Improve Availability through greater Site Redundancy

Correct Distortions with Adaptive Optics

Don’t Shoot through Clouds – Pulse Spreading