Free Space Laser Communications

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Free Space Laser Communications
25 June 2001

• Phillip Dykstra
• WareOnEarth Communications Inc.
• phil_at_sd.wareonearth.com

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Introduction

• Free space laser communications
• a.k.a. Free Space Optics, Optical Wireless
• Basically a laser and a telescope
• Most are protocol independent
• Typical range lt 4 km (.5, 1, 2, 4, 8km)
• Typical bandwidths 10, 100, 1000 Mbps
• Lucent 10 Gbps DWDM (TeraBeam)

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Example Systems
LightPointe Flight series
ICS Sky series
LSA Supraconnect
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Transmitters

• LED (single or multiple)
• 1 mW typical
• Laser (single or multiple)
• 10-20 mW typical, up to 100 mW
• Usually 785 or 850 nm, some 1310 nm
• Eye safety limit 1.5 mW/cm2 at 785 nm
• Multiple transmitters boost power, may also
  reduce scintillation fades

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Detectors

• PIN Diodes
• -43 dBm typical
• Avalanche Photo Diodes (APD)
• -53 dBm typical (also greater dynamic range)
• Single or multiple detectors
• Larger aperture increases receive power, reduces
  scintillation fades
• Design goal BER lt 10-9 typical

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Laser Link GeometryCritical Design Parameters
Receive area 200 cm2
Receiver sensitivity
Beam Divergence 3 mrad
Distance 1km
Transmit Power
Diameter 3 m Beam area 70686 cm2
Adapted from a LightPointe Technology slide
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System Comparison(OC3 1km products)

OpticalAccess ICS
LightPointe Transmit Power (mW) 10 (10 dBm)
100 (20 dBm) 4 (6 dBm) Beam Diverg (mrad)
2.5 11
3 Receive Area (cm2) 52
232 200 Min
Recv Power (nW) 100 (-40 dBm) 32 (-45 dBm)
50 (-43 dBm) Price
19.5k 25k
24k
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Example Loss BudgetOptical Access T1000X
10 dBm
Tx Power (dBm) 10.00 Distance 686
meters Beam Diameter 1.72 meters Beam Area
2.31 meters2 Geometric loss -26.65 dB Rx
Max (dBm) -16.65 Rx Min (dBm)
-40.00 Margin (dBm) 23.35
-3 dB mispointing
- 10 dB scintillation
10 dB for atmospheric attenuation
-17 dBm
-20 dBm
-30 dBm
-40 dBm
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Equipment Delivery
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T1000X Interface Box
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T1000X Full View
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Platform Mount
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Eastward View
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Westward View
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Concrete Pedestal Base
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Front of T1000X
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(No Transcript)
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Fog Event (19 June 01)
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Fog Event (cont)
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Packet Loss
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Availability

• Availability Prob (Visibility gt Vmin)

V gt 1.5km V gt
0.5km San Diego 98
99.1 Las Vegas 99.9 100

• 99.999 is nearly impossible, except in the
  desert
• Some products add backup microwave (lower speed)

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For More Info

• http//wcisd.hpc.mil/
• Served by the laser link
• Laser and Weather stats
• Links to laser company web sites
• phil_at_sd.wareonearth.com