1. Free Space Laser Communications
25 June 2001
Phillip Dykstra
WareOnEarth Communications Inc.

2. 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 < 4 km (.5, 1, 2, 4, 8km)
Typical bandwidths: 10, 100, 1000 Mbps
Lucent 10 Gbps DWDM (TeraBeam)

3. Example Systems LightPointe Flight series ICS Sky series LSA
Supraconnect

4. Transmitters LED (single or multiple) Laser (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/cm^2 at 785 nm
Multiple transmitters boost power, may also reduce scintillation fades

5. Detectors PIN Diodes Avalanche Photo Diodes (APD)
-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 < 10^-9 typical

6. Laser Link Geometry Critical Design Parameters
Receive area = 200 cm2
Receiver sensitivity
Beam Divergence = 3 mrad
Distance = 1km
Transmit Power
Diameter = 3 m
Beam area = cm2
Adapted from a LightPointe Technology slide

7. System Comparison (OC3 1km products)
OpticalAccess ICS LightPointe
Transmit Power (mW) 10 (10 dBm) (20 dBm) (6 dBm)
Beam Diverg (mrad)
Receive Area (cm^2)
Min Recv Power (nW) (-40 dBm) 32 (-45 dBm) (-43 dBm)
Price $19.5k $25k $24k

8. Example Loss Budget Optical Access T1000X
Tx Power (dBm) =
Distance = meters
Beam Diameter = meters
Beam Area = meters^2
Geometric loss = dB
Rx Max (dBm) =
Rx Min (dBm) =
Margin (dBm) =
-3 dB mispointing
- 10 dB scintillation
= 10 dB for atmospheric attenuation
+10 dBm
-17 dBm
-20 dBm
-30 dBm
-40 dBm

9. Equipment Delivery

10. T1000X Interface Box

11. T1000X Full View

12. Platform Mount

13. Eastward View

14. Westward View

15. Concrete Pedestal Base

16. Front of T1000X

18. Fog Event (19 June 01)

19. Fog Event (cont)

20. Packet Loss

21. Availability Availability = Prob (Visibility > Vmin)
V > 1.5km V > 0.5km
San Diego % %
Las Vegas % %
99.999% is nearly impossible, except in the desert
Some products add backup microwave (lower speed)

22. For More Info http://wcisd.hpc.mil/ Served by the laser link
Laser and Weather stats
Links to laser company web sites