1. Free Space Optics (FSO) Technology Overview
John Schuster
Chief Technology Officer
Terabeam Corporation

2. Presentation Overview
Why Free Space Optics?
Challenges
Transceiver Design
Predicting Availability
Eye Safety
Applications & Network Integration
The Future of FSO

3. Why Free Space Optics (FSO)? The “Last Mile” Bottleneck Problem
Wide Area Networks between major cities are extremely fast
Fiber based
>2.5 Gbps
Local Area Networks in buildings are also fast
>100Mbps
The connections in between are typically a lot slower
Mbps
Only about 5% of commercial buildings are lit with fiber

4. Why Free Space Optics? Why Not Just Bury More Fiber?
Cost
Rights of Way
Permits
Trenching
Time
With FSO, especially through the window, no permits, no digging, no fees

5. Why Free Space Optics? How Fiber Optic Cable Works
Glass Fiber Strands
Light Source
Detector
Detector
Light Source
Network Device
Network Device
Pulses of light communicate the data
“ON” = 1
“OFF = 0
Capable of more than 40 Gbps
>7 CDs a second

6. Why Free Space Optics? How FSO Works3
A receiver at the other end of the link collects the light using lenses and/or mirrors 2
Transmitter projects the carefully aimed light pulses into the air 5
Reverse direction data transported the same way.
Full duplex 1
Network traffic converted into pulses of invisible light representing 1’s and 0’s 4
Received signal converted back into fiber or copper and connected to the network
Anything that can be done in fiber can be done with FSO

7. Why Free Space Optics? Very Narrow and Directional Beams
Beams only a few meters in diameter at a kilometer
Allows VERY close spacing of links without interference
No side lobes
Highly secure
Efficient use of energy
Ranges of 20m to more than 8km possible

8. Why Free Space Optics? Deployment Behind Windows
Rapid installations without trenching and permitting
Direct connection to the end user
Bypasses the building owner
No roof rights
No riser rights

9. Why Free Space Optics? The FSO “Value Proposition”
No interference
Unlicensed
Easy to install
Through the window (or from the rooftop)
No trenching, no permits
Fiber-like data rates
Many deployment options
Fungible equipment

10. Fundamental Concepts Small Angles - Divergence & Spot Size
1° ≈ 17 mrad → 1 mrad ≈ °
1 mrad
1 km
1 m
Small angle approximation:
Angle (in milliradians) * Range (km)= Spot Size (m)
Divergence
Range
Spot Diameter
0.5 mrad
1.0 km
~0.5 m (~20 in)
2.0 mrad
~2.0 m (~6.5 ft)
4.0 mrad (~ ¼ deg)
~4.0 m (~13.0 ft)

11. Fundamental Concepts The Decibel - dB
A logarithmic ratio between two values
In the optical world of Power in mW,
dB=10*Log(power2/power1)
3 dB = ratio of 2/1
6 dB = ratio of 4/1
10 dB = ratio of 10/1
20 dB = ratio of 100/1
50 dB= ratio of 100,000/1
Gain/Loss
Multiplier
+30 db
+20 db
+10 db
0 db
-10 db
-20 db
-30 db
1000
100 10 1 .1
.01
.001

12. Challenges Environmental factors
Sunlight
Window
Attenuation
Fog
Building Motion
Alignment
Scintillation
Range
Obstructions
Low Clouds
Each of these factors can “attenuate” (reduce) the signal. However, there are ways to mitigate each environmental factor.

13. Challenges Atmospheric Attenuation - FOG
Absorption or scattering of optical signals due to airborne particles
Primarily FOG but can be rain, snow, smoke, dust, etc.
Can result in a complete outage
FSO wavelengths and fog droplets are close to equal in size
(Mie Scattering)
Typical FSO systems work 2-3X further than the human eye can see
High availability deployments require short links that can operate in the fog

14. Challenges Low Clouds, Rain, Snow and Dust
Very similar to fog
May accompany rain and snow
Rain
Drop sizes larger than fog and wavelength of light
Extremely heavy rain (can’t see through it) can take a link down
Water sheeting on windows
Heavy Snow
May cause ice build-up on windows
Whiteout conditions
Sand Storms
Likely only in desert areas; rare in the urban core

15. Challenges Scintillation
Beam spreading and wandering due to propagation through air pockets of varying temperature, density, and index of refraction.
Almost mutually exclusive with fog attenuation.
Results in increased error rate but not complete outage.

16. Challenges Window Attenuation
Uncoated glass attenuates 4% per surface due to reflection
Tinted or insulated windows can have much greater attenuation
Possible to trade high altitude rooftop weather losses vs. window attenuation
WAM

17. Challenges Building Motion
Type
Cause(s)
Magnitude
Frequency
Tip/tilt
Thermal expansion
High
Once per day
Sway
Wind
Medium
Once every several seconds
Vibration
Equipment (e.g., HVAC), door slamming, etc.
Low
Many times per second

18. Challenges Compensating for Building Motion – Two Methods
Automatic Pointing and Tracking
Allows narrow divergence beams for greater link margin
System is always optimally aligned for maximum link margin
Additional cost and complexity
Large Divergence and Field of View
Beam spread is larger than expected building motion
Reduces link margin due to reduced energy density
Low cost
0.2 – 1 mrad divergence
= 0.2 to 1 meter spread at 1 km
2 – 10 mrad divergence
=2 to 10 meter spread at 1 km

19. Challenges Building Motion – Thermal Expansion
Results from Seattle Deployment:
15% of buildings move more than 4 mrad
5% of buildings move more than 6 mrad
1% of buildings move more than 10 mrad