Free Space Optics

Copyright © 2002 Terabeam Corporation. All rights reserved. 1
Free Space Optics (FSO)
Technology Overview
Dr.S.SYED JAMAESHA
Asspciate Professor
Karpagam Institute of Technology

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

Why Free Space Optics (FSO)?
The “Last Mile” Bottleneck Problem
Only about 5% of commercial
buildings are lit with fiber
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
• 0.3-1.5 Mbps

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

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

How FSO Works
1 Network traffic
converted into
pulses of invisible
light representing
1’s and 0’s
2 Transmitter projects the
carefully aimed light pulses
into the air
5 Reverse direction data
transported the same way.
• Full duplex
3 A receiver at the other end of the
link collects the light using lenses
and/or mirrors
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

• 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
Very Narrow and Directional Beams

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

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

Fundamental Concepts
Small Angles - Divergence & Spot Size
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 1.0 km ~2.0 m (~6.5 ft)
4.0 mrad (~ ¼ deg) 1.0 km ~4.0 m (~13.0 ft)
1° ≈ 17 mrad → 1 mrad ≈ 0.0573°

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

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

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

Challenges
Low Clouds, Rain, Snow and Dust
• Low Clouds
– 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

• 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.
Challenges
Scintillation

• 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
Challenges
Window Attenuation
WAM

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

1. 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
2. Large Divergence and Field of View
– Beam spread is larger than expected building motion
– Reduces link margin due to reduced energy density
– Low cost
Challenges
Compensating for Building Motion – Two Methods
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

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

Free Space Optics

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