Free Space Optics (FSO) Technology Overview

Copyright © 2002 Terabeam Corporation. All rights reserved. 1

Free Space Optics (FSO)Technology Overview

John SchusterChief Technology OfficerTerabeam Corporation


Presentation Overview

• Why Free Space Optics?Why Free Space Optics?• ChallengesChallenges• Transceiver DesignTransceiver Design• Predicting AvailabilityPredicting Availability• Eye SafetyEye Safety• Applications & Network IntegrationApplications & Network Integration• The Future of FSOThe 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 With FSO, especially through the window, no permits, no the window, no permits, no

digging, no fees digging, no fees


Why Free Space Optics?How Fiber Optic Cable Works

Light Source

Glass Fiber Strands

Detector

NetworkDevice

• Pulses of light communicate the data• “ON” = 1• “OFF = 0

• Capable of more than 40 Gbps• >7 CDs a second

Light Source

Detector

NetworkDevice


Why Free Space Optics?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

Why Free Space Optics?Very Narrow and Directional Beams


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


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


Fundamental ConceptsSmall Angles - Divergence & Spot Size

1 mrad

1 km

1 m

Small angle approximation:

Angle (in milliradians) * Range (km)= Spot Size (m)

Divergence Range Spot Diameter0.5 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 ConceptsThe 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


ChallengesEnvironmental factors

Sunlight

Building Motion

Alignment

WindowAttenuation

Fog

Each of these factors can “attenuate” (reduce) the signal. However, there are ways to mitigate each environmental factor.

Scintillation

RangeObstructions

Low Clouds


ChallengesAtmospheric 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


ChallengesLow 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.

ChallengesScintillation


• 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

ChallengesWindow Attenuation

WAM


ChallengesBuilding Motion

Type Cause(s) Magnitude FrequencyTip/tilt Thermal

expansionHigh 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

ChallengesCompensating 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


ChallengesBuilding Motion – Thermal Expansion

Results from Seattle Deployment:

• 15% of buildings move more than 4 mrad



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Free Space Optics (FSO) Technology Overview