Free Space Optics Technology Overview.ppt
- 1. Copyright © 2002 Terabeam Corporation. All rights reserved. 1 Free Space Optics (FSO) Technology Overview John Schuster Chief Technology Officer Terabeam Corporation
- 2. Copyright © 2002 Terabeam Corporation. All rights reserved. 2 Presentation Overview • Why Free Space Optics? Why Free Space Optics? • Challenges Challenges • Transceiver Design Transceiver Design • Predicting Availability Predicting Availability • Eye Safety Eye Safety • Applications & Network Integration Applications & Network Integration • The Future of FSO The Future of FSO
- 3. Copyright © 2002 Terabeam Corporation. All rights reserved. 3 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
- 4. Copyright © 2002 Terabeam Corporation. All rights reserved. 4 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
- 5. Copyright © 2002 Terabeam Corporation. All rights reserved. 5 Why Free Space Optics? 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
- 6. Copyright © 2002 Terabeam Corporation. All rights reserved. 6 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
- 7. Copyright © 2002 Terabeam Corporation. All rights reserved. 7 • 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
- 8. Copyright © 2002 Terabeam Corporation. All rights reserved. 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. Copyright © 2002 Terabeam Corporation. All rights reserved. 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. Copyright © 2002 Terabeam Corporation. All rights reserved. 10 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°
- 11. Copyright © 2002 Terabeam Corporation. All rights reserved. 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. Copyright © 2002 Terabeam Corporation. All rights reserved. 12 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
- 13. Copyright © 2002 Terabeam Corporation. All rights reserved. 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. Copyright © 2002 Terabeam Corporation. All rights reserved. 14 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
- 15. Copyright © 2002 Terabeam Corporation. All rights reserved. 15 • 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
- 16. Copyright © 2002 Terabeam Corporation. All rights reserved. 16 • 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
- 17. Copyright © 2002 Terabeam Corporation. All rights reserved. 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. Copyright © 2002 Terabeam Corporation. All rights reserved. 18 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
- 19. Copyright © 2002 Terabeam Corporation. All rights reserved. 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