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

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 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 Only about 5% of commercial buildings are lit with fiber

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

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

Why Free Space Optics? How FSO Works 3 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

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

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 Concepts Small Angles - Divergence & Spot Size 1° ≈ 17 mrad → 1 mrad ≈ 0.0573° 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)

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 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.

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 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

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.

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

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

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

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