Copyright © 2002 Terabeam Corporation. All rights reserved. 1 Challenges Solar Interference Direct or reflected sunlight can saturate receivers or tracking sensors Primarily affects East/West links Duration and frequency of outages dependent upon global location of link Outages typically on the order of minutes Outage frequency and duration wavelength dependent Sunlight

Copyright © 2002 Terabeam Corporation. All rights reserved. 2 Challenges Obstructions to Line of Sight

Copyright © 2002 Terabeam Corporation. All rights reserved. 3 Transceiver Design Basic Design Transceiver Trades Design Constraints –Price Point –Range and Availability –Data Rate Major Technical Choices –Wavelength –Wide Divergence Vs. Automatic Pointing and Tracking –Single-Transmitter Vs. Multi-Transmitter –Direct Coupling Vs. Fiber Coupling

Copyright © 2002 Terabeam Corporation. All rights reserved. 4 Transceiver Design Choosing a Wavelength X-Ray Gamma Ray Ultra- Violet Infrared Radio Visible Light Millimeter Wave Microwave 300 GHz ~ nm FSO ( THz) 10 GHz30 GHz MMDS, U-NII, ISM Typical FSO Wavelengths nm~380 THz 1550 nm~190 THz 10000nm (10 micron)~30 THz FSO wavelengths selected for low atmospheric molecular absorption

Copyright © 2002 Terabeam Corporation. All rights reserved. 5 Transceiver Design Choosing a Wavelength Wavelength (nm) AdvantagesDisadvantages Low Cost High Sensitivity/Large Area Silicon Detectors Relatively Easy to Detect Close to Visible Wavelengths 1550Higher Power Vs. Eye-safety Same Components as Fiber Compatible w/ EDFAs High Cost Small InGaAs Detectors 10000Better Fog Penetration???? Higher Power Vs. Eye-safety Doesn’t Penetrate Glass Not Compatible w/ Fiber Components Limited Sources and Detectors

Copyright © 2002 Terabeam Corporation. All rights reserved. 6 Transceiver Design Divergence Vs. Pointing and Tracking Wide Divergence –Easier to keep on target –Low cost Narrow Divergence –Stronger signal at the receiver –Longer range –Higher availability –Requires automatic pointing and tracking for building motion Security Considerations –Wider beams are easier to intercept Laser Pointer – Narrow Divergence Flashlight – Wide Divergence

Copyright © 2002 Terabeam Corporation. All rights reserved. 7 Transceiver Design Field of View (FOV) FOV is the receiver’s equivalent of divergence If light doesn’t originate within the defined cone, the receiver won’t see it (e.g., Laser 2) Smaller FOV means less noise entering system Small FOV makes interference highly unlikely High data rate detectors typically have small FOVs FOV also drives the need for pointing and tracking Laser 1 Receiver Field of View Cone Laser 2

Copyright © 2002 Terabeam Corporation. All rights reserved. 8 Transceiver Design Automatic Pointing & Tracking Systems Laser Steering Mirror Compensates for base motion Adjusts to maintain alignment of receiver and transmitter Essential for narrow divergence/FOV systems Receiving Optics Gimbal Motors

Copyright © 2002 Terabeam Corporation. All rights reserved. 9 Transceiver Design Single Transmitter Vs. Multi-Transmitter Spot Size = X Spot Size = X+Y Distance Between Centers = Y

Copyright © 2002 Terabeam Corporation. All rights reserved. 10 Transceiver Design Single Transmitter Vs. Multi-Transmitter At long range, the multiple beams merge into one No inherent power or alignment advantages Does offer significant advantages for overcoming the effects of scintillation Typically not important for high availability fog limited installations

Copyright © 2002 Terabeam Corporation. All rights reserved. 11 Transceiver Design Direct Coupling Vs. Fiber Coupling Fiber Detector Direct Coupling Light is focused directly onto the detector Allows for a large FOV Electronics closely coupled to optics Low cost Fiber Coupling Light is focused onto the end of a fiber Eventually the fiber connects to a detector ­ May be after several FSO links Fiber requires a small FOV ­ Pointing and tracking required Not required for data rates <2.5 Gbps The Ultimate vision of FSO – Fiber through the air