copyright © 2002 terabeam corporation. all rights reserved. 1 challenges solar interference direct...
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Copyright © 2002 Terabeam Corporation. All rights reserved. 1
ChallengesSolar InterferenceChallengesSolar 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
0.0
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1.0
1.2
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1.6
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Wavelength (nm)
Irra
dia
nc
e (
W/m
^2
-nm
)
Sunlight
Copyright © 2002 Terabeam Corporation. All rights reserved. 2
ChallengesObstructions to Line of SightChallengesObstructions to Line of Sight
Copyright © 2002 Terabeam Corporation. All rights reserved. 3
Transceiver DesignBasic Design Transceiver TradesTransceiver DesignBasic 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 DesignChoosing a WavelengthTransceiver DesignChoosing a Wavelength
X-Ray
Gamma Ray
Ultra-Violet
Infrared Radio
Visible Light
MillimeterWave
Microwave
300 GHz~400-700nm
FSO (30-400 THz)
10 GHz30 GHz
MMDS,U-NII, ISM
Typical FSO Wavelengths
785-850 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 DesignChoosing a WavelengthTransceiver DesignChoosing a Wavelength
Wavelength(nm)
Advantages Disadvantages
785-850 • Low Cost• High Sensitivity/Large Area
Silicon Detectors
• Relatively Easy to Detect• Close to Visible Wavelengths
1550 • Higher Power Vs. Eye-safety• Same Components as Fiber• Compatible w/ EDFAs
• High Cost• Small InGaAs Detectors
10000 • Better 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 DesignDivergence Vs. Pointing and Tracking
Transceiver DesignDivergence 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 DesignField of View (FOV)Transceiver DesignField 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 DesignAutomatic Pointing & Tracking Systems
Transceiver DesignAutomatic 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
GimbalMotors
Copyright © 2002 Terabeam Corporation. All rights reserved. 9
Transceiver DesignSingle Transmitter Vs. Multi-Transmitter
Transceiver DesignSingle Transmitter Vs. Multi-Transmitter
Spot Size = X
Spot Size = X+Y
Distance Between Centers = Y
Copyright © 2002 Terabeam Corporation. All rights reserved. 10
Transceiver DesignSingle Transmitter Vs. Multi-Transmitter
Transceiver DesignSingle 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 DesignDirect Coupling Vs. Fiber Coupling
Transceiver DesignDirect 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
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