copyright © 2002 terabeam corporation. all rights reserved. 1 challenges solar interference direct...

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

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

100

300

500

700

900

1100

1300

1500

1700

1900

Wavelength (nm)

Irra

dia

nc

e (

W/m

^2

-nm

)

Sunlight


ChallengesObstructions to Line of SightChallengesObstructions to Line of Sight


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


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


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


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


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


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


Transceiver DesignSingle Transmitter Vs. Multi-Transmitter


Spot Size = X

Spot Size = X+Y

Distance Between Centers = Y




• 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


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

copyright © 2002 terabeam corporation. all rights reserved. 1 challenges solar interference direct...

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