1. Free Space Optical (FSO)
Communication Systems
E E 421 FinalProject
ThisdocumentcontainsinformationonFSOcommunication
as well assome of the challengesmetwhendesigningsuch
systems. Furthermore,thisdocumentproposestoaddress
the atmosphericattenuationproblemforthe Space Photonic
proprietaryLaserFire FSOcommunicationsystem.
Chad Weiss
JosephCourtright
5/2/2016

2. 1
Abstract:
Free space optical (FSO) communicationisaformof communicationthathasbeeninuse since the dawn
of earlyhumancivilization. Fire signalswereusedinordertosendalertmessagesorsignalsof warover
verylongdistanceswithverylittletime oreffortinvolved. Now,communicationhasevolvedtoits
presentdayform;i.e.a meshnetworkof interconnecteddevicesenablinglongdistance communication
overguided/unguidedmedia.
Although presentdayFSOcommunicationtechnologyallowsforeffortlesscommunication,designing
such systemsisanythingbuttrivial. Engineersare facedwiththe huge problemthatisatmospheric
attenuationwhendesigningFSOcommunicationsystems. ManufacturerslikeSpace Photonicshave
beenable toachieve wirelessoptical communicationatdistancesupto5 km but remainvulnerable to
adverse weatherconditionslike fog,rainorsnow.
Thispaperseeksto addressthe vulnerabilitiesthatSpace Photonicsface regardingFSOcommunication
systems. Furthermore,thispaperproposesasolutiontothe problemthatisatmosphericattenuation
on the LaserFire FSOcommunicationsystem(Space Photonicsproprietary). Finally,the paperwill
conclude withashort discussionabout the future of FSOcommunication.

3. 2
Table of Contents
Section Page
i.Abstract 1
ii.Contents 2
I. Introduction 3
II.Advantages 3
III.Last Mile Problem 4
IV.How itWorks 4
V.SystemSolution 5
VI.Proposal 8
VII.Future 8
VIII.WorksCited 10
List of Tables
Table Page
Table 1: MitigationTechniques 7

4. 3
Introduction
Free space optical communication(FSOCommunication) islaserbasedcommunicationsystem. Similar
to fiberopticscommunication, FSOcommunication hasaveryhighdata rate whencomparedwithradio
transmissionorcopperlines. Since complicatedcableinstallationsare unnecessaryforFSOsystems,the
systemsare much cheaper.Itisthe combinationof highspeedandlow costwhichmakesFSO
technologysouseful.
Optical signalsare one of the earliestformsof longdistance communication.In800 BC and perhaps
earlier,fire beaconswouldbe flashedbythe Romanstosendmessagesoverlongdistances. OnJune 3,
1880 AlexanderGrahamBell testedthe Photophone.The Photophonemodulatedvoice signalswitha
mirrorusedto direct sunlight.The systemhada range of about 700 feet,butfailedtoworkwhenitwas
cloudyor at night.
Free space optical communicationsystemsintheirmodernformoriginatedinthe late 60s,withthe
inventionof laser. Dr.Erhard Kube is consideredone of the founders of fibersopticnetworks.InJune of
1968, Dr. Kube published"Informationtransmissionbylightbeams throughthe atmosphere"the first
journal article proposingoptical transmissionwith lasersystems [1] [2].
Early applicationsinvolvedmilitarysatellitesandNASA.Thesespace basedapplications rallied
communicationsbetweensatellites. Because atmosphericinterference isthe primaryobstacle toFSO
systems, space providesanexcellent environmentforapplication. FSOhas beenwidelyreplacingother
formsof wirelesscommunication.Somebusinesseshave found FSOsystems tobe a suitable
replacementforEthernetnetworkingcables. Furthermore, FSOsystems are beingusedtotransmitlive
television andcell signals.Ashigherresolutionsof videobecome more commonplace,the needto
switchto higherspeedscausesgroupstorely more andmore on optical communicationsystemssuchas
free space optics.
Advantages
One of the primaryadvantagesof the FSO communicationsystem isthe highspeed atwhichthe system
transmitsdata. All FSOsystems are guaranteedtohave a data rate of at least2.5 Gbps.Some newer
systemsevenhave speedsof upto160 Gbps [1] [3]. Comparatively, radiowaves tendtofunctionata
maximumof only15Kbps. Signal modulation isusedto encode the datawithinthe waves.Dependingon
the type of modulation,the numberof bit sentina single wave canbe varied.However, datarate is
alwaysinverselyproportional tothe wavelength. FSOprimarily usesinfraredlightasa carrierwhichhas
a smallerwavelengththanRadioorTV waves.
Anotherimportantadvantage of FSOsystemsiscost.Fiberopticcablesare able to rival FSOwithspeed.
However, fiberopticscable networkscanbe expensive.Anysignificantinfrastructure of fiberoptic
cableswill require timeandmanpowertoinstall. Howeverfree space optical systems onlyrequiresa
transmitteranda receiver.Finallyradioandtelevisionwavesare regulatedandspecifictransmission
frequenciesare owned.Because infraredtransmissionsare unregulatedone wouldnotneedtoowna
sectionof the radiospectrum[3].

5. 4
The beam divergenceisproportionaltowavelength asindicatedbythe equationbelow.
θdiv =
2.44λ
Dr
FSO useswavelengthsbetween700nmand 1600nm whichissignificantlysmallerthanthe 30mm to 3m
whichisusedinRF communications.The FSOsignal propagatesata much smallerangle. The signal
takeslesspowertotransmitovera longer distances,because the powerismore focused overasmaller
area at the same distance.
Generally,wirelesstechnologyisasecurityrisk,because itpropagateswithinalargerfield.However,
FSO propagatesata muchsmallerangle thanradioor televisionwaves. Thissmall fieldof propagation
makesitnearlyimpossible totapintoa communicationwithoutalsocuttingthe communication.
Additionally,FSOsignalscannotbe detectedusing aspectrumanalyzerorRF meter.
Finally,FSOsystemsare notsubjecttointerference fromotherFSOsystems. FSOoperatesona narrow
beamunlike RF.Itusesa highlydirectionalbeamandtherefore manyFSOsystemscanoperate onthe
same frequencywithinclose proximityandnothave to worryabout interferingwitheachother.
Last Mile Problem
Since the 1960s the worldhasbuiltup a massive networkof highspeedfiberopticcablesbetweencities
and majorcenters;however,installingfiberopticcables, especiallyinurbanareas,posesmany
problems. There are several obstacles regardingthe costsandpermits required toinstall fiberoptic
cables. Inthe UnitedStatesonlyaboutfive percentof all buildings have adirectconnectiontofiber
opticsinternet.Whileatthe same time 75% of all businessesexistwithinone mileof one of these
centers[4].Many of these businessesare connectedwith slowerEthernetcables. The speedof any
connectionisonlyasfast as the slowestpart,sothese corporationsdonotreceive the speedbenefits
associatedwithfiberopticsandinsteadrelyonmuchslowerconnectionspeeds;thisisknownare the
lastmile problem. One of the majorapplications of FSOsystemsistosolve the lastmile problem,
because theymaintainahighspeedconnectionwithoutthe needforexpensive,highcostinstallations.
By creatinga FSO network, otherbusinessesinthe regioncanbenefit fromhighspeedconnections
withoutneedingtobuildupexpensive infrastructure.
How It Works
Like manyfiberopticssystemsFSOusespulsesof lighttocarrydata. However,unlikefiberoptics
systems, FSOsystemseliminate the needfora guidingmedium suchthatthe lightsignal canpass safely
fromthe transmittertothe receiver.Instead,the message isdirectedthroughthe openairbyusing
unguided line of sighttoreachthe receiver.
The data transmissioninfree space opticssystemsisdone entirelybylaser. The energyof electronsis
quantized,meaningthatitcan onlyexistatparticularevery level. A laseriscreatedbygettingan
electrontoabsorb an electriccharge movingupto a higherenergylevel. Ina shortperiodof time the
electronwill rerelease the energyasa wave of lightat a specificfrequency. The frequencycanbe found

6. 5
by the RydbergequationshownbelowwhereRisthe Rydbergconstantand n1 and n2 are propertiesof
the atom.
f = c × R(
1
n1
2
+ n2
2)
The lasercan be modulatedbycontrollingthe amountof currentusedto create the laser.By usinga
largercurrent,the laserwill be more intense.However,since thisrelationshipisnon-lineardigital
modulationispreferred [5]. The wavelength of the lightisanimportantfactorin determininghow
much isscatter or absorbedinthe atmosphere.
The lightfromthe laseristo be focusedontoa beamusingsurroundingmirrorsandlenses. Thenthe
selectionof the lenswill determine the diversionof the beam andthe laser‘sfocuslength.A more
focusedbeam hasa higherpowerconcentration andthe focuslengthcanbe carefullychosenbasedon
howfar the lightisto travel.
The unguided laserlightwillneedtobe aimed atthe receiveroverthe openair. Inthe openair the
signal can be scattered,absorbedorblocked. Once atthe receiveranotherlenswillfocusinontoa
photodetector. The selectionof the lenscandeterminethe acceptance angle intoreceiver.The
combinationof the acceptance angle andthe size of the aperture openingdeterminethe amountof
lightwhichentersthe photoreceptor. The photoreceptorthenconvertsthe signal backintoan
electronicsignal andsendsthe communication.
System Solution
The primarychallenge whendesigningaFSOcommunicationsystemisthe abilitytocompensate for
atmosphericattenuationsandatmosphericdisturbances,especiallywhendesigningforlongrange and
highspeed. Toaccomplishthis,one musttake intoaccount the effectsof atmosphericabsorption,
scatteringandturbulence,whichleadtoahigherattenuationfactor. Due tothe interactionbetween
lightandmatter,atmosphericabsorptionandscattering canbe minimizedbychoosingasource of
propagatinglightwithanappropriate wavelength. Furthermore,itisimperative tochoose agoodlight
source,suchas a laserdiode andnota poorsource such as a LED. Otherfactors that affectthe overall
performance of FSOsystemsinclude:atmosphericattenuation,scintillation,buildingalignment,
vibrations,solarinterference andline-of-sightobstructions.
In orderto minimize the scatteringandabsorptioneffectof the atmosphere hasonthe propagatinglight
signal,one mustselectthe wavelengthwisely. If the wavelengthistooshort,toomuch of the signal will
scatter duringtransmission,causingthe biterrorrate (BER) to become increasinglydetrimentaltothe
fidelityof the signal. Asforthe absorption,one mustchoose awavelengththatfallswithinthe
appropriate atmospherictransmissionwindow,i.e.the range of wavelengthvaluesthatwill minimize
absorption. The absorptionvaluesfordifferentwavelengthscanbe foundindatabaseslike HITRAN
(HighResolutionTransmission) orwithcomputerprogramslike MODTRAN (Moderate Resolution
AtmosphericTransmission). MODTRAN simulatesvariouswavelengthsof the electromagneticspectrum
as theypropagate throughthe atmosphere undersetatmospheric conditionssuchasfog,rain or snow.

7. 6
These simulationsprovidesystemengineerswiththe informationtomake decisionsaboutthe overall
design. Itisimperative thatone alsoconsidersthe probabilityandstatisticsof weatherphenomena
withinthe natural environmentwhere the FSOcommunicationwilloperate.
Aside fromatmosphericattenuation,additionallyone mustconsideratmosphericturbulence asamajor
systeminterrupt. Turbulentcells,alsoknownaseddies,ultimatelyensuesconstructive and
deconstructive interferencesonthe propagatinglightthatpassesthroughthem. Due tocorrelations
betweenpressure,temperature andindex of refraction,highertemperaturesandgreaterpressures
pocketshave a higherindex of refractionthanlowerpressure andlowertemperature pockets. Asthe
propagatinglightpassesthroughthe non-uniformmedium,constructiveordeconstructiveinterference
may occur. The resultof thisinterference isaredirectionof the propagatingsignalwhichleadsto
fluctuations inintensity,alsoknownasscintillation. Turbulence inducedbeamwanderingand
turbulence inducedbeamspreadingalsooccurs. Engineersare facedwithahuge dilemmabecause the
effectsof turbulence cannotbe avoided. Turbulence inducedbeamspreadingoccurswhenthe beam
size islargerthan the eddies,whenthe beamsizeissmallerthanthe eddiesandwhenthe beamsize is
approximatelythe same size of the eddies.
The last three thingsthatcan affectthe overall performance of FSOcommunication systemsare:Beam
divergence,ambientlightandmisalignment/buildingsway. Beamdivergence occurswhenthe length
of the linkisverylarge incomparisontothe aperture of the receiver. Divergence occursatthe receiving
endwhichleadstosome geometriclossof light. Again,thislossincreasesasthe lengthof the link
increasesanddecreasesasthe size of the receiveraperture isincreased. Ambientlightreferstothe sun
or moonlightthatinterfereswiththe signal byaddingsomethingcalledshotnoise. Shotnoise canbe
avoidedif the wavelengthisincreased. Increasingthe wavelengthreducesthe probabilitythatnoise will
be addedto the system. Lastly,there isbuildingswayormisalignment. Thiscancause serious
problems,especiallyif the beamcannotsee the receiver. Buildingswaycanoccur formany reasons.
Thermal linearexpansionof materialsinhightemperature environmentsmaycause the buildingto
expandorcontract in coldweatherenvironments. Also,tremorsandearthquakes maycause vibrations
inthe superstructure of the buildingwhichwouldultimatelyrattle the signal transmitter. Additionally,
there isa chance that highwindspeedscouldshake the FSOcommunicationsystemanddisruptthe link
also.
Systemengineershave manyobstaclestoovercome whendesigningaFSOcommunicationsystemand
despite all the effortstomaintainaviable linkbetweentwolocations,thereisalwaysthe chance that
something,whetheritisabird,a plane orSuperman,will obstructthe signal pathbyblockingthe line-
of-sight(LOS). Engineershave tocome upwitha way to mitigate the effectsof all these nuances.
Moreover,itisimperative todevelopasystemthatisadaptable toany situationimaginable;otherwise
the system’sexecution isextremelylikelytofail. Thatiswhymuch thoughtisinvolvedwhendesigninga
FSO system. FSOcommunicationsystemsare veryvulnerableandneedtobe as strongas Mother
Nature itself.
Some of the designparametersconsideredwhendesigningaFSO communicationsysteminclude the
beamdivergence,transmitterpower,operatingwavelength,andtransceiverfield-of-view (FOV). For

8. 7
Space Photonics,anadvancedtracking,acquisitionandpointingsystem(TAP) isusedtocompensate for
all the adverse weathereffectsandlongdistance challenges. The subjectof interestisthe LaserFire FSO
communicationsystem. Thissystemisahighlysecure communicationsystemthatprovides
uninterrupted,secure wirelesscommunicationwithultra-highwirelessbandwidth. Ithas an automatic
tracking,acquisitionandpointingsystemwhichallowsforsmallerspotsize transmissionbeamswhich
make it virtuallyimpossibletodetectorintercept. The LaserFire systemoperatesinthe nearinfrared
spectrumandprovidesafast continuouslinksynchronizationthatcorrectsforatmosphericturbulence
and beamfading. Forextrasecurity,if the beamisblockedforany reason,the transmitterwill
automaticallystopsendingdataas to ensure informationsecurity. The highfidelitysystemcanprovide
up to 1.0 Gbpsat distancesupto5 km; furthermore,the systemhasbeendesignedtobe compatible
withcommerciallyavailable wavelengthdivisionmultiplexing(WDM) componentsformuchhigher
aggregate bandwidths. LaserFirehasa low mass, low powerrequirementandcanbe deployedrelatively
easyinpoint-to-pointandmultimode configurations. Itusesa small,highlycollimated,infraredbeam
and GUI to assure a reliable connection.
Despite all of the greatthingsaboutLaserFire,itisstill mildlysusceptible toadverse weatherconditions
such as snow,heavyrain,fogor dust. Much researchhasbeendone totry and mitigate the effectsof
such impairments. Developmentsonthe actual physical layeraswell assome of the upper layersinthe
system,like the linkornetworklayerare pushingthe limitsforFSOcommunicationsystems.
Here is a table thatincludessome of the alreadyexistingmethodsforaddressingFSOcommunication
systemdesignchallenges:(Redreferstophysical layertechniques,bluereferstothe upperTCP layers)
Table 1: Mitigation Techniques
MitigationTechnique DesignChallenge Justification
Aperture Averaging AtmosphericTurbulence Increasingthe receiveraperture
createsa largeracceptance
angle whichcan helpaverage
out the fastfluctuations,or
scintillations,causedbyeddies,
thusreducingchannel fading.
Adaptive Optics AtmosphericTurbulence A closedloopfeedbacksystem
compensatesforthe beam
misalignmentdue toturbulence.
Adaptive Thresholding AtmosphericTurbulence Codingandmodulation
techniquescanbe usedto
simplifythe detectionof a1 or 0
at the receiver. If dispersionor
scintillationwere tooccur,
adaptive thresholdingcould
cleanup the BER of the system.
BackgroundNoise Rejection SolarInterference Spatial filters,polarizingfilters
and modulatingtechniqueswith
highpeak-to-average power

9. 8
couldeliminateanynoise
enteringthe receiver.
HybridRF/FSO AtmosphericAttenuation If weatherconditionsmake it
impossible forFSO
communication,the system
couldswitchto a backupRF.
PacketRe-transmission IncreasedBER,single-biterrors
and bursterrors
Re-transmissionprotocolssuch
as automaticrepeatrequest
(ARQ) can improve the overall
performance of the link.
ReconfigurationandRerouting LOS obstructions,faulty
equipment
Path configurationanddata
reroutingisissuedinorderto
increase the reliabilityand
availabilityof the link.
Qualityof Service Control AtmosphericAttenuation,
Distortion,Turbulence,Solar
Interference,Scattering,
Absorption,Hardware Failure,
LOS obstructions,FOV
limitations,misalignmentand
weather
Implementationof routing
protocolsandcontrol algorithms
can be usedtoimprove the
overall qualityof the service
beingprovidedbymeasuringthe
data rate,latency,delayjitter,
data loss,energyconsumption,
reliabilityandefficiency.
Proposal
Regardingall of the mitigationtechniqueslistedinTable 1onlyone addressesthe atmospheric
attenuationfactorsuch,i.e.the hybridRF/FSOsystem. UponthoroughlyinvestigatingSpace Photonics’
LaserFire,itwasdiscoveredthatthe systemwasvulnerable toadverse weatherconditionslike fog,rain
and snowwithabsolutelynobackupavailable. Therefore,itissuggestedtoimplementahybridRF/FSO
systeminorderto compensate forthe effectsof atmosphericattenuation. Increasingthe datarate will
involve researchinvestigatingthe propertiesof semi-conductormaterialsinrelationtothe fastforward
recombinationtimesaswell asmanyothersubjects. Inorderto improve the range of currentFSO
systems,largerbeamsandmore advancedtransmitterreceiverswill have tobe researchedand
developed. Since the definitionof aFSOcommunicationsystemincludesthe factthatitis an unguided
transmissionlink,one wouldhave tocreate avacuum or rarefactioninthe transmissionpath. One
mightbe able to accomplishrarefactionusingsoundwavesbut,itisstill very unlikelythatitwill actually
increase the range of the FSO system. Sounddoesn’ttravel veryfarbefore attenuatinganditisvery
difficulttocontrol. Laserpoweramplifierswouldhave tobe integratedinordertoachieve further
distances.
Future
The future for FSOcommunicationisverypromising. Itprovidesuserswithhigherbandwidths,greater
securityandincreaseddataratesfor lessmoney,lesspowerandlesspermissionwhichmakessetup

10. 9
easyand quick. The applicationsforFSOtechnology are endless;theycanbe usedindevelopingthird
worldcountries,establishingsecure communicationincombatsituations,promotingsustainabilityand
much more. The main challenge iscontrollingthe atmospherictransmissionchannel. Until weathercan
be controlledandlossymediaeradicatedfromthe transmissionchannel,FSOcommunicationsystems
will remainlimited.

11. 10
Works Cited
[1] Laseroptronics,"FSOHistoryandTechnology,"Laseroptronics,2016.[Online].Available:
http://www.laseroptronics.com/index.cfm/id/57-66.htm.[Accessed16April 2016].
[2] LightPointe,2006.[Online].Available:http://www.freespaceoptics.org/freespaceopticshome.html.
[Accessed16 April 2016].
[3] Sona Optical,"FSOGuide,"SonaOptical,[Online]. Available:
http://www.fsona.com/technology.php?sec=fso_guide. [Accessed17April 2016].
[4] P. S.J. Rajput,Director, Free SpaceOptical Communication. [Film].EandC Engg Dept.,2015.
[5] M. Carter,"LaserBeam Modulation,"October2015. [Online]. Available:
http://www.maxmcarter.com/lasrstuf/lasermodulator.html.[Accessed19April 2016].