LED

2. 9/12/2013 Mahesh J. vadhavaniya 1

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4. Objectives…Objectives…Objectives…Objectives…Objectives…Objectives…Objectives…Objectives…
IntroductionIntroduction
ExplainExplain withwith helphelp ofof energyenergy bandband structure,structure, thethe processprocess
ofof opticaloptical emissionemission fromfrom semiconductorsemiconductor..
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LEDLED StructuresStructures
LEDLED CharacteristicsCharacteristics
AdvantagesAdvantages && disadvantagesdisadvantages ofof ILDILD overover LEDLED forfor
longlong haulhaul opticaloptical fiberfiber cc00mmunicationmmunication

5. Point of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of Discussion
ExplainExplain withwith thethe helphelp ofof energyenergy bandband
structure,structure, thethe processprocess ofof opticaloptical emissionemissionstructure,structure, thethe processprocess ofof opticaloptical emissionemission
fromfrom semiconductorssemiconductors..
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6. Process of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical Emission
ToTo allowallow considerationconsideration ofof semiconductorsemiconductor opticaloptical
sourcessources itit isis necessarynecessary toto reviewreview somesome ofof thethe propertiesproperties ofof
semiconductorsemiconductor materials,materials, especiallyespecially withwith regardregard toto pp--nn
junctionjunction..
semiconductorsemiconductor
IntrinsicIntrinsic ExtrinsicExtrinsic
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7. Process of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical Emission
AA perfectperfect semiconductorsemiconductor crystalcrystal containingcontaining nono
impuritiesimpurities andand latticelattice effecteffect..
Intrinsic Semiconductor :Intrinsic Semiconductor :
Extrinsic Semiconductor :Extrinsic Semiconductor :
AA semiconductorsemiconductor crystalcrystal whichwhich isis mademade upup byby thethe
processprocess ofof doping,doping, ii..ee.. addingadding impurityimpurity toto itit..
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8. Process of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical Emission
The energy band structureThe energy band structure
The FermiThe Fermi--Dirac probability distributionDirac probability distribution
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9. Process of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical Emission
ForFor aa semiconductorsemiconductor inin thermalthermal equilibriumequilibrium thethe energyenergy
levellevel occupationoccupation isis describeddescribed byby fermifermi diracdirac distributiondistribution
functionfunction..
ConsequentlyConsequently thethe probabilityprobability P(E)P(E) thatthat anan electronelectron
gainsgains sufficientsufficient thermalthermal energyenergy atat anan absoluteabsolutegainsgains sufficientsufficient thermalthermal energyenergy atat anan absoluteabsolute
temperaturetemperature TT suchsuch thatthat itit willwill bebe foundfound occupyingoccupying aa
particularparticular energyenergy levellevel E,E, isis givengiven byby thethe FermiFermi--DiracDirac
distributiondistribution..
KTEE
EP
F )exp(1
1
)(
−+
=
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10. Process of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical Emission
TheThe FermiFermi levellevel isis defineddefined asas thethe highesthighest occupiedoccupied molecularmolecular
orbitalorbital inin thethe valencevalence bandband atat 00 K,K, soso thatthat therethere areare manymany statesstates
availableavailable toto acceptaccept electrons,electrons, ifif thethe casecase werewere aa metalmetal..
ThisThis isis notnot thethe casecase inin insulatorsinsulators andand semiconductorssemiconductors sincesince thethe
valencevalence andand conductionconduction bandsbands areare separatedseparated..
ThereforeTherefore thethe FermiFermi levellevel isis locatedlocated inin thethe bandband gapgap..
TheThe probabilityprobability ofof thethe occupationoccupation ofof anan energyenergy levellevel isis basedbased onon thethe
FermiFermi functionfunction..
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11. Process of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical Emission
EnergyEnergy
Energy Band DiagramsEnergy Band Diagrams
nn–– type semiconductortype semiconductor pp–– type semiconductortype semiconductor
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12. Process of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical Emission
TheThe pp--nn junctionjunction diodediode isis formedformed byby creatingcreating adjoiningadjoining
pp andand nn typetype semiconductorsemiconductor layerslayers inin singlesingle crystalcrystal..
AA thinthin depletiondepletion regionregion isis formedformed atat thethe junctionjunction
throughthrough carriercarrier recombinationrecombination..
ThisThis establishesestablishes aa potentialpotential barrierbarrier betweenbetween thethe pp andand nnThisThis establishesestablishes aa potentialpotential barrierbarrier betweenbetween thethe pp andand nn
typetype regionsregions whichwhich restrictsrestricts thethe interinter diffusiondiffusion ofof
majoritymajority carrierscarriers fromfrom theirtheir respectiverespective regionsregions..
AnAn externalexternal appliedapplied voltagevoltage formform currentcurrent flowflow throughthrough
thethe devicedevice asas theythey continuouslycontinuously diffusediffuse awayaway fromfrom thethe
interfaceinterface..
However,However, thisthis situationsituation inin suitablesuitable semiconductorsemiconductor
allowsallows carriercarrier recombinationrecombination withwith thethe emissionemission ofof lightlight..
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13. Process of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical Emission
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15. AA PNPN junctionjunction (that(that consistsconsists ofof directdirect bandband gapgap
semiconductorsemiconductor materials)materials) actsacts asas thethe activeactive oror
recombinationrecombination regionregion..
WhenWhen thethe PNPN junctionjunction isis forwardforward biased,biased, electronselectrons andand
Semiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light Sources
holesholes recombinerecombine eithereither radiativelyradiatively (emitting(emitting photonsphotons)) oror
nonnon--radiativelyradiatively (emitting(emitting heat)heat).. ThisThis isis simplesimple LEDLED
operationoperation..
InIn aa LASER,LASER, thethe photonphoton isis furtherfurther processedprocessed inin aa
resonanceresonance cavitycavity toto achieveachieve aa coherent,coherent, highlyhighly directionaldirectional
opticaloptical beambeam withwith narrownarrow lineline widthwidth..
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16. The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )
LEDsLEDs isis usedused toto convertconvert electricalelectrical energyenergy intointo lightlight
AA lightlight emittingemitting diodediode (LED)(LED) isis essentiallyessentially aa PNPN
junctionjunction optoopto--semiconductorsemiconductor thatthat emitsemits aa monochromaticmonochromatic
(single(single color)color) lightlight whenwhen operatedoperated inin aa forwardforward biasedbiased
directiondirection..
LEDsLEDs isis usedused toto convertconvert electricalelectrical energyenergy intointo lightlight
energyenergy byby recombinationrecombination ofof holesholes andand electronselectrons atat thethe PP NN
JunctionJunction..
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17. Light EmissionLight EmissionLight EmissionLight EmissionLight EmissionLight EmissionLight EmissionLight Emission
BasicBasic LEDLED operationoperation ::
WhenWhen anan electronelectron jumpsjumps fromfrom aa higherhigher energyenergy statestate
((EcEc)) toto aa lowerlower energyenergy statestate ((EvEv)) thethe differencedifference inin energyenergy
EcEc -- EvEv isis releasedreleased eithereither ……EcEc -- EvEv isis releasedreleased eithereither ……
asas aa photonphoton ofof energyenergy EE == hhνν ((radiativeradiative
recombination)recombination)
asas heatheat (non(non--radiativeradiative recombination)recombination)
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18. The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )
ForFor fiberfiber--optics,optics, thethe LEDLED shouldshould havehave aa highhigh radianceradiance
(light(light intensity),intensity), fastfast responseresponse timetime andand aa highhigh quantumquantum
efficiencyefficiency
EmittedEmitted wavelengthwavelength dependsdepends onon bandband gapgap energyenergy
λν /hchEg ==
EmittedEmitted wavelengthwavelength dependsdepends onon bandband gapgap energyenergy
(eV)
2399.1
m)(
gE
=µλ
λλ = wavelength in microns= wavelength in microns
h = Planks constanth = Planks constant
c = speed of lightc = speed of light
E = Photon energy inE = Photon energy in eVeV
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19. ForFor photonicphotonic communicationscommunications requiringrequiring datadata raterate isis
100100--200200 Mb/sMb/s withwith multimodemultimode fiberfiber withwith tenstens ofof
microwatts,microwatts, LEDsLEDs areare usuallyusually thethe bestbest choicechoice
LEDLED configurationsconfigurations beingbeing usedused inin photonicphotonic
communicationscommunications::
The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )
communicationscommunications::
11 -- SurfaceSurface EmittersEmitters (Front(Front Emitters)Emitters)
22 -- EdgeEdge EmittersEmitters
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20. Surface Emitting LEDSurface Emitting LEDSurface Emitting LEDSurface Emitting LEDSurface Emitting LEDSurface Emitting LEDSurface Emitting LEDSurface Emitting LED
Schematic of high-radiance surface-emitting LED. The active region is limitted to a circular cross section that
has an area compatible with the fiber-core end face.
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21. Edge Emitting LEDEdge Emitting LEDEdge Emitting LEDEdge Emitting LEDEdge Emitting LEDEdge Emitting LEDEdge Emitting LEDEdge Emitting LED
Schematic of an edge-emitting double heterojunction LED. The output beam is lambertian in the plane of
junction and highly directional perpendicular to pn junction. They have high quantum efficiency & fast
response.
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22. Light Source MaterialLight Source MaterialLight Source MaterialLight Source MaterialLight Source MaterialLight Source MaterialLight Source MaterialLight Source Material
MostMost ofof thethe lightlight sourcessources containcontain IIIIII--VV ternaryternary &&
quaternaryquaternary compoundscompounds
byby varyingvarying xx itit isis possiblepossible toto controlcontrol thethe
bandband--gapgap energyenergy andand therebythereby thethe emissionemission wavelengthwavelength
overover thethe rangerange ofof 800800 nmnm toto 900900 nmnm.. TheThe spectralspectral widthwidth
AsAlGa xx1−
isis aroundaround 2020 toto 4040 nmnm
byby changingchanging 00<x<<x<00..4747;; yy isis approximatelyapproximately
22..22x,x, thethe emissionemission wavelengthwavelength cancan bebe controlledcontrolled overover thethe
rangerange ofof 920920 nmnm toto 16001600 nmnm.. TheThe spectralspectral widthwidth variesvaries
fromfrom 7070 nmnm toto 180180 nmnm whenwhen thethe wavelengthwavelength changeschanges
fromfrom 13001300 nmnm toto 16001600 nmnm.. TheseThese materialsmaterials areare latticelattice
matchedmatched..
y1yxx1 PAsGaIn −−
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24. Spectral width of LED typesSpectral width of LED typesSpectral width of LED typesSpectral width of LED typesSpectral width of LED typesSpectral width of LED typesSpectral width of LED typesSpectral width of LED types
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25. PLANAR LEDPLANAR LEDPLANAR LEDPLANAR LEDPLANAR LEDPLANAR LEDPLANAR LEDPLANAR LED
Light
Output
Ohmic
Contacts
p – type epitaxial layer
n – type substrate Light
Output
SimplestSimplest ofof thethe structuresstructures availableavailable..
PP typetype diffusiondiffusion intointo nn typetype substratesubstrate..
LambertianLambertian spontaneousspontaneous emissionemission..
LightLight emitsemits fromfrom allall surfacessurfaces..
TotalTotal internalinternal reflectionreflection.. RadianceRadiance isis lowlow..
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26. DOME LEDDOME LEDDOME LEDDOME LEDDOME LEDDOME LEDDOME LEDDOME LED
Semiconductor
material is shaped
like a hemisphere
HemisphereHemisphere ofof nn typetype
GaAsGaAs isis formedformed aroundaround pp
typetype regionregion..
HigherHigher amountamount ofof internalinternal
n+
Electrodes
p
HigherHigher amountamount ofof internalinternal
emissionemission reachingreaching thethe
surfacesurface withinwithin thethe criticalcritical
angleangle ofof GaAsGaAs airair interfaceinterface..
HigherHigher externalexternal efficiencyefficiency
thanthan thethe PlanarPlanar LEDLED..
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27. Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?
Semiconductor
material is shaped
like a hemisphere
Plastic Dome
n+
Electrodes
p
pn Junction
Electrodes
To reduce TIR …To reduce TIR …
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28. How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?
GaAsGaAs--airair interface,interface, thethe θθ == 1616oo whichwhich meansmeans thatthatGaAsGaAs--airair interface,interface, thethe θθcc == 1616oo whichwhich meansmeans thatthat
muchmuch ofof thethe lightlight sufferssuffers TIRTIR..
ToTo solvesolve thethe problemproblem wewe couldcould::ToTo solvesolve thethe problemproblem wewe couldcould::
1.1. ShapeShape thethe surfacesurface ofof thethe semiconductorsemiconductor intointo aa1.1. ShapeShape thethe surfacesurface ofof thethe semiconductorsemiconductor intointo aa
domedome oror hemispherehemisphere soso thatthat lightlight raysrays strikestrike thethedomedome oror hemispherehemisphere soso thatthat lightlight raysrays strikestrike thethe
surfacesurface anglesangles << θθcc thereforetherefore doesdoes notnot experienceexperience
TIRTIR.. ButBut expensiveexpensive andand notnot practicalpractical toto shapeshape pp--nn
junctionjunction withwith domedome--likelike structurestructure..
22.. EncapsulationEncapsulation ofof thethe semiconductorsemiconductor junctionjunction22.. EncapsulationEncapsulation ofof thethe semiconductorsemiconductor junctionjunction
withinwithin aa domedome--shapedshaped transparenttransparent plasticplastic mediummedium
(an(an epoxy)epoxy) thatthat hashas higherhigher refractiverefractive indexindex thanthan airair..
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29. Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )
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30. TheThe surfacesurface layerlayer isis keptkept asas thinthin asas possiblepossible ((1010--1515 µµm)m)TheThe surfacesurface layerlayer isis keptkept asas thinthin asas possiblepossible ((1010--1515 µµm)m)
toto minimizeminimize reabsorptionreabsorption..
Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )
AA wellwell isis etchedetched inin aa GaAsGaAs substratesubstrate toto preventprevent heavyheavyAA wellwell isis etchedetched inin aa GaAsGaAs substratesubstrate toto preventprevent heavyheavy
absorptionabsorption ofof thethe emittedemitted radiationradiation && toto physicallyphysically
accommodateaccommodate thethe fibrefibre..
PhotonsPhotons areare generatedgenerated inin thethe thinthin pp--GaAsGaAs && regionregionPhotonsPhotons areare generatedgenerated inin thethe thinthin pp--GaAsGaAs && regionregion
emissionemission fromfrom thethe toptop surfacesurface isis ensuredensured heterostructureheterostructure &&
PhotonsPhotons areare generatedgenerated inin thethe thinthin pp--GaAsGaAs && regionregion
emissionemission fromfrom thethe toptop surfacesurface isis ensuredensured heterostructureheterostructure &&
reflectionreflection fromfrom thethe backback crystalcrystal faceface.. ThusThus forwardforward radianceradiance
ofof thesethese devicedevice isis veryvery highhigh..
TheThe toptop nn--GaAsGaAs contactcontact layerlayer ensuresensures lowlow contactcontactTheThe toptop nn--GaAsGaAs contactcontact layerlayer ensuresensures lowlow contactcontact
resistanceresistance && thermalthermal resistance,resistance, thereforetherefore allowingallowing forfor highhigh
currentcurrent densitiesdensities && highhigh raditionradition intensityintensity..
TheThe internalinternal absorptionabsorption inin thisthis devicedevice isis veryvery lowlow duedue totoTheThe internalinternal absorptionabsorption inin thisthis devicedevice isis veryvery lowlow duedue toto
thethe largerlarger bandgapbandgap confiningconfining layerslayers..
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31. Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )
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32. N+- GaAlAs
GaAs(n) substrate
Metal contact
Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )
Active layer n- GaAlAs
N GaAlAs
N+- GaAlAs
Metal contact
P GaAlAs
P+ GaAlAs
n- GaAlAs
Light emits
from the edge
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33. ActiveActive layerlayer isis usuallyusually lightlylightly dopeddoped oror undopedundoped && aa veryveryActiveActive layerlayer isis usuallyusually lightlylightly dopeddoped oror undopedundoped && aa veryvery
largelarge populationpopulation forfor recombinationrecombination isis createdcreated inin thisthis regionregion byby
forwardforward biasbias injectioninjection..
Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )
PhotonsPhotons areare generatedgenerated inin thethe veryvery thinthin activeactive regionregion &&PhotonsPhotons areare generatedgenerated inin thethe veryvery thinthin activeactive regionregion &&
spreadspread intointo thethe guidingguiding layers,layers, withoutwithout reabsorptionreabsorption becausebecause ofof
theirtheir largelarge bandgapsbandgaps..
MostMost ofof thethe propagatingpropagating lightlight isis emittedemitted atat oneone edgeedge ofofMostMost ofof thethe propagatingpropagating lightlight isis emittedemitted atat oneone edgeedge ofof
structurestructure byby puttingputting aa reflectivereflective coatingcoating atat thethe nonnon emittingemitting endend &&structurestructure byby puttingputting aa reflectivereflective coatingcoating atat thethe nonnon emittingemitting endend &&
puttingputting anan antireflectiveantireflective coatingcoating atat thethe emittingemitting endend..
TheThe waveguidewaveguide reducesreduces thethe divergencedivergence ofof thethe emittedemittedTheThe waveguidewaveguide reducesreduces thethe divergencedivergence ofof thethe emittedemitted
radiationradiation..
InIn directiondirection perpendicularperpendicular toto thethe planeplane ofof layerslayers ,the,the halfhalfInIn directiondirection perpendicularperpendicular toto thethe planeplane ofof layerslayers ,the,the halfhalf
powerpower widthwidth isis approxapprox.. 3030 degdeg..
InIn thethe planeplane ofof thethe layerlayer thethe outputoutput isis stillstill lambertianlambertian && halfhalfInIn thethe planeplane ofof thethe layerlayer thethe outputoutput isis stillstill lambertianlambertian && halfhalf
powerpower widthwidth isis approxapprox.. 120120 degdeg..
TheThe waveguidewaveguide andand reducedreduced beambeam divergencedivergence allowsallows moremoreTheThe waveguidewaveguide andand reducedreduced beambeam divergencedivergence allowsallows moremore
efficientefficient couplingcoupling ofof thethe radiatedradiated beambeam intointo fibrefibre..
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34. Advantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LED
AdvantagesAdvantages ::
EfficientEfficientEfficientEfficient
ColorColor
SizeSize
OnOn // OffOff timetime
CyclingCyclingCyclingCycling
DimmingDimming
CoolCool LightLight
SlowSlow FailureFailure
LifeLife timetime
ShockShock resistanceresistance
FocusFocus
RobustRobust
LinearLinear
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35. LargeLarge lineline widthwidth ((3030--4040 nm)nm)LargeLarge lineline widthwidth ((3030--4040 nm)nm)
LargeLarge beambeam widthwidth (Low(Low couplingcoupling toto thethe fiber)fiber)
LowLow outputoutput powerpower
LowLow E/OE/O conversionconversion efficiencyefficiency
Advantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LED
DrawbacksDrawbacks ::
LowLow E/OE/O conversionconversion efficiencyefficiency
TemperatureTemperature dependancedependance
VoltageVoltage sensitivesensitive
HighHigh initialinitial priceprice
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36. Point of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of Discussion
AdvantagesAdvantages && disadvantagesdisadvantages ofof ILDILD
overover LEDLED forfor longlong haulhaul OpticalOptical fiberfiberoverover LEDLED forfor longlong haulhaul OpticalOptical fiberfiber
communicationcommunication..
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37. ILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LED
AdvantagesAdvantages ::
1.1. Modulation CapabilitiesModulation Capabilities
2.2. High radianceHigh radiance
3.3. Narrow line widthNarrow line width
4.4. Relative temporal coherenceRelative temporal coherence
5.5. Good spatial coherenceGood spatial coherence
6.6. More focused radiation pattern, small fiberMore focused radiation pattern, small fiber6.6. More focused radiation pattern, small fiberMore focused radiation pattern, small fiber
7.7. Much higher radiant power, longer spanMuch higher radiant power, longer span
8.8. Faster ON, OFF time; higher bit rates possibleFaster ON, OFF time; higher bit rates possible
9.9. Monochromatic light, reduces dispersionMonochromatic light, reduces dispersion
DisadvantagesDisadvantages ::
1. Much more expensive
2. Higher temperature, shorter lifespan
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38. LEDLEDLEDLED ILDILDILDILD
• Lower Efficiency •Higher Efficiency
• Slow response rate •High response rate
• Lower data transmission
rate
•Higher data transmission rate
ILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LED
• Simple construction •Construction is complicated
• Higher distortion level at
output
•Lower distortion level at
output
• Higher dispersion •Lower dispersion
• In coherent beam •Coherent beam
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