Optical Communication Unit 1 - Part 2

Optical Communication - Ray Optics

1. R.M.K. COLLEGE OF ENGINEERING AND TECHNOLOGY
DEPARTMENT OF ECE
OPTICAL COMMUNICATION
DR.K.KANNAN
ASSISTANT PROFESSOR
DEPARTMENT OF ECE

2. ●Opticalcommunicationwavelengthwindowisbetween800nmto1300nm.Today
thewavelengthrangeis1550nm
●CharacteristicsofLight
-IntensityofLight
-FrequencyandWavelengthofLight
-SpectralwidthofLight
●LightactasanElectromagneticWaves-Polarization
●LightStructure
-Simplestmodelofray-RAYModel
-AdvancedversionofRaymodel–WAVEModel
-MoreadvancedVersuinofWaveModel–QUANTUM Model
OPTICAL FIBER COMMUNICATION

3. RayOptics
Rayisanidealizedmodeloflight,obtainedbychoosingalinethatis
perpendiculartothewavefrontsoftheactuallight,andthatpointsinthedirectionof
energyflow
TypesofRayOptics
-Meridionalray
-Skewray
-Guidedray
-Leakyray
RAY THEORY TRANSMISSION
RAY MODEL

4. ●AMeridionalrayisaraythatpassesthroughtheaxisofanopticalfiber
●ASkewrayisaraythattravelsinanon-planarzigzagpathandnevercrossesthe
axisofanopticalfiber
●AGuidedray,Boundray,orTrappedrayisarayinamulti-modeopticalfiber,
whichisconfinedbythecore.Forstepindexfiber,lightenteringthefiberwillbe
guidedifitmakesananglewiththefiberaxisthatislessthanthefiber'sacceptance
angle
●ALeakyrayorTunnelingrayisarayinanopticalfiberthatgeometricoptics
predictswouldtotallyreflectattheboundarybetweenthecoreandthecladding,but
whichsufferslossduetothecurvedcoreboundary
RAY OPTICS

5. ●Ameridionalrayisaraythat
passesthroughtheaxisofanoptical
fiber.
●Types
-BoundedRay-Raysthatare
propagatedthroughthefiberbyTIR
-UnboundedRay–Raysthatare
refractedfromthecore
Cladding imperfections
preventtotalinternalreflection
Unboundedrayseventually
escapefromthecable
MERIDIONAL RAY

6. ●Lightentryintothetipofthefiber
core
●TIR–SustainedPropagation
-obeysSnell’slaw
-n
1>n
2&θ
1>θ
2
●SamePlane–Incidentrayandfiber
core
MERIDIONAL RAY

7. ●Tworayincidence–Maximumintensityatameetingpointoftworay
duringpropagation
●Meridionalraystakecomparativelylesserlightraypathbecauseoflesser
acceptanceangle
●Theserayareconfinedtothemeridionalplanesofthefiberwhicharethe
planesthatcontaintheaxisofsymmetryoffibers
MERIDIONAL RAY

8. ●Raysnevermeettheaxisofthefiber
●Itdoesnotlieintheplanecontainingtheaxisofthefiberanditwillgoin
differentdirection
●Raywillgosprileariundthefiberaxis–Multiplereflectionintheinterface
●Lightintensityofthisrayisverylow–Nevermeettheaxis
●Twopossibleintensitydistribution
●Maximumintensitydistributionatthecenter–axisofthefiber
●Minumumintensitydistributionatthecenter
SKEW RAY

9. ●Dependsonlightcollecting
efficiencyofthefiber
●DependsonOpticalSourceandtypes
offiberused
●θ
0andθ
max
θ
max=Sin
-1
(n
1
2
-n
2
2
)
1/2
-NA
●NAshouldbelargeaspossiblei.e
(n
1
2
-n
2
2
)
1/2
shouldbelarge
Heren
1isfixed–Glass(1.5)andin
generaln
2≥1
SKEW RAY -ACCEPTANCE ANGLE

10. Raymodelanalysis
✓TotalInternalReflectionexplainsthattheraycompletelyreflectedatthe
boundarybetweencoreandcladding.
✓Doesnotdiscussaboutwhathappenedinthecladding
✓Zerointensityinthecladding
✓DoesnotgivethecorrectpictureofTIRbecoseattheinterfacethe
elecromagneticfieldintermsoflightwillgotozero
SKEW RAY –MODEL ANALYSIS

11. Wavemodelanalysis
➢Inthecladdingmediumthelightintensityisnotzero,thefieldsarepresentin
thesecondmediumanditisexponentiallydecayinthecladding
➢Thefieldinthecladdingmustbeconsiderotherwiseitwilldisturbthefieldinside
thecore–Protectthefieldinthecladding
➢Propercladdingshouldbedonetodieoutthefieldandthatshouldnotgoawayto
theoutsidetheworld
ThereforeatTIR
-Standingwavetypeoffieldsinthecore
-Decayingfieldsincladding
-Therayundergophasechangeatthereflectingboundary

12. ●Sustained constructive
interferencebythephasefront
isachievedbyseparation
betweenthem
●Theseparationbetweenthese
shouldbemultiplesof2??????
●Thenonlythephasefront
movinginthisdirectionand
gets reflected(constructive
interference)
SKEW RAY -TIR

13. ●Forasustainedconstructive
interference,thedistance
betweenthesetwophase-
frontsmustbemultiplesof2??????
●S
1=d/Sinθ&S
2=ADCosθ
S
2=(Cos
2
θ–Sin
2
θ)d/Sinθ
2??????n
1/λ(S
1-S
2)+2??????=2??????m
2??????n
1dSin??????/λ+??????=??????m
●Theaboveconditionismust
satisfiedforthelightray
propagatedinsidethefiber
SKEW RAY CONT…

14. ●Forsustainedpropagationincidentangleoftheray<??????
0??????????????????(TIR)
●Withtheabovetheincidentraymustsatisfythefollowingconditionis
2??????n
1dSin??????/λ+??????=??????m
●Otherwisetheraycan'tpropagateinsidethefiber
●Herem=integerand2??????n
1dSin??????/λgivesdiscreteangles
●Departurefromasolidconeoflightincontinuous
??????&#3627408413;&#3627408424;&#3627408422;????????????&#3627408423;??????&#3627408423;??????&#3627408424;&#3627408413;??????&#3627408412;??????&#3627408414;??????&#3627408414;??????domain
●ThischangewillleadtoMODESinsidetheopticalfiber–Discretepatternsof
intensityoflightinsidetheopticalfiber
SKEW RAY CONT…

15. ●Skewrayfollowshelicalpaththroughthefiber.thispathtracedthroughthe
fibergivesachangeindirectionof2??????(γ+??????)ateachreflection
●ThenumberofReflectionsinsidethecorewillgivetheoutputdistributionnot
dependsontheinputconditionsofthefiber
●Nonuniformlightinput–Uniformlightoutputbymultiplereflection
●TheincidentandreflectedraysatthepointBareinthesameplanerepresented
byCos??????
SKEW RAY –ACCEPTANCE ANGLE

16. ●Thus to resolve the ray path AB relative to the radius BR in these two perpendicular
planes requires multiplication by Cos ??????and Sin ??????
SKEW RAY –ACCEPTANCE ANGLE

19. ●DependonCos??????value,theaxialanglesarelargerthanmeridionalrays
●InmeridionalraysCos??????=1and??????
????????????=????????????
●Hence,Skewraystendstopropagateonlyinannularregionneartheouter
surfaceofthecoreanddonotfullyutilizethecorefortransmission

20. COMPARISION BETWEEN
MERDIONAL AND SKEW RAYS
MERIDIONAL RAY SKEW RAY
Meridional rays enter into the optical
fiber through it's axis
Skewrays enter into the optical fiber not
it's axis
These rays cross the fiber axis at each
reflection
Skew rays do not cross the fiber axis and
propagate around the optical fiber axis
Acceptance angle is minimum Acceptance angleis maximum
Lightcollecting efficiency is
minimum
Lightcollecting efficiency is maximum
Propagation in ZigZagPath Propagation in Helical Path

21. ●Aleakymodeorleakyraysortunneling
modeisamodehavinganelectricfield
thatdecaysmonotonicallyforafinite
distanceinthetransversedirection
●Thepropagationoflightthroughoptical
fiberbyskewrayssufferonlypartial
reflectionwhilemeridionalraysare
completelyguided
●Thusthemodesallowingpropagationof
skewraysarecalledleakymodes.Some
opticalpowerislostintocladdueto
thesemodes.
LEAKY RAYS

22. AnopticalfiberinairhasanNAof0.4.Comparetheacceptanceangleformeridionalrays
withthatforskewrayswhichchangedirectionby100°ateachreflection.
Solution:Theacceptanceangleformeridionalraysisgivenby
NA=n
0sinθ
a=(n1
2
−n2
2
)
1/2
ForAirmediumn
0=1
NA=θ
a=sin
−1
NA=sin
−1
0.4
=23.6°
Theskewrayschangedirectionby100°ateachreflection,thereforeγ=50°.Theacceptance
angleforskewraysis:
θ
as=sin
−1
(NA/Cosγ)=sin
−1
(0.4/Cos50°)=38.5°
Inthisexample,theacceptanceanglefortheskewraysisabout15°greaterthanthe
correspondingangleformeridionalrays
PROBLEM

23. TYPES OF OPTICAL FIBER –
REFRACTIVE INDEX

24. ●Therefractiveindexofthecoreisuniformthroughoutandundergoesonabrupt
changeatthecorecladdingboundary
●TheRIProfilen(r)=n1;r<a-Core
=n2;r≥a-Cladding
STEP INDEX FIBER

25. ●Thediameterofthecoreisabout50-200μminthecaseofmultimodefiberand
10μminthecaseofsinglemodefiber
●Lowerbandwidthandlowerattenuation
●Thelightraypropagationisintheformofmeridionalrays(ZigZag)andit
passesthroughthefiberaxis

26. SinglemodeorMonomodestepindexfiber
-OnlyoneEMModeandthecorediameterintheorderof2to10??????m
-Lowintermodaldispersion(Broadeningoftransmittedlightpulse)
andlowertolerancerequirementsonfiberconnectors
-Maximumbandwidth
-LargerNA,Largercorediameterandeasiercouplingtoopticalsource
SINGLE MODE STEP INDEX FIBER

27. Amultimodestep-indexfiberhasacoreofradius(a)andaconstantrefractive
indexn
1.Acladdingofslightlylowerrefractiveindexn
2surroundsthecore.
-Diameter of the core is 50 -200????????????
-Lower bandwidth
-Light propagation in ZigZagpath along the fiber
-Short distance communication
-Large dispersion
-Low performance
MULTI MODE STEP INDEX FIBER

28. ●Therefractiveindexofthecoreismade
tovarygraduallysuchthatitis
maximumatthecenterofthecore.
●Thediameterofthecoreisabout50μm
inthecaseofmultimodefiber
●Thepathoflightishelicalinmanner
●Attenuationisless
●Thisfiberhashigherbandwidth
●Thelightpropagationisintheformof
skewraysanditwillnotcrossfiber
axis.
GRADED INDEX FIBER

29. RefractiveIndexprofileofGradedindexfiberis
Here∆=RelativeRIdeference
??????=Profileparameter
??????=1-Triangularprofile
=2-Parabolicprofile
=∞-StepIndexprofile
GRADED INDEX FIBER

30. ●Here you can describe the wrong
uses of your logo
●Here you can describe the wrong
uses of your logo
●Here you can describe the wrong
uses of your logo
●Here you can describe the wrong
uses of your logo
GRADED INDEX FIBER

31. ●Here you can describe the wrong
uses of your logo
●Here you can describe the wrong
uses of your logo
●Here you can describe the wrong
uses of your logo
●Here you can describe the wrong
uses of your logo
GRADED INDEX –MULTI MODE
FIBER

32. STEP INDEX AND GRADED INDEX FIBER

33. COMPARISION
Feature Step-Index Fiber Graded-Index Fiber
Bandwidth Size Lower bandwidth Higher bandwidth
Diameter of the Core 50-200 µm About 50 µm
Application Scenarios
Normally used in short-distance (within
a few kilometers) and low-speed (8
Mb/s or less) communication systems
Usually used in medium-distance
(10~20 km) and relatively higher-
speed (34~140 Mb/s)
communication systems
Data Transmission Form
Light propagates in the shape of a
zigzag along the fiber/core axis
Light travels forward in the form
of sinusoidal oscillation/curves
Modal Dispersion
Affects the transmission capacity of the
fiber and limits the relay distance
Greatly decreased dispersion than
step-index multimode fiber,
making a higher bandwidth
NumericalAperture High Low
ReflectionLoss Yes No
Attenuation Maximum Minimum