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TransmissionCharacteristicsof

OpticalFiber

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TransmissionWindowsinOpticalFiber

st

Lowloss

10Kmrepeaterspacing u mo e erswereuse

140Mbps

g erattenuat onShortdistance

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TransmissionWindowsinOpticalFiber

nd

LowlossSilicafibers

40Kmrepeaterspacing

Sin lemodeaswellasMultimodefiberswereused

2.5Gbps

Comparativelylessattenuationlongdistance

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TransmissionWindowsinOpticalFiber

rd

Lowloss

90Kmrepeaterspacing

ng emo ean u mo e erswereuse

Upto10Gbps

owestattenuat onlongdistance

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AttenuationCharacteristicsofanOpticalFiber

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FiberAttenuation

Lossorattenuationisalimitingparameterinfiberoptic

systems

electricaltransmissionlinesonlywhenlosseswerereducedtoallowsignaltransmissionoverdistancesgreaterthan10km

Fiberattenuationcanbedescribedbythegeneralrelation:

PdP =

whereisthepowerattenuationcoefficientperunitlength

in ,afterpropagatingalengthLwithinthefiberPoutis

LPP inout = exp

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FiberAttenuation

Attenuat on sconven ent yexpresse ntermso B m

( ) log10

10

=P

P

LkmdB out

log10 10

=

PeP

L in

L

in

( ) ( )

34.4

log 10

=

= eLL

dBmmW

mWmWP 10

1

10log1010 10 =

==

mWmWdBmP 50110127 1027

===

PowerisoftenexpressedindBm (dBm isdBfrom1mW)

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Attenuation

Intrinsic

Extrinsic Duetoscattering

MicrobendingMacrobending

Atomicdefects near

Mie

Nonlinear

SRS

SBS

CPM

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Absorption

Absorption is caused by three different mechanisms:

1- Impurities in fiber material: from transition metal ions (must

be in order of ppb) & particularly from OH ions with

absor tion eaks at wavelen ths 2700 nm 400 nm 950 nm &

725nm.

2- Intrinsic absorption (fundamental lower limit): electronica sorpt on an reg on atom c on v rat on an

(IR region) in basic SiO2.

3- Radiation defects

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ExternalLosses

Bendingloss

Radiationlossatbendsintheo ticalfiber

InsignificantunlessR

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B n in l Macrobendin

Microbending

cladding

core

23

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Transmission medium erformance

z=0 z=L

Attenuation

z=0 z=L

Dispersion

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Dispersionofthetransmitted

opticalsignalcausesdistortionfor

transmissionsalongopticalfibers. Indigitalsystems,thedispersion

mec anismscausea roa ening

ofthetransmittedpulsesasthey

travelalongthefiber. Thisreducesthefiberbandwidth

andcancauseintersymbol

interference(ISI).

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Dispersion

Dis ersivemedium:velocit of ro a ationdependsonfrequency

Dispersioncausestemporalpulsespreading

Pulseoverlapresultsinindistinguishabledata Intersymbolinterference(ISI)

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Di r i n m h ni mIntermodal Intramodal / Chromatic

materialAtomicresonances Electronic

1

resonances

multimode fiber! 1010 1011 1012 1013 1014 1015 1016 1017(Hz)

wavegu e

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Higherordermodeshavealongerpathlength Longerpathlengthhasalongerpropagationtime

TemporalpulseseparationL=

vgisusedasthepropagationspeedfortheraystotake

gv

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In rm l i r i nL

n1 c

X

2

n

Lc

L

T1

min

distance

===2

cos nc =

n1

1max

nLT=

1

t

2

2

1

nc

Ln

c

=

[ ]kmnscn

LNA

c

nLn

c

n

n

LnTTT s /

2 1

2

1

2

1minmax

==

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1/Ts

Ts/2 Ts/2 time

dttpdttp

sT

T

ii )(1)(2

==

s

2

rms pulsebroadeningduetointermodaldispersion

1

0

2

1

12

mmeanzero

mm

T

s

s

=

=

( )1 22

2

2

2

NALT

dtT

tm

s

sTs

==

=

34)(

32

223

1

2

1

1

cnNAL

cLn

cn

s

s

==

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Dispersionhaslesseffectinsinglemodefibers

BL = 20 MHz.km

BL = 1 GHz.km

= .

2

=TB

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e.g.AMMGIFiberexhibitstotalpulsebroadeningof0.1soveradistanceof15Km.Estimate:

1. Max.possiblebandwidthonthelinkassumingnoISI

.

3. Bopt.L productforthefiber.

1) 5MHz

2) 6.67ms/Km

3) 75MHz.Km

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Duetothedispersion,thereispulsebroadening

BisthebitrateandTthebitduration.

Togetanorderofmagnitudeofthedispersioneffect,oneusesthefollowingcriteria:thepulsebroadeningTmustbelessthanthepulsewidthT.

1=< TT

B1 0 0 1 1 1* 1 1

uecer a n vaa1 0 0 1 1 0 1 1

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Dispersion has two origins

Intramodal orchromaticdispersion:describesthepulse

broadeningduetothefinitespectrallinewidthofthesources

andoccursinalltypesoffibers:

thecladdingchangewithwavelength)

Waveguidedispersion(Echangeswithwavelength) Profiledispersion(indexprofilechangeswithwavelength)

Intermodaldispersion:describesthepulsebroadeningduetothepropagationdelaydifferencesbetweenthepropagation

modes in multimode fibers.

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The group velocity is given by:

d

d

g =

d

l

l

== g

v udepend both on frequency & the propagation

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Materialdispersion

Itoccurswhenthephasevelocityofaplane

wave ro a atin inthedielectricmedium

variesnonlinearlywithwavelength.nd

2

n

cv

d

p

==

2

N

c

d

dv

g

g ==

dnn

cdvdelayGroup

g

g

g

=

====

11

Ld

dnncdelaydispersionmaterial

dc

m

=

11

1

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Pulsebroadeningduetomaterialdispersion

12

ndL

m =c

2

Lc

=

2

1

d

Materialdispersionparameter

Kmnmpsd

nd

cM = /21

2

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Materialdispersiontendstobecome0

atlongerwavelengths

200

150

dispersion

50

50

00.6 0.8 1 1.2 1.3 1.4 1.6 1.8 2

100

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ChromaticdispersionandIntermodal dispersion

22nCT

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Polarization Mode dispersionIntensity

zOutput light pulse

t

ore

n // x

1y

E

Ex

x

Ey= Pulse spread

x y

E

t

Input light pulse

Su ose that the core refractive index has different values alon two ortho onal

directions corresponding to electric field oscillation direction (polarizations). We cantakexandyaxes along these directions. An input light will travel along the fiber withExandEypolarizations having different group velocities and hence arrive at the output at

1999 S.O. Kasap,Optoelectronics (Prentice Hall)

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Temporal changes in a narrow optical pulse that is subjected to Kerr nonlinearity in

.

Innn 20+= Kerr nonlinearity in fiber, where I is the intensity of.

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Di r i nfl n fi r

40

Dm SiO21.55

0

20

D(ps/nm/km)

Dtot DFF

\

-20

w

n(r)

-40

1.2 1.3 1.4 1.5 1.6

wavelength (m)

51

rW-fiber

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