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8/14/2019 Fundamentals of FO, Types of Fibers
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Fundamentals of Fiber Optics
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Optical Communication
Why Optical Communication??
IT Revolution - Need for exchange of more and more
information
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D e m a n d o f B a n d w i d t h
A p r i l 2 0 0 0
A p r i l 2 0 0 3 1 6 M i l l i o n T b / m o n t h
3 , 5 0 , 0 0 0 T b / m o n t h
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Input Received
signal Over one Kilometer distance signalStrength Strength
1000 UTP : 30dB 1
1000 Microwave : 10 dB 100
1000 STP and coaxial cable :20dB 10
1000 Fiber : 2 dB 950
1000 Experimental fiber : 0.0005dB 999.99
TYPICAL SIGNAL LOSSES
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EM Spectrum
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Optical Communications: What does it
offer?
Uses an optical carrier: 1013 - 1014Hz
can carry 1013 - 1014Hz( 10 to 100 THz) of information
- analog voice: 20KHz bandwidth 500million
channels- digitized voice at 64kbps 160 million channels
- analog video:5MHZ 2 million channels
- digitized voice at 100Mbps 100k channels
Unguided Optical Communication
atmospheric link: requires line of sight
high attenuation
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What is the difficulty in using light wave???
Requirement of a Suitable media to carry light
Which is the most suitable medium to carry light???
Air?????????????????
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Air is vulnerable,which leads to interference of signals
with other light waves present in the atmosphere
Due to the presence of fog,moisture etc in the atmosphere
there will be a lot of distortion introduced to light waves
Which is the most suitable medium to carry light???
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Glass is known since ancient times as the most suitable
transmission medium for light
To use light for long distance transmission,light is
required to be carried in glass
Light should have enough power so that signal can be
sustained for long distance
Glass
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Evolution of Optical
Communication
Problem1 is solved with the invention of glass fiber which
is popularly known as Optical fiber
Problem2 is solved with the invention of LASER and LEDs
How Light transmits throughFiber??
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Principle of Light Transmission
Light Transmission
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Structure Of Optical Fiber
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Structure Of Optical Fiber(Contd..)
Schematic representation of Optical
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Schematic representation of OpticalFiber
Why is Cladding required??
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Why Cladding is
required??
Mechanical protectionGuard against electromagnetic interference
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Acceptance
Angle
Acceptance Angle
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Numerical Aperture
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Numerical Aperture
Numerical aperture (NA): NA= (n12 n2
2)1/2
Typical NA values are 0.1 to 0.4 which correspond to
acceptance angles of 11 degrees to 46 degrees
Acceptance angle of a fiber: a = sin-1NA
Light that enters at an angle equal to or less than the
acceptance angle will be guided
NA is more means more light gathering power
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Concept of
Modes
Modes
C f
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Concept of V-number
Concept of V-Number :
v= 2 * * (a / ) * NA
Number of modes directly proportional to V-number
No. of modes M v2 /2
If M is large
Fiber is Single Mode, if
v
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Common designs of
fiber
Step Index Fiber
1 2 3 4
n1
n2
n1
n2
R.I.
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Core
Cladding
rr
n2
n1
Refractive
Index n (r ) a
Graded Index Fiber
Types of Optical Fiber
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Jul 26, 2008Source From: Internet
Types of Optical Fiber
R I Di t ib ti f Diff t
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Jul 26, 2008
R.I. Distribution of Different
Optical Fiber
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Why SingleMode Fiber Is always Step Index ?
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Single mode and Multimode
fiber
Single mode and Multimode
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Single mode and Multimode fiber
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Core Cladding Jacket
Multimode 50 micron 125 micron 250micron62.5micron 125 micron 250 micron
Single mode 9micron 125 micron 250 micron
Ad t f O ti l i ti
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Advantages of Optical communication
Explosive demand for higher bandwidth
Low bandwidth of copper
Nearly 25THz possible with fiber
Low Loss-Longer distance transmission(Less Repeaters)
No EMI in fiber-based telecom
Less cross-talk,more reliability
More secure communications
Lighter than copper
Lower cost per unit bandwidth(made of silica which is very
cheap)
Safer and more advantages
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Very light weight and compact
Comparison of copper cable & Optical fiber cable with
same information carrying capacity
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Copper Fiber
Diameter (inches) 2.8 0.5
Weight (lb/1000-ft length) 4800 80
Data capacity (megabits/sec) 3.15 417
Characteristics of Cables Based on copper
wire and fiber optics
Limitations of Fiber Optics
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Communication over optical fiber is limited by two
factors:
Loss
Dispersion
Limitations of Fiber Optics
Loss and dispersion
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Reasons for Attenuation
Because of the following factors:
Rayleigh scattering (Attenuation decreased with
wavelength)
Attenuation absorption peaks associated with the hydroxyl
ion (OH-)
Attenuation to increase at wavelength above 1.6 micron
due to
bending induced loss due to silica absorption
Attenuation for SM fiber is typically 0.20 to 0.35 dB/Km
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o
Attenuation
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Attenuation
There should be enough optical power at the receiver
for error free detection
Bit Error Rate (BER), typically less than 10-12
To travel long distances, we need to amplify or regenerate
the optical signal
~1 mW 80 km of fiber
0.25 dB/km
~10 W
Transmitter Receiver
Electrical
signal
Electrical
signal
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Loss
Mechanisms
Density
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Density
Fluctuations
Loss in a
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Loss in a
Fiber
0.10.2
0.5
1.02.0
5.0
1020
50
100
800600 12001000 16001400 1800
Early 1970s
First
Window
SecondWindow Third
Window1980s
Wavelength (nm)
At
tenuation
(dB/km)
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Loss due to external
reasons
Micro Bending
Macro Bending
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Macro Bending
If the radius of a bend is relatively large (say 10 cm
or so) there will be almost no loss of light. However,if the bend radius is very tight (say 1 cm) then some
light will be lost.
Figure : Propagation
around a Bend in the
Fiber
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Micro Bending
Micro Bends
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Micro Bends
Micro-bends can be an important source of loss.If the fiber is pressed onto an irregular surface
you can get tiny bends in the fiber as illustrated
in the figure
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Micro Bends
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DISPERSION
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Dispersion is of two types
3. Intermodal dispersion or Modal dispersion
5. Intramodal dispersion or Chromatic dispersion
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Step Index Fiber
1 2 3 4
n1
n2
M d l
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Modal
Dispersion
Modal dispersion is the spreading of optical signals indifferent modes
Multimode fiber has large number of modes and each
mode travel with different distances, which results inmodal dispersion
Multimode fiber is not used for long distancecommunication due to this large modal dispersioncoefficient
Graded-index multimode fiber have less modaldispersion coefficient, thus can be used for longerdistance than multimode fiber
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Core
Cladding
rr
n2
n1
Refractive
Index n (r ) a
Chromatic Dispersion
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Chromatic Dispersion
Chromatic Dispersion
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p
Different frequency components within the optical
pulse (different wavelength) travels with different groupvelocities
Chromatic dispersion occurs only in single mode fiber
since it has only one mode of propagation
High chromatic dispersion broadens the optical pulsesin time and lead to inter-symbol interference that can
produce an unacceptable bit error rate
Chromatic Dispersion
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p
Chromatic Dispersion (Contd)
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There are two contributions to the chromatic dispersion:
The material dispersion of the glassWhen velocity variation is caused by some property of the
wave guide materials - Effect is called Material Dispersion
p ( )
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MFD
MFD
Waveguide DispersionWaveguide Dispersion
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Waveguide Dispersiong p
When velocity variation is caused by structure of the wave
guide itself - Effect is called Wave guide Dispersion
The power distribution of a mode between the core &
cladding is a function of wavelength
Hence if wavelength changes,power distribution changes,
causing the effective index of the mode change.
This causes light energy of a mode propagates partly in core
and partly in cladding, this is called wave guide dispersion
Waveguide dispersion is usually smaller than material dispersion
and depends on the index profile of the fiber.
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1200 1300 1400 1500 1600
-20
-10
10
20
0
Material
Total
Wavelength (nm)
D
ispersion[ps/(nmk
m)]
Waveguide
Positive and Negative Dispersion
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TYPES OF FIBERTYPES OF FIBER
FirstEVALUATIONEVALUATION
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0.1
0.2
0.5
1.0
2.0
5.0
10
2050
100
800600 12001000 16001400 1800
Early 1970s
Window
Second
Window
ThirdWindow
1980s
Wavelen th nm
Atten
uation(dB/km)
p cap caFiberFiber
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FiberFiber
Mostly SM fiber is used long distance
communication typically 5 Km to 170 Km
with out any problem
MM fiber is only used for the low data
rates and short distance communicationtypically 100 meter to 1 Km
Distance of reach depends on so many
arameters
Typical SM FibersTypical SM Fibers
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Typical SM FibersTypical SM Fibers
Normal Single Mode Fiber
DSF (Dispersion shifted fiber)
NZ-DSF (Non-Zero dispersionshifted fiber )
DCF (Dispersion compensatingfiber)
LEAF (Larger effective area fiber)
yp cayp caFibersFibers
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FibersFibers
Dispersion is zero at 1310 nm wavelength
At 1310 nm the losses in the fiber is high
While Losses minimum at 1550 nm while the
dispersion parameter is +17 ps/nm/Km
Typical SM FiberTyp ca M F erParametersParameters
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ParametersParameters
Zero dispersion wavelength (nm)
Cutoff wavelength (nm)
Attenuation (dB/Km)
Dispersion (ps/nm Km) PMD coefficient (ps/Km1/2)
Mode field diameter (micro meter)
Effective area (micro meter2)
Typical SM FiberParameters
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Parameters
Parameter at different wavelengths are
Attenuation slope (dB/Km/nm)
Dispersion slope (ps/nm2 Km)
Mode field diameter
Typical Value forTyp ca Va ue orSM FiberSM Fiber
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SM FiberSM Fiber
1 Attenuation only in fiber (dB/km) 1550 nm 0.25
2 Attenuation vs. wavelength (dB/km) 0.05
Max Delta from 1550nm value between (1525-1625 nm)
3 Dispersion slope (ps/nm 2 -km) mean At 1550 (nm) 0.092
4 Zero dispersion wavelength (nm) 1310 or 15505 Dispersion (ps/nm.km) mean @1550nm (P or N)
1530 to 1565 nm 2.6 to 6.0 P
1565 to 1625 nm 4.5 to 11.2 P
6 Mode field diameter (m) At 1550 nm 9.2 to 10
7 Max Effective area (m2) Norminal 728 Cutoff Wavelength (nm) 1247
9 PMD Coefficient (ps/km1/2), max mean, @1550 nm 0.08
10 Effective Group Index of Refraction @ 1550 nm 1.469
an ar san ar s(Optical Fiber)(Optical Fiber)
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(Optical Fiber)(Optical Fiber)
G.650 Definition and test methods for the
relevant parameters of single
mode fibers
G.651 Characteristics of a 50/125 m
multimode graded index optical fiber
cable
G.652 Characteristics of a single-mode optical
fiber cable
G.653 Characteristics of a dispersion-shifted
single-mode optical fiber cable.
ITU StandardsITU Standards(Optical Fiber)(Optical Fiber)
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(Optical Fiber)(Optical Fiber)
G.654 Characteristics of a 1550 nm
wavelength loss- minimized
single-mode optical fiber cable
G.655 Characteristics of a non-zerodispersion single- mode optical fiber
cable.
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fiberfiber
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fiberfiber
ITU-recommendation G.652
SMF hasZero chromatic dispersion at 1310
High chromatic dispersion
(approx. 17ps/nm-km) at1550nm
AdvantageSupport WDM
Low in cost
DisadvantageSuitable only for short and
medium distances
G6525fiberfiber
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fiberfiber
15501310
Dispersion(ps/
nm.Km)
0
-10
-20
10
20
nm
EDFA Gain Spectrum
1530 1610
spers on espers on eFiberFiber
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FiberFiber
ITU-recommendation G.653
Wave guide dispersion and material dispersioncancel out each other at 1310nm
Same cancellation is used at 1550nm band
The reasons are principally:
Fiber attenuation is a lot lower in the 1550 nmband
Erbium doped fiber amplifiers operate in thisband
Done by increasing the waveguide dispersion
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1200 1300 1400 1500 1600
-20
-10
10
20
0
Material
Total
Wavelength (nm)
D
ispersion[ps/(nmk
m)]
Waveguide
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spers onspers onShifted FiberShifted Fiber
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Shifted FiberShifted Fiber
AdvantageSuitable for DWDM applications, with
broad channel spacing
Dispersion compensation is required after
long distances
DisadvantageNot suitable for higher channel count
Suffers from strong nonlinear effectsUnsuitable for narrow channel spacing,due to four wave mixing
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Non Zero DispersionNon Zero D spers onshifted Fibershifted Fiber
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shifted Fibershifted Fiber
ITU-recommendation G.655
Low positive value of dispersion
(4 ps/nm/km in the 1530-1610 nm band)
Advantages
Minimizes unwanted effects Four-Wave-Mixing(FWM)
More distance than SMF
Disadvantage
Not able to carry large optical power
on- ero spers on-Shifted FiberShifted Fiber
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DSF
Dispersio
n(ps/
nm.Km
)
0
-5
-10
5
10
nm
EDFA GainSpectrum
1530
1610
NZ-DSF
1550
NZ-DSF
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spers on a enespers on a eneFiberFiber
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Here dispersion over range from 1300to 1700 is reduced i.e 3ps/nm/km
Advantages
Very less dispersion change within EDFAspectrumEfficient for DWDM systems with less
number of channels
DisadvantagesExtremely high attenuation (2dB/Km)Severe Four Wave Mixing problems
spers on a enespers on a eneFiberFiber
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Dispersion(ps/
nm.K
m)
0
-10
-20
10
20
nm
EDFA GainSpectrum
1530 1610
DSF
1550
DispersionFlattened
Large Effective Area FiberLarge E ect ve Area F er(LEAF) :(LEAF) :
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( )( )
Large Effective Area Fiber (LEAF) :Large Effective Area Fiber (LEAF) :
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Advantages:
Fiber effective is increased to 72 to 80 micro meter2
from 50 micro meter2
This type of fiber can carry large amount of theoptical powerNonlinear interactions will be reducedGenerally used in Undersea applications
Disadvantages
Difficult fiber design
Cost is very high
Large Effective Area Fiber (LEAF)
:
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:
DispersionDispersionCompensated FiberCompensated Fiber
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Compensated FiberCompensated Fiber
(DCF)(DCF)
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