Fundamentals of FO, Types of Fibers

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??
http://d/Documents%20and%20Settings/Administrator/Desktop/Chennai/ANIMATIONS/Cledding.exe


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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
http://d/Documents%20and%20Settings/Administrator/Desktop/Chennai/ANIMATIONS/fig-movie.exe


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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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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



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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


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


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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