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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 em an d o f B an d w id th

A p ril 2 0 0 0

A p ril 2 0 0 3 1 6 M illio n T b / m o n th

3 , 5 0 , 0 0 0 T b / m o n th

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Input Received

signal Over one Kilometer distance signal

Strength 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 through Fiber??

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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..)

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Schematic representation of Optical Fiber

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-1 NA

• 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

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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 <= 2.405

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Common designs of fiber

Step Index Fiber

n1

n2

n1

n2

R.I.

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Common designs of fiber

Total internal reflection

Refraction

Core

n1

n2n3

n4

n5

n6

Claddingn1>n2>n3>n4>n5>n6 etc.

Graded Index Fiber

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CoreCladding

rr

n2

n1

RefractiveIndex n (r ) a

Graded Index Fiber

Apr 15, 2023Source From: Internet

Types of Optical Fiber

Apr 15, 2023

R.I. Distribution of DifferentOptical 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 250micron

62.5micron 125 micron 250 micron

Single mode 9micron 125 micron 250 micron

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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

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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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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 fiber0.25 dB/km

~10 W

Transmitter Receiver

Electricalsignal

Electricalsignal

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Loss Mechanisms

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Density Fluctuations

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Loss in a Fiber

0.10.20.51.02.05.0102050100

800600 12001000 16001400 1800

Early 1970s

First Window

Second Window Third

Window1980s

Wavelength (nm)

Att

enu

atio

n

(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

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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

1. Intermodal dispersion or Modal dispersion

2. Intramodal dispersion or Chromatic dispersion

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Step Index Fiber

n1

n2

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Modal Dispersion

Modal dispersion is the spreading of optical signals in different modes

Multimode fiber has large number of modes and each mode travel with different distances, which results in modal dispersion

Multimode fiber is not used for long distance communication due to this large modal dispersion coefficient

Graded-index multimode fiber have less modal dispersion coefficient, thus can be used for longer distance than multimode fiber

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CoreCladding

rr

n2

n1

RefractiveIndex n (r ) a

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Chromatic Dispersion

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Chromatic Dispersion

Different frequency components within the optical pulse (different wavelength) travels with different group velocities

Chromatic dispersion occurs only in single mode fiber since it has only one mode of propagation

High chromatic dispersion broadens the optical pulses in time and lead to inter-symbol interference that can

produce an unacceptable bit error rate

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Chromatic Dispersion

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There are two contributions to the chromatic dispersion:

     The material dispersion of the glass

When velocity variation is caused by some property of the wave guide materials - Effect is called “Material Dispersion”

Chromatic Dispersion (Contd…)

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MFD

MFD

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Waveguide DispersionWaveguide Dispersion

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)

Dis

per

sion

[ p

s/ (

nm

km

) ]

Waveguide

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Positive and Negative Dispersion

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TYPES OF FIBERTYPES OF FIBER

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0.1

0.2

0.5

1.0

2.0

5.0

10

20

50

100

800600 12001000 16001400 1800

Early 1970s

First Window

Second Window Third

Window1980s

Wavelength (nm)

Att

enua

tion

(dB

/km

)EVALUATIONEVALUATION

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Optical Fiber Optical Fiber

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 communication typically 100 meter to 1 Km

Distance of reach depends on so many parameters

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Typical SM FibersTypical SM Fibers

Normal Single Mode Fiber

DSF (Dispersion shifted fiber)

NZ-DSF (Non-Zero dispersion shifted fiber )

DCF (Dispersion compensating fiber)

LEAF (Larger effective area fiber)

DFF (Dispersion Flattened Fiber)

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Typical SM FibersTypical SM Fibers

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

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Typical SM Fiber ParametersTypical SM Fiber Parameters

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)

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Typical SM Fiber Parameters

Parameter at different wavelengths are

Attenuation slope (dB/Km/nm) Dispersion slope (ps/nm2 Km) Mode field diameter

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Typical Value for SM FiberTypical Value for SM Fiber

1 Attenuation only in fiber (dB/km) 1550 nm ≤0.252 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) ≤ 0.0924 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 P1565 to 1625 nm 4.5 to 11.2 P

6 Mode field diameter (µm) At 1550 nm 9.2 to 107 Max Effective area (µm2) Norminal 728 Cutoff Wavelength (nm) 12479 PMD Coefficient (ps/km1/2), max mean, @1550 nm ≤0.0810 Effective Group Index of Refraction @ 1550 nm 1.469

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ITU Standards (Optical Fiber)ITU Standards (Optical Fiber)

G.650 – Definition and test methods for the relevant parameters of single mode fibers

G.651 – Characteristics of a 50/125 μm 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.

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ITU Standards (Optical Fiber)ITU Standards (Optical Fiber)

G.654 – Characteristics of a 1550 nm wavelength loss- minimized single-mode optical fiber cable

G.655 – Characteristics of a non-zero dispersion single- mode optical fiber cable.

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G652 fiberG652 fiber

ITU-recommendation G.652SMF has

Zero chromatic dispersion at 1310 High chromatic dispersion (approx. 17ps/nm-km) at 1550nm

Advantage Support WDMLow in cost

DisadvantageSuitable only for short and medium distancesNeeds Dispersion Compensation modules

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G652 fiberG652 fiber

15501310

Dis

per

sio

n (

ps/

nm

.Km

)

0

-10

-20

10

20

nm

EDFA Gain Spectrum

1530 1610

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Dispersion Shifted FiberDispersion Shifted Fiber

ITU-recommendation G.653 Wave guide dispersion and material dispersion cancel 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 nm band Erbium doped fiber amplifiers operate in this band

Done by increasing the waveguide dispersion

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1200 1300 1400 1500 1600

-20

-10

10

20

0

Material

Total

Wavelength (nm)

Dis

pers

ion

[ ps

/ (nm

km

) ]

Waveguide

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Dispersion Shifted Fiber (DSF)Dispersion Shifted Fiber (DSF)

1550

1310

Dis

per

sio

n (

ps/

nm

.Km

)

0

-10

-20

10

20

nm

EDFA Gain Spectrum

1530 1610

NDSF

DSF

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Dispersion Shifted FiberDispersion Shifted Fiber

AdvantageSuitable for DWDM applications, with broad channel

spacingDispersion compensation is required after long distances

DisadvantageNot suitable for higher channel countSuffers from strong nonlinear effectsUnsuitable for narrow channel spacing, due to four wave

mixing

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Non Zero Dispersion shifted FiberNon Zero Dispersion shifted 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

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Non-Zero Dispersion Shifted FiberNon-Zero Dispersion Shifted Fiber

DSF

Dis

pers

ion

(ps/

nm

.Km

)

0

-5

-10

5

10

nm

EDFA Gain Spectrum

1530

1610

NZ-DSF

1550

NZ-DSF

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Dispersion Flattened FiberDispersion Flattened Fiber

Here dispersion over range from 1300 to 1700 is reduced i.e 3ps/nm/km

Advantages Very less dispersion change within EDFA spectrumEfficient for DWDM systems with less number of channels

DisadvantagesExtremely high attenuation (2dB/Km)Severe Four Wave Mixing problems

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Dispersion Flattened FiberDispersion Flattened FiberD

ispe

rsio

n (p

s/ n

m.K

m)

0

-10

-20

10

20

nm

EDFA Gain Spectrum

1530 1610

DSF

1550

Dispersion Flattened

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Large Effective Area Fiber (LEAF) :Large Effective Area Fiber (LEAF) :

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Large Effective Area Fiber (LEAF) :Large Effective Area Fiber (LEAF) :

Advantages:

Fiber effective is increased to 72 to 80 micro meter2 from 50 micro meter2

This type of fiber can carry large amount of the optical power Nonlinear interactions will be reducedGenerally used in Undersea applications

Disadvantages

Difficult fiber designCost is very high

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Large Effective Area Fiber (LEAF) :

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Dispersion Compensated Fiber Dispersion Compensated Fiber (DCF)(DCF)

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