Optical Communication - Unit 1 - introduction

ec 8751 optical communication
unit 1
INTRODUCTIONTO OPTICAL FIBERS
DR.K.KANNAN
ASP/ECE
RMKCET

objectives
 To study about the various optical fiber modes, configuration, Fiber materials and
fiber Fabrication
 To learn about transmission characteristics of optical fibers
 To learn about the various optical sources and detectors
 To learn about optical receiver and to explore various idea about optical fiber
measurements and also various coupling techniques
 To enrich the knowledge about optical communication systems and networks

Syllabus
UNIT I INTRODUCTION TO OPTICAL FIBERS 9
Introduction-general optical fiber communication system- basic optical laws and definitions optical modes and configurations -mode analysis for optical
propagation through fibers modes in planar wave guide-modes in cylindrical optical fiber-transverse electric and transverse magnetic modes- fiber materials-
fiber fabrication techniques-fiber optic cables classification of optical fiber-single mode fiber-graded index fiber.
UNIT II TRANSMISSION CHARACTERISTIC OF OPTICAL FIBER 9
Attenuation-absorption --scattering losses-bending losses-core and cladding losses-signal dispersion –inter symbol interference and bandwidth-intra model
dispersion-material dispersion- waveguide dispersion-polarization mode dispersion-intermodal dispersion dispersion optimization of single mode fiber-
characteristics of single mode fiber-R-I Profile cut-off wave length-dispersion calculation-mode field diameter.
UNIT III OPTICAL SOURCES AND DETECTORS 9
Sources: Intrinsic and extrinsic material-direct and indirect band gaps-LED-LED structures surface emitting LED-Edge emitting LED-quantum efficiency and
LED power-light source materials-modulation of LED-LASER diodes-modes and threshold conditions-Rate equations-external quantum efficiency-resonant
frequencies-structures and radiation patterns single mode laser-external modulation-temperature effort. Detectors: PIN photo detector Avalanche photo
diodes-Photo detector noise-noise sources-SNR-detector response time Avalanche multiplication noise-temperature effects comparisons of photo detectors.
UNIT IV OPTICAL RECEIVER, MEASUREMENTS AND COUPLING 9
Fundamental receiver operation-preamplifiers-digital signal transmission-error sources-Front end amplifiers-digital receiver performance-probability of
error-receiver sensitivity-quantum limit. Optical power measurement-attenuation measurement-dispersion measurement- Fiber Numerical Aperture
Measurements- Fiber cutoff Wave length Measurements- Fiber diameter measurements-Source to Fiber Power Launching-Lensing Schemes for Coupling
Management-Fiber to Fiber Joints-LED Coupling to Single Mode Fibers-Fiber Splicing Optical Fiber connectors.
UNIT V OPTICAL COMMUNICATION SYSTEMS AND NETWORKS 9
System design consideration Point – to –Point link design –Link power budget –rise time budget, WDM –Passive DWDM Components-Elements of optical
networks-SONET/SDH Optical Interfaces-SONET/SDH Rings and Networks-High speed light wave Links-OADM configuration-Optical ETHERNET-Soliton.

Course outcomes
At the end of the course, the you should be able to:
🠶 Realize basic elements in optical fibers, different modes and configurations
🠶 Analyze the transmission characteristics associated with dispersion and
polarization techniques
🠶 Design optical sources and detectors with their use in optical
communication system
🠶 Construct fiber optic receiver systems, measurements and coupling
techniques
🠶 Design optical communication systems and its networks

SOURCES
TEXT BOOKS:
1. P Chakrabarti, "Optical Fiber Communication”, McGraw Hill Education (India)Private Limited,
2016. (UNIT I, II, III)
2. Gred Keiser,"Optical Fiber Communication”, McGraw Hill Education (India) Private Limited. Fifth
Edition, Reprint 2013. (UNIT I, IV
, V)
REFERENCES:
1. John M.Senior, “Optical Fiber Communication”, Pearson Education, Second Edition.2007.
2. Rajiv Ramaswami, “Optical Networks", Second Edition, Elsevier , 2004.
3. J.Gower, “Optical Communication System”, Prentice Hall of India, 2001.
4. Govind P.Agrawal, “Fiber-Optic Communication Systems”, Third Edition, John Wiley & 2004.

UNIT1
INTRODUCTION TO OPTICAL FIBERS
 Introduction- Optical fiber communication system
 Optical laws and definitions
 Optical modes and configurations
 Mode analysis for optical propagation through fibers
 Modes in planar wave guide
 Modes in cylindrical wave guide
 Transverse electric and transverse magnetic modes
 Fiber materials-Fiber fabrication techniques
 Classification of optical fiber

What is Optical fiber Communication?
OFC is a technique of transmitting information from one
place to another with the help of light travelling through
optical fibers.
“NEAR ZERO LOSS & NEAR
INFINITE BANDWIDTH”

What is Light ?
 Light may be a particle was first advocated by Sir Issac Newton
 In 19th century, Albert Einstein revived the view. He argued that properties such as the
reflection and refraction of light could only be explained if light was made up of
particles.
 Light was energy, instead of matter.
 Light was a wave Fresnel, Young and Maxwell, are credited with investigating the wave-
like properties of light.
 Einstein then proposed that light is actually made up of tiny packets of energy that
travel or propagate in a wave-like manner.
 Light is an Electromagnetic Wave !!!

electromagnetic spectrum
Wavelengths of visible light: 400 nm (violet) to 700 nm (red)

LightwaveTechnology:ApplicationAreas
 Majority Applications:
– Telephone networks
– Data communication systems
– Cable TV distribution
 NicheApplications:
– Optical sensors, Medical equipment, Textile, Illumination…

Communication
Developments & Issues
 Communication – exchange of information
 Telecommunication – exchange of information over a distance –
using some type of equipment
1896
2010s
Informat
ion
Informat
ion
Transmit
ter
Link
Receiv
er
Transmis
sion
medium
• Generally three basic types of information to be exchanged
Voice,Video and Data
 Information is often carried by an EM carrier - frequency varying from few
MHz to several hundred THz.

TelecomSystemsof 1970s
 Transmission Medium
• Twisted pair
• Coaxial cable
• Radio and Microwave
• Satellite
 Signal Type
• Analog—continuous
• Digital-- discrete
• HighAttenuation  20 dB/km
• Limited Bandwidth  KHz to MHz
Attenuation
and BW
limitations

OpticalFiber CommunicationSystem
TRANSMITTER
RECIEVER
 Transmitter : Electrical signal converted into Optical signal (E/O) using an
optical source (optical modulation).
 Transmission Medium: Modulated optical signal transmitted through optical fibers
to the receiver.
 Receiver : Optical signal reconverted to the electrical signal (O/E) for further
processing (demodulation) before passing onto destination.

Why Fiber OpticTechnology?
 During past 3-4 decades, remarkable and dramatic changes in electronic
communication industry.
 A phenomenal increase in voice, data and video
communication
 demands for larger capacity &
more economical communication systems.
 Lightwave Technology: ATechnological route for achieving
this ever-increasing goal
 Most cost-effective way to move huge amounts of information
(voice, video, data) quickly and reliably.

Why Fiber Optic Technology?
Capacity ! Capacity ! and More Capacity !
 A technical revolution in Communication Industry to explore for large
capacity, high quality & economical systems for communication at
Global level.
 Radio -waves and Trrestrial Microwave systems have long since reached
their capacity
 Satellite Communication Systems can provide, at best, only a temporary
relief to the ever-increasing demand.
Extremely high initial cost of launching the geometry of suitable orbits,available microwave
frequency allocations and if needed repair is nearly impossible
Next option: OPTICAL COMMUNICATION SYSTEMS !

HowOptical Transmission fulfill the need ?
 Information carrying capacity of a communications system is
directly proportional to its bandwidth;
 Wider the bandwidth, the greater its information carrying capacity.
👉Theoretically ; BW is 10% of the carrier frequency
System with light as carrier has excessive bandwidth (100,000 times than
achieved with microwave frequencies)  THz Bandwidth
 Meet the present communication needs or that of the foreseeable future.
Signal Carrier
 VHF Radio system; 100
MHz.
 Microwave system; 10 GHz
 Lightwave system; 1015 Hz
or 106 GHz
Bandwidth
10 MHz
1.0 GHz
105 GHz
C= BWlog2 (1+SNR);

NeedforOptical FiberCommunication

History of fiber optic
 In 1790s the French Chappe brothers invented the first “optical telegraph.” It was a system
comprised of a series of lights mounted on towers where operators would relay a message
from one tower to the next.

 In the 1840s, physicists Daniel Collodon and Jacques Babinet showed that light could be directed along
jets of water for fountain displays.
 In 1854, John Tyndall, a British physicist, demonstrated that light could travel through a curved stream of
water thereby proving that a light signal could be bent. He proved this by setting up a tank of water with
a pipe that ran out of one side. As water flowed from the pipe, he shone a light into the tank into the
stream of water. As the water fell, an arc of light followed the water down.

 Alexander Graham Bell patented an optical telephone system called the photo phone in
1880.
 That same year, William Wheeler invented a system of light pipes lined with a highly
reflective coating that illuminated homes by using light from an electric arc lamp placed
in the basement and directing the light around the home with the pipes.
 Doctors Roth and Reuss, of Vienna, used bent glass rods to illuminate body cavities in
1888.
 In the 1920s, John Logie Baird patented the idea of using arrays of transparent rods to
transmit images for television and Clarence W. Hansell did the same for facsimiles.
 Heinrich Lamm, however, was the first person to transmit an image through a bundle of
optical fibers in 1930
 In 1954, the “maser” was developed by Charles Townes and his colleagues at Columbia
University. Maser stands for “microwave amplification by stimulated emission of
radiation.”

 The laser was introduced in 1958 as a efficient source of light. The concept was
introduced by Charles Townes and Arthur Schawlow to show that masers could be
made to operate in optical and infrared regions.
 In 1960, the first continuously operating helium-neon gas laser is invented and
tested. That same year an operable laser was invented which used a synthetic pink
ruby crystal as the medium and produced a pulse of light.
 Charles Kao and George Hockham, of Standard Communications Laboratories in
England, published a paper in 1964 demonstrating, theoretically, that light loss in
existing glass fibers could be decreased dramatically by removing impurities.

 Charles Kao known as Father of Fiber optic communications
 Awarded Nobel Prize

 In 1970, the goal of making single mode fibers with attenuation less then 20dB/km was
reached by scientists at Corning Glass Works. This was achieved through doping silica
glass with titanium.
 In 1973, Bell Laboratories developed a modified chemical vapor deposition process that
heats chemical vapors and oxygen to form ultra-transparent glass that can be mass-
produced into low-loss optical fiber.
 The first non-experimental fiber-optic link was installed by the Dorset (UK) police in
1975. Two years later, the first live telephone traffic through fiber optics occurs in Long
Beach, California.
 In the late 1970s and early 1980s, telephone companies began to use fibers extensively
to rebuild their communications infrastructure.
 Sprint was founded on the first nationwide, 100 percent digital, fiber-optic network in
the mid-1980s.

 The Erbium-doped fiber amplifier, which reduced the cost of long-distance fiber systems by
eliminating the need for optical-electrical-optical repeaters, was invented in 1986 by David Payne
of the University of Southampton and Emmanuel Desurvire at Bell Labratories. Based on
Desurvire’s optimized laser amplification technology, the first transatlantic telephone cable went
into operation in 1988.
 In 1991, Desurvire and Payne demonstrated optical amplifiers that were built into the fiber-optic
cable itself. The all-optic system could carry 100 times more information than cable with
electronic amplifiers. Also in 1991, photonic crystal fiber was developed. This fiber guides light by
means of diffraction from a periodic structure rather then total internal reflection which allows
power to be carried more efficiently then with conventional fibers therefore improving
performance.
 The first all-optic fiber cable, TPC-5, that uses optical amplifiers was laid across the Pacific Ocean
in 1996. The following year the Fiber Optic Link Around the Globe (FLAG) became the longest
single-cable network in the world and provided the infrastructure for the next generation of
Internet applications.

FiberOpticTimeline
 1951: Light transmission through bundles of fibers- flexible fibrescope used in medical field.
 1957 : First fiber-optic Endoscope tested on a patient.
 1960 : Invention of Laser (development, T Maiman)
 1966: Charles Kao et al; proposed cladded fiber cables with lower losses as a communication medium.
 1970: (Corning Glass, NY) developed fibers with losses below 20 dB/km.
 1972: First Semiconductor diode laser working at room temp. developed
 1977: GTE in Los Angeles and AT&T in Chicago sends live telephone signals through fiber optics
(850nm,MF, 9km ) - World’s first FO link
 1980s: 2nd generation systems; 1300nm, SM, 0.5 dB/km, O-E-O
3rd generation systems; 1550nm, SM, 0.2 dB/km, EDFA, 5Gb/s
 1993 : Bell Labs sends 10 Billion bits/s through 20,000 km of fibers using a WDM systems and Soliton pulses.
 1996 : NTT, Bell Labs and Fujitsu able to send Trillion bits per second through single optical fiber.
 2000 : Towards achieving, Tb/s of data, leading to all Optical Networks

Generation of Optical Fiber
First generation - Bit rate of 45 Mb/s with repeater spacing of up to 10 km
Second generation - Developed for commercial use in the early 1980s, operated at 1.3 μm
and used InGaAsP semiconductor lasers.
 Multi mode fiber dispersion, and in 1981 the single-mode fiber was revealed to greatly
improve system performance
 By 1987, these systems were operating at bit rates of up to 1.7 Gb/s with repeater
spacing up to 50 km (31 mi).
Third generation - Fiber optic systems operated at 1.55 μm & had losses of about 0.2 dB/km
 Indium GalliumArsenide photodiode
 Overcame earlier difficulties with pulse-spreading at that wavelength using
conventional InGaAsP semiconductor lasers.
 Dispersion shifted fibers designed to have minimal dispersion at 1.55 μm
 Third generation systems to operate commercially at 2.5 Gb/s with repeater spacing in
excess of 100 km (62 mi)

Generation of Optical Fiber Contd…
Fourth generation – Optical communication systems used optical amplification to reduce
the need for repeaters and wavelength-division multiplexing to increase
data capacity
• Doubling of system capacity every six months starting in 1992 until a bit rate of 10
Tb/s was reached by 2001
• In 2006 a bit-rate of 14 Tb/s was reached over a single 160 km (99 mi) line using
optical amplifiers
Fifth generation - Optical communication extending the wavelength range over which a WDM
system can operate
• The conventional wavelength window, known as the C band, covers the wavelength
range 1.53 –1.57 μm, and dry fiber has a low-loss window promising an extension of
that range to 1.30–1.65 μm

OPTICALFIBERCOMMUNICATIONSYSTEM

Elementsof OpticalCommunicationSystem
Electronics
Optical
Transmitter
Regenerator
Optical
Receiver
Drive
Circuit
Light
Source
Fiber
flylead
Transmitter
Electrical Input signal
Connector
Optical coupler
or beam splitter
Optical
Splicer
Optical Fiber
To other
equipment
Electrical Output signal
Fiber
flyl
ead Photo
Detector
Signal
Restorer
Amplifier
Receiver
Optical
Amplifier
Electrical signal
Optical signal
Optical Fiber

Advantages of Optical Fiber Communication
1. Information bandwidth is more.
2. Optical fibers are small in size and light weighted.
3. Optical fibers are more immune to ambient electrical noise, electromagnetic
interference.
4. Cross talk and internal noise are eliminated in optical fibers.
5. There is no risk of short circuit in optical fibers.
6. Optical fibers can be used for wide range of temperature.
7. A single fiber can be used to send many signals of different wavelengths using
Wavelength Division Multiplexing (WDM).
8. Optical fibers are generally glass which is made up of sand and hence they are
cheaper than copper cables.
9. Optical fibers are having less transmission loss and hence less number of
repeaters are used.
10. Optical fibers are more reliable and easy to maintain.

1.Large Transmission Capacity
2.Low Attenuation
3.Easy Amplification
4.Low Cost

5.LightWeight
8.Greater BW
6.High Speed
7.Better Reliability
9.No Interference
10.Longer Distance

TelephoneSignals
Transmission
Internet
Communication
Detect
NuclearRadiations
CableTelevision
SignalTransmission
MedicalDiagnosis
Military
Applications
Railway
Monitoring
APPLICATIONS OF OPTICAL FIBER

Fiber Optics
Fiber optic lines are strands of glass or transparent fibers
that allows the transmission of light and digital information
over long distances.
They are used for the telephone system, the cable TV
system, the internet, medical imaging, and mechanical
engineering inspection.
Optical fibers have many advantages over copper wires
like less expensive, thinner, lightweight, and more
flexible.
They aren’t flammable since they use light signals instead
of electric signals.
Light signals from one fiber do not interfere with signals
in nearby fibers, which means clearer TV reception or
phone conversations. A fiber optic wire
spool of optical fiber

Fiber Optics
Fiber optics are often long strands of
very pure glass. They are very thin,
about the size of a human hair.
Hundreds to thousands of them are
arranged in bundles (optical cables) that
can transmit light great distances.
There are three main parts to an optical
fiber:
• Core - The thin glass center where light travels.
• Cladding - Optical material (with a lower index of refraction than the core) that
surrounds the core that reflects light back into the core.
• Buffer Coating - Plastic coating on the outside of an optical fiber to protect it
from damage.

• consists of three concentric sections
Plastic jacket Glass or Plastic
cladding
Fiber core

Optics:
• Physics of light
• Different layers of understanding/describing light
Geometric Optics:
• Light consists of rays, moves in straight line until it hits interface
• Arose in ancient Greece ~ 300BC
• Greatly developed in Persia in the middle ages
Optical laws anddefinitions

Wave Optics:
• Light is a wave phenomenon (Huygens 1690)
• New effects beyond geometric optics: interference
• Later: light is electromagnetic wave
• Unified with theory of electromagnetism by Maxwell (1860s)
Modern (Quantum) Optics:
• Light is not just a wave but at the same time consists of particles, the
photons
• Started by Planck and Einstein around 1900
• Many new phenomena, e.g., the LASER

• light consists of rays (infinitely thin beams of light)
• in vacuum or in uniform medium,
ray is a straight line
• if medium is not uniform (for example at a surface), ray
can be curved or bent
• we can see an object if rays emitted by the
object enter our eyes
(if you can see something, it must be a
source of light!)
Ray optics

Basic Definition of Frequency and Wavelength
 The radio waves and light are electromagnetic waves. The rate at which they alternate in polarity is
called their frequency(f) measured in Hz.
 The speed of electromagnetic wave in free space is approximately 3*108 m/sec.
 The distance travelled during each cycle is called as Wavelength(λ).
• In fiber optics, it is more convenient to use the
wavelength of light instead of the frequency with
light frequencies, wavelength if often stated in
micron or nanometres

RayTheory(Lawsof Optics)Transmission
 Reflection
 Refraction
 Refractive Index(n)
 Snell’s Law
 Critical Angle (φC)
 Total Internal Reflection (TIR)
 Acceptance Angle (φa)
 Numerical Aperture (NA)

• Light striking a surface may be reflected, transmitted, or absorbed
• Reflection from a smooth
surface is Specular
(mirror- like)
• Reflection from a rough
surface is Diffuse (not
mirror-like).
reflection

 
i r
=
i r
Real Important Note: The angles are measured relative to the surface normal.
• Reflected light leaves surface at the same angle it was incident on surface:
Specular reflection:

reflection
Incident Angle:
The angle between the incident
wave and the normal is called the
angle of Incidence (φ1)
Reflected Angle:
The angle between the reflected
wave and normal is called the angle
of reflection (φ2)
Law of Reflection:
The angle of incidence is equal to
the angle of reflection
Law of Reflection

• Light rays change direction (are
“refracted”) when they move from
one medium to another
• Refraction takes place because light
travels with different speeds in
different media
• The speed of light in free space is
higher than in water or glass.
Speed of light in vacuum:
C = 2.9979x108 m/s (just use 3x108 m/s)
REFRACTION

Refractive index
Based on material density, the refractive index is expressed as the ratio of the
velocity of light in free space to the velocity of light of the dielectric material
(substance). i.e
n = Index of Refraction
c = Speed of light in vacuum
v = Speed of light in medium
Note that a large index of refraction
corresponds to a relatively slow light speed in
that medium.
Medium
Vacuum
Air (STP)
Water (20º C)
Ethanol
Glass
Diamond
n
1
1.00029
1.33
1.36
~1.5
2.42
c
n =
v

Because light never travels faster than c, n  1.* For water, n = 1.33 and for
glass, n 1.5.
c
v =
n
8
3×10 m/s
v =
2.42
8
v = 1.24×10 m/s
Example: calculate the speed of light in diamond (n = 2.42).
Refractive index

3/7/20
20
optical fiber communication-
session-2
7
4

Snell’s Law
Snell’s law states that a ray of light bends in
such a way that the ratio of the sine of the
angle of incidence to the sine of the angle of
refraction is constant. Mathematically,
ni sin i = nr sin r
Here ni is the index of refraction in the original
medium and nr is the index in the medium the
light enters. i and r are the angles of incidence
and refraction respectively.
i
r
ni
nr
Wille brord Snell

Snell’s Law
Refractive Model for Snell’s Law

Snell’sLaw
 If φ1 and φ2 be the angles of incidence and angle of refraction respectively. Then
according to Snell’s law, a relationship exists between the refractive index of both
materials given by, n1 sin φ1 = n2 sin φ2
 The refracted wave will be towards the normal when n1 < n2 and will away from it
when n1 > n2
 Snell’s law states how light ray reacts when it meets the interface of two media
having different indexes of refraction
 Let the two medias have refractive indexes n1 and n2, where n1 > n2

CriticalAngle(φC)
The critical angle is defined as the minimum angle of incidence (φ1)
at which the ray strikes the interface of two media and causes an
angle of refraction (φ2) equal to 90o . It is denoted as φC.

The critical angle is the angle of incidence
that produces an angle of refraction of 90º.
If the angle of incidence exceeds the critical
angle, the ray is completely reflected and
does not enter the new medium.
A critical angle only exists when light is
attempting to penetrate a medium of higher
optical density than it is currently traveling
in.
c = sin-1
nr
ni
ni
nr
c
Since sin 90 = 1, we have n1 sinc = n2 and
the critical angle is
CriticalAngle(φC)
Hence at critical angle, φ1 = φC and φ2
= 90
Using Snell’s law : n1 sin φ1 = n2 sin φ2

Critical Angle Sample Problem
Calculate the critical angle for the diamond-air boundary.
c = sin-1 (nr / ni)
= sin-1 (1 / 2.42)
= 24.4
Any light shone on this boundary beyond
this angle will be reflected back into the
diamond.
c
air
diamond
Refer to the Index of Refraction chart for the information.

Total Internal Reflection
Total internal reflection occurs when light attempts to pass from a more optically
dense medium to a less optically dense medium at an angle greater than the
critical angle. When this occurs there is no refraction, only reflection.
n1
n2
 > c

n1
n2 >

Conditionfor TIR
Total Internal Reflection at the fiber wall can occur only if two conditions are
satisfied:
Condition 1
The index of refraction of glass fiber must be slightly greater than the index
refraction of material surrounding the fiber (cladding).
If refractive index of glass fiber = n1 and refractive index of cladding = n2 then n1 > n2
Condition 2
The angle of incidence (φi)of light ray must be greater than Critical Angle (φc)

max

max
A
B
  c for total
internal reflection
Lost by
radiation
Acceptance
cone
ACCEPTANCE ANGLE

Large diameter fiber Small diameter fiber

Fiber
axis
Core
n1
Claddin
g n2
A
B
C
θ
r
θ
i
θ
r
θ

cos.............(2)
from the right angle triangle ABC
r   900
r  90 
0
n sin  n sin(900
)
0 i 1
n0 sini  n1 cos
n0 sini  n1 sinr ..............(1)
0
1
n
n
i
sin 

1
1
2
0
1
2
0
1
n1
n
n
sin90
n
n
n
n
c
c
c
c
c
m
n 2
n 2
0
According to law of refraction
n1 sini  n2 sin r

cos  1 2
............(4)
1sin2
  1(
n2
)2
n
cos 
sin 
sin 
i c  r  90
Cos  ................(3)
sin
when   critical angle(c )
i m

2
1
max
2
1
0 1
n 2
n 2
n
n 2
n
n
substitute equation (4) in (3)
m
m
n 2
n 2
if the medium surrounding the fiber is air,then n0 1
n 2
 1 1 2
  sin1
sin 
sin

no sin1  n1
sin2
The NAdefines a cone of acceptance for light that will be
guided by the fiber

1
Air
n0
n2
n1
A 2
B
C

=90-2
> c

  2
2

1
NA  n1 2
NA  n 2
2
 
n1  n2
n1
NA  n1(n1  n2 )
n1  n2
1 2
(n1  n2 )(n1  n2 )
NA  n 2
n 2
NA
NA  sinmax
 The numerical aperture (NA) of a fiber is a figure of merit
which represents its light gathering capability.
 Larger the numerical aperture, the greater the amount
of light accepted by fiber.
 The acceptance angle also determines how much light is
able to be enter the fiber and hence there is relation
between the numerical aperture and the cone of
acceptance.
Numerical aperture (NA) = sinmax

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•
•
•
•
•
•
•

ELECTROMAGNETIC MODE THEORY
Propagation of light in optical waveguide:
• The Ray theory: to get a clear picture of the propagation of light
inside the cable.
• The Mode theory: to understand the behavior of the light inside the
cable (comprehending of the properties of absorption, attenuation
and dispersion).
• MODE: EM WAVES TRAVELS I N A WAVEGUIDE W I T H
DIFFERENT SPEED

TE,TM & TEM MODES
• Transverse Electric mode (TE): Electric field is
perpendicular to the direction of propagation,
(Ez = 0), but a corresponding component of the
magnetic field H in the direction of
propagation(Z).
• Transverse Magnetic (TM) mode: A component
of E field is in the direction of propagation(Z),
but Hz=0.
• Modes with mode numbers; TEm and TMm
• Transverse Electro Magnetic (TEM) : Total
field lies in the transverse plane both Ez and
Hz are zero.

MODE T H E O RY FOR C I R C U L A R
WAVEGUIDES
7
• To understand optical power propagation in fiber it is necessary to solve Maxwell’s
Equation subject to cylindrical boundary conditions
• When solving Maxwell’s equations for hollow metallic waveguide, only transverse
electric (TE) and transverse magnetic (TM) modes are found
• In optical fibers, the core cladding boundary conditions lead to a coupling
between electric and magnetic field components. This results in hybrid modes.
• Hybrid modes EH means (E is larger) or HE means H is larger

The equations 7 and 8 are known as standard wave equations. The modes can be found by
solving the wave equation subjected to the core - cladding boundary condition.

Modes in Cylindrical Fibers –Weakly Guided
Approximation

Linearly Polarized modes
• Fibers are constructed so that n1-n2 << 1. The field components are called linearly polarized (LP) modes
and are labeled LPjm where j and m designate mode solutions.

Single Mode fibers
• Only one mode of propagation
• Core diameter 8-12 μm and V = 2.4
• Δ varies between 0.2 and 1.0 percent
• Core diameter must be just below the cut off of the first higher order
mode
• LP01 mode alone exists
• 0 <V<2.405

 Total number of guided modes in graded index fiber is
given by
 M = (α / α + 2 ) ( n1 k a)2 Δ
Normalized frequency
V = n1 k a (2 Δ) 1/2
M = (α / α + 2 ) (V2 / 2)
For a parabolic refractive index profile α = 2
M = V2 / 4

10/7/2
020
session-3
14
9

μ
μ
10/7/2
020
session-3
15
0

λ Δ
10/7/2
020
session-3
15
1

μ
10/7/2
020
session-3
15
4

λ π √ Δ) )=
1214
λ
10/7/2
020
session-3
15
5

OPTICAL FIBER MANUFACTURING
An optical fiber is manufactured from silicon dioxide by either of two
methods.
crucible method - powdered silica is melted, produces fatter,
multimode fibers suitable for short-distance transmission of many
light wave signals.
vapor deposition process - creates a solid cylinder of core and
cladding material that is then heated and drawn into a thinner,
single-mode fiber for long-distance communication.

There are three types of vapor deposition techniques:
Outer Vapor Phase Deposition
Vapor Phase Axial Deposition
Modified Chemical Vapor Deposition (MCVD)

Vapor Phase Axial Deposition

Modified Chemical Vapor Deposition (MCVD)

Outer Vapor Phase Deposition

An optical fiber in air has an NA of 0.4. Compare the acceptance angle for meridional rays
with that for skew rays which change direction by 100° at each reflection.
Solution: The acceptance angle for meridional rays is given by
NA = n0 sin θa = (n12 − n2 2 )1/2
For Air medium n0 = 1
NA = θa = sin−1 NA = sin−1 0.4
= 23.6°
The skew rays change direction by 100° at each reflection, therefore γ = 50°.The acceptance
angle for skew rays is:
θas = sin−1 (NA/Cos γ) = sin−1 (0.4/Cos 50°)
= 38.5°
In this example, the acceptance angle for the skew rays is about 15° greater than
the corresponding angle for meridional rays
PROBLEM

Determine the cutoff wavelength for a STEP INDEX fiber to exhibit SINGLE MODE operation
when the core refractive index and radius are 1.46 and 4.5 μm with relative index difference
being 0.25%
Solution
Given n1 = 1.46, a = 4.5 and Δ = 0.25%
We know that V = 2 𝝅 a(n1
2 – n2
2)1/2 ; where V = 2.405 (Single Mode
Fiber)
λ
λ = 2πan1(2Δ)1/2
V
λ = 2 x πx4.5x1.46x(2x.25/100)1/2
2.405
= 1.214𝝁m.

Calculate the number of modes supported by a Graded Index fiber having a core radius of
25μm and operating at 820nm.The fiber has a refractive index of 1.48 at the core radius and a
cladding index of 1.46.Assume a parabolic index fiber
Solution
Given
a = 25x10-6m , λ = 820x10-9m , n1 = 1.48, n2 = 1.46
WKT, Normalized Frequency V = 2 𝝅 a(n1
2 – n2
2)1/2
λ
= 2 x π x 25x10-6 (1.482-1.462)1/2
820x10-9
= 46.45
No of Modes M = α/(α+2) x (V2/2) ; Where α = 2 for Parabolic
profile
= 2/4 x (46.45)2
= 539.42
≈ 540 modes

Exercise
(1) A GI fiber with parabolic RI profile core has a RI at the core axis of 1.5 and RI difference
Δ=1%.Estimate the maximum core diameter which allows single mode operation at λ = 1.3μm
(2) Calculate the NA, Cut off parameter and number of modes supported by a fiber having n1 =
1.54,n2 = 1.5.core radius 25μm and operating wavelength 1300nm
(3) Single mode step index fiber has core and cladding refractive indices of 1.498 and 1.495
respectively. Determine the core diameter required for the fiber to permit its operation over the
wavelength range 1.48 and 1.6μm.
(4) Silica optical fiber has a core index of 1.5 and cladding index of 1.47.Determine the
acceptance angle in water(RI is 1.33)of the fiber
(5) A step index fiber has n1 = 1.5 and n2 = 1.47 .Determine the solid acceptance angle

Optical Communication - Unit 1 - introduction

Optical Communication - Unit 1 - introduction

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