Optical Communication (REC-075)
 Overview of Optical Fiber Communication System
 General Optical System
 Advantages of Optical Fiber Communication
Optical Communication Overview:
Optical
Communication
Transmission of Information in form of
light signal through channel
Optical Fiber Communication Overview
 Invention of telegraph by Samuel F.B. Morse in 1838 laid down the foundation stone of
electrical communication.
 In Electrical systems, data is transmitted by superimposing information onto the high
frequency carrier signal (sinusoidally varying electro-magnetic wave).
 Amount of Information Proportional
Increasing Carrier frequency
frequency range over which the carrier separates
Increase in transmission bandwidth
Increase in
information
carrying capacity
 Due to this, in communication system we continuously got
moving towards high frequency ( shorter wavelength) carrier for
getting higher bandwidth or larger information capacity.
Optical Communication Spectrum
S.NO.
Frequency
Range
Wavelength
Named
as:
1.
3Hz -30 Hz
10,0000-10,000
KM
ELF
2.
30 Hz-300 Hz
10,000-1000 KM
SLF
3.
300 Hz-3 KHz
1000-100 KM
Voice Frequency
4.
3 KHz- 30 KHz
100-10KM
VLF
5.
30 KHz-300 KHz
10- 1 KM
LF
6.
300 KHz- 3 MHz
1KM- 100m
MF
7.
3 MHz-30 MHz
100m-10m
HF
8.
30 MHz- 300 MHz
10m- 1m
VHF
9.
300 MHz-3 GHz
1m-10 cm
UHF
GSM, CDMA , Mobile communication
10.
3 GHz- 30 GHz
10 cm- 1 cm
SHF
Microwave band, (1 -100 GHz)
11.
30 GHz- 300 GHz
1cm- 1mm
EHF
12.
300 GHz- 300 THz
1mm-1 micro
INFRARED
13.
300 THz – 3 PHz
1 micro-100 nm
VISIBLE
14.
3 PHz -30 PHz
100nm-10nm
UV
15.
30 PHz-300 PHz
10nm-1nm
X-RAYS
Radio Frequency spectrum (300 KHz -300
GHz)
AM spectrum ( 540 kHz to 1650 KHz)
FM Spectrum ( 88 to 108 MHz)
100µ (far infrared) -50 nm (
ultraviolet)
Optical Spectrum
Evolution of optical fiber communication

Medium of transmission used for frequencies upto 1 GHz-----Co-axial
Cable
Loss of 20
db/km
 When operating frequencies increased further the coaxial cables proved to be inadequate and
lossy, thereby giving rise to the need of another medium called waveguides.
 But at operating frequency further increased to few hundreds of gigahertz these waveguides too proved
to be inadequate as there were no supporting electronic circuitry available that could operate at such
high frequencies.
 Optical communication was considered to be a solution at such high
frequency ( 3*10^12 to 6 * 10^15) in which medium through which light
signal will propagate was considered to be a glass .
 Initially Glass showed an attenuation of 1000 db/km , but after intensive
purification techniques in
glass fabrication , scientists achieved an
attenuation of 20 db/km and also 0.01db/km after employing newer
purification techniques.
Optical Communication System
Fig. Block Diagram of Optical System
Optical Communication System ……..contd..
Transmitter
1.
2.
Channel
1.
2.
3.
Drive circuit--- converts electrical signal to the form suitable for optical
source .
Optical Source– converts electrical to an optical signal, ex: LED, LASER
Optical Fiber Cable; Single mode/ Multi mode
Repeaters--- Regenerate an optical signal by converting optical to
electrical and converting back to optical after processing , also known as
regenerators.
Optical Amplifiers—Raman Amplifier, EDFA(Erbium doped fiber
amplifier, SOA( Semiconductor optical amplifier) .
Receiver
1.
2.
Photo detector; pn junction, PIN, APD,RAPD
Signal Restorer---It amplifies and reshapes the
electrical signal before passing it to destination.
Advantages of optical communication
1. Low Transmission Loss: Optical fiber has low transmission loss compared to copper
wires which makes it suitable for transmission of more data with lesser number of
required repeaters for long distance communication.
2. Higher Bandwidth : As available transmission bandwidth is directly proportional to
carrier frequency and in optical communication carrier frequency of the order of
10^12 is being used so enormous potential transmission bandwidth is provided by
optical fiber.
3. No electromagnetic interference :As in optical fiber light signal propagates through
glass which is a dielectric , so optical fiber does not pick up electromagnetic radiation.
.
4. Small size and weight :Diameter of an optical fiber is of the
order of micrometers which is comparable to the diameter of
human hair.
5.
High Signal Security :Optical fiber does not radiate out energy due to which
highly secured transmission is achieved by an optical fiber communication.
6. Greater Safety: If the optical fiber is damaged , it does not cause any spark /fire
hazards as it does not carry electric current so are more safer then copper cables.
7. Electrical Isolation: No short circuiting issues are there with optical
communication as information is transmitted through glass which is a dielectric.
8. Low Cost: Optical fiber glass is fabricated with silica (sand) as raw material
which is available at almost free of cost , only purification of sand to obtain a low
attenuated glass makes the major costing.
9. Long Distance Communication :With attenuation lesser then .01db/km of an
optical fiber ,long distance communication is achieved with an optical fiber.
10. Higher transmission rate of information :As transmission rate is proportional to
bandwidth and optical fiber provides higher enormous potential
bandwidth.
Optical Fiber Waveguide
It consists of 3 layers:
1. Core: It is the central region of fiber and it
consists material of refractive index n1
which is greater than the refractive index of
cladding n2, Light signal is guided through
the core.
2. Cladding: It is the outer optical material
surrounds core , and its refractive index is
less than core i.e. n1>n2.
3. Buffer Coating/Plastic Jacket: It is the
plastic coating that protects the fiber , it does
not have any optical property . It provides
mechanical strength to the fiber.
Ray theory transmission in optical fiber
In Ray theory , Light is considered to be a simple ray represented by a line and we
know speed of light in free space is 3*10^8 m/s.
To analyze the propagation of light signal through optical fiber , all laws of
reflection and refraction will be exhibited in an optical fiber.
Optical characteristics of optical fiber waveguides
Refractive Index: It is defined as the ratio of velocity of light in free space to the
velocity of light in optically transparent material.
n= velocity of light in free space/ velocity of light in optical material
n= c/v where c is speed of light in free space and v is the velocity of
light in optical material.
Note: Refractive Index shows the amount of refraction at the interface of two
different mediums.
Material
Refractive
Index
Air
1
Glass
1.5
Diamond
2
Silicon
3.5
GaAs
3.7
Refraction:
According to Snell's
law, at the interface
AlGaAs
3.4
sinø1 /sinø2 =n2/n1
Higher Refractive
Index
Lesser speed of light in that medium
n1 sinø1 =n2 sinø2
According to law of
reflection
Angle of incidence = Angle of
reflection
Critical Angle : At particular angle of incidence , the angle of refraction becomes
90 degree and refracted ray becomes parallel to the plane of
incidence , that angle of incidence is known as critical angle.
From Snell's law , at the interface
n1 sinø1 =n2 sinø2
………..(i)
If ø1 = øc i.e. critical angle, ø2= 90 degree ………..(ii)
n1 sinøc = n2 sin 90 ……………….(iii)
n1 sinøc = n2 * 1
TIR
Ø1 > Ø c
Critical angle
sinøc = n2 / n1……….(iv)
Øc = inv( sin n2 / n1 )
Signal gets reflected back completely in the same
medium and refracted signal becomes zero , this is
known as total internal reflection (TIR).
Acceptance Angle (θa)
Acceptance angle (θa) is the
maximum angle that a light
signal can make with core axis at
the entering point of an optical
fiber so that signal can be
propagated down the length
with the process of total internal
reflection inside the core of an
optical fiber .
cladding
Core
cladding
If θ > θa , where θ is the angle of optical signal
(clockwise or anticlockwise) with respect to core axis
at the entering point O,
Then, angle with respect to normal made by the refracted ray at the
core cladding interface will be lesser than critical angle and the
signal will get refracted into the cladding or we can say that signal
will not be guided inside the core of an optical fiber.
Relationship of acceptance angle with refractive indices of outer
medium, core and cladding
n2
A
no
C
B
n1
In this, we have considered that light is entering
into the fiber core at point A from a medium having
cladding refractive index no.
θ1 is the angle made by entering beam with respect
to core axis, θ2 is the refracted angle at point A and ø
core
is the angle made by light signal at core-cladding
interface; B with respect to normal.
cladding
Now , at point A, using Snell's law
no sinθ1 = n1 sinθ2 …………..(i)
Now in triangle ACB,
Π/2 + ø + θ2 = Π …………… .(ii)
θ2 = Π/2 – ø …………………………(iii)
Relationship of acceptance angle with refractive indices of outer
medium, core and cladding ………………………contd..
n2
A
no
C
Using (iii) in (i) we get,
n1
B
…………………..(iv)
we get,
…………………..(v)
Now , in (v) we shall use the limiting condition of propagation i.e θ1 = θa and ø = øc ,
and sinøc = n2 / n1 ( from critical angle relation)
So we get,
Required Relation .
Numerical Aperture of an optical fiber (NA)
Light collecting ability of an optical fiber.
Now, we have
so in the relation,
; Relative refractive index difference , which is given by:
we will use relative refractive index difference
And , the final relation will come out to be
=
Phase Velocity and Group Velocity
Phase Velocity (vp) of a wave is the rate at which the wave propagates in some
medium. It is the rate at which points of constant phase in the wave travels in
space .
Rate at which envelope of
wave comprises of two or
more frequencies
propagates is known as
group velocity ,vg.
Rate at which constant
phase point in wave
travels is phase velocity.
Vp= ω/β………..(i)
Where, ω= angular frequency of wave and β
is the propagation constant in the given
medium .
Vg=dω/d β………...(ii)
Propagation constant
Group Velocity is the rate at which
envelope propagates in a medium.
β in a medium of
refractive index n1
is given by 2Π n1 /λ
Phase Velocity and Group Velocity
β = 2Π n1 /λ
…………..(ii)
We know that f=c/ λ
2Πf = ω = 2Π c/ λ => β = ω n1/ c
vp= ω/β= c/ n1
contd……..
………….(iii)
…………………….(iv)
Now, Group Velocity = vg=dω/d β= dω / d λ * dλ / dβ ……….(v)
dω / d λ = - ω/ λ and dλ / dβ = inverse of (dβ/ dλ )
…….(vi)
……..(vii)
vg
Required Relation of Group Velocity
in terms of c, n1 and λ , Ng is group
index of waveguide
Meridional and Skew Ray Propagation
Meridional Ray Propagation: In Meridional Ray Propagation after each reflection
from core cladding interface the light signal passes through core axis.
Skew Ray Propagation: In skew Ray Propagation , light signal moves down the
length of an optical fiber in form of helical path enveloping the core axis but
never pass through the core- axis .
Numerical aperture in
Meridional ray
propagation
(c)
no sinθa cosƳ = Numerical aperture
in skew ray propagation
Fig. a and b Propagation of skew rays , fig. c
propagation of Meridional rays
Materials Used for Fabrication of Optical Fiber
For Fabrication of optical fiber material must satisfy certain requirements:
(i) It must be possible to obtain long thin and flexible fibers from the material.
(ii) Physically compatible materials that have slightly different refractive indices for the core
and cladding must be available .
(iii) The Material must be transparent at a particular optical wavelength in order for the
fiber to guide light efficiently.

Glass Fibers: Glass Fibers are made of fusing mixtures of metal oxides,
sulphides , and selenides.
 Metal Oxide SiO2 i.e Silica is prominently used to fabricate glass of an
optical fiber .
 To obtain compatible material for core and cladding certain dopants are added
to SiO2.
Materials Used for Fabrication of Optical Fiber
Dopants GeO2 and P2O5 are used to increase the refractive index of SiO2 to obtain core
and B2O3 is used to decrease the refractive index of SiO2 to obtain cladding .
There are certain combinations of material used for fabrication of core and cladding glass as
follows:
Table of materials used for fabrication of oxide glass fibers
Glass composed of pure
silica is known as silica
glass, fused silica or
vitreous silica.
S.NO
CORE MATERIAL
CLADDING MATERIAL
1.
GeO2- SiO2
SiO2
2.
SiO2-P2O5
SiO2
3.
SiO2
SiO2-B2O3
4.
GeO2-SiO2-B2O3
SiO2-B2O3
Materials Used for Fabrication of Optical Fiber
Glass composed of pure silica is resistant to deformation at temperature as high as 1000
degree centigrades but its high melting point is the disadvantage if glass is prepared from
molten state.
Halide Glass: In 1975 ,researchers at the university of Rennes discovered Fluoride glasses
that have extremely low transmission losses at mid infrared wavelength
( 0.2-0.8 µm). Fluorides belongs to the category of halides in which anions are from the
elements in group VII of the periodic table namely Fluorine, chlorine, bromine.
In Halide Glasses material used to compose the core of fiber are ZrF4
,BaF2 ,LaF3, AlF3, and NaF and we partially replace ZrF4 by HaF4 to
obtain the material for cladding .
These glasses offers intrinsic minimum losses of .01dB/km or .001dB/km
but these glasses are prone to divitrification.
Materials Used for Fabrication of Optical Fiber
Active Glass fibers : Incorporating rare earth elements ,atomic number 57-71 into
normally passive glass gives the resulting material new optical and properties, these new
properties allows the materials to perform amplification , attenuation and phase retardation
on the light passing through it.
Two materials used for active glass fibers are erbium and neodymium.
Plastic optical Fibers (POF) : Plastic optical fibers serves the purpose in high speed
data transmission.
The core of this fiber is usually polymethylmethacrylate or a per fluorinated fiber .These
fibers are known as PMMA POF and PFP POF.
The attenuation of these fibers is more compared to glass fibers but these are tough and durable.
Compared to glass fibers , diameter of plastic fibers is 10 to 20 times
greater which offers relaxation of connectors tolerances and inexpensive
plastic injection technologies can be used to fabricate the connectors for
these fibers.
Active Glass fibers : Incorporating rare earth elements ,atomic number 57-71 into
normally passive glass gives the resulting material new optical and properties, these new
properties allows the materials to perform amplification , attenuation and phase retardation
on the light passing through it.
Two materials used for active glass fibers are erbium and neodymium.
Plastic optical Fibers (POF) : Plastic optical fibers serves the purpose in high speed
data transmission.
The core of this fiber is usually polymethylmethacrylate or a per fluorinated fiber .These
fibers are known as PMMA POF and PFP POF.
The attenuation of these fibers is more compared to glass fibers but these are tough and durable.
Compared to glass fibers , diameter of plastic fibers is 10 to 20 times
greater which offers relaxation of connectors tolerances and inexpensive
plastic injection technologies can be used to fabricate the connectors for
these fibers.
Modes in an Optical Fiber
Mode represents the field pattern of the propagating wave .
Free space modes : Plane waves in which the electric and magnetic fields are both
orthogonal to the direction of propagation.
Modes in waveguides and transmission lines: Transverse modes, modes that have at east
one of the electric and magnetic field entirely in transverse direction.
Transverse electromagnetic mode ( TEM), as with a free space plane wave , both electric
and magnetic field are entirely transverse. (EZ =0 , HZ=0)
Transverse Electric (TE) modes, only the electric field is entirely
transverse ; (EZ =0) but (HZ will not be zero).
Transverse Magnetic ( TM) mode, only the magnetic field is entirely
transverse ; HZ = 0 but Ez will not be zero.
Modes in an Optical Fiber
Order of a mode: Order of a mode is the number of field zeros across the guide.
Field pattern shows that
the electric field of
guided modes is not
completely confined to
the centre of fiber instead
they extend partially into
the cladding
Lower
order
modes are guided
nearby core axis
but higher order
modes
are
propagated near to
the core cladding
interface.
Fig. Electric field pattern of zeroth, first and second order mode in an
optical fiber
Modes in an Optical Fiber
Types of mode in an optical fiber:
(a) Guided Mode :These modes are guided well inside the core of an optical fiber
for these modes θ1( angle which light signal makes with respect to core axis at the
entering point ) < Acceptance angle (θa); θ1< θa.
Note: A mode remains guided if n2k<β< n1k .
(a) Radiant mode or Refracted Mode :for these modes θ1( angle which light signal makes
with respect to core axis at the entering point ) > Acceptance angle (θa) and optical
energy gets refracted into the cladding if light energy follows the field pattern of
radiant mode;θ1> θa.
(c) Leaky Mode : These modes are partially confined to the core region
and are attenuated by continuously radiating power out into the cladding
as these modes propagate along the fiber, this power radiation out of the
waveguide results from a quantum mechanical phenomenon known as
tunnel effect.