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OPTICAL FIBRE COMMUNICATION
2011 AUGUST
Pcm equipment
Pcm equipment(2) contd
Global Capacity Trend
WORLD INTERNET USAGE AND POPULATION STATISTICS
World Regions
Population
(Est 2007)
Million
Africa
Populatio
n % of
the World
Internet
Usage,
Latest Data
(Million)
% Population
( Penetration )
Usage %
of World
Usage
Growth
20002007
933
14.2%
34
3.6%
2.9%
643.1%
3,713
56.5%
437
11.8%
37.2%
282.1%
Europe
810
12.3%
322
39.8%
27.4%
206.2%
Middle East
193
2.9%
20
10.1%
1.7%
494.8%
North America
335
5.1%
233
69.5%
19.8%
115.2%
Latin America/Caribbean
557
8.5%
110
19.8%
9.4%
508.6%
34
0.5%
19
54.5%
1.6%
146.7%
6,575
100.00%
1,173
17.84%
100.00%
225.00%
Asia
Oceania / Australia
WORLD TOTAL
Source: www.internetworldstats.com
Current Estimated Capacities between
Continents
Europe
810 Million
North
America
6Tbps
335 Million
Middle East
193 Million
Asia
6Tbps
3.7 Billion
Africa
Latin America/
933 Million
Oceania/
Caribbean
Australia
557 Million
34 Million
Internet Usage by World Region
EXPLOSIVE GROWTH OF INTERNET
SEA-ME-WE 4 Cable System Configuration
Diagram
FTTH, PON(passive Optical
Network)
Power Line Communication System
LIGHT
Light is an is electromagnetic radiation of a
wavelength that is visible to the human eye
CHARACTERISTICS OF LIGHT
• Light is made up of , either electromagnetic
wave or particles called photons. Light can be
considered as rays that follows straight line
between or within optical elements, bending
only at surfaces.
• Light is composed of electrical & magnetic
fields, which vary in amplitude as they move
through space together at the speed of the
light. The two fields are perpendicular to each
other and to the direction on which the light
travels.
MEDIUM
Medium is a material substance which can
transmit energy waves.
E.g. Air, Liquid, Solid
Air
Liqui
d
Soli
d
What is Optical Fibre?
An optical fibre consists 3 different parts.
– Core
– Cladding
– Buffer Coating
Basic Theory :
Light has to be confined to the core so that the
digital signal can be transmitted from one place
to another through light.
Basic Elements of the Optical Fibre System :
ELECTRICITY
LIGHT
Transmitter
LED/Laser
Light
Source
Light
Source
ELECTRICITY
Receiver
Glass Fibre
Avalanche
Photo Diode
Basic Theorems behind Optical Fibre (a)
• Refractive Index
Light changes its speed when it travels from one material to another, such as from air into glass.
This cause an effect called refraction. Hence bending of the light at the surface of a material is
expected. The speed of the light in the vacuum is highest.
• Snell’s Law
Snell's law states that the ratio of the sines (Sin) of the angles of incidence and
refraction is equivalent to the ratio of velocities in the two media, or equivalent to the
opposite ratio of the indices of refraction.
Medium 1
n1
Ө1
Medium 2
n2
Ө2
n1< n2
Sin Ө 1
Sin Ө 2
n2
n1
Sin Ө 1
n2
Sin Ө 2
n1
n1 SinӨ1 = n2 SinӨ2
Basic Theorems behind Optical Fibre (b)
• Critical Angle & Total Internal Reflection
Medium 2
n2
Φ2
Medium 1
n1
ΦC
• According to the
increase of the angle of
incidence, the angle of
refraction increases.
Φ1
Total Internal Reflection
• When the refraction angle reaches 90°, there is no refraction and the
incident angle reaches its critical angle (ΦC)
• When incident angle reaches the critical angle, there is no refraction.
• Beyond the critical angle, light ray becomes totally internally reflected .
Special points to be considered in optical fibre
(a)
1. Numerical Aperture (NA)
– NA is defined as the sine of half the angle of a fibre’s light
acceptance cone (see Figure).
– All modes of light entering the fiber at angles less than that which
correspond to the NA, will be bound or confined to the core of the
fiber.
– The larger the NA of a fiber, the larger the light acceptance cone.
Numerical Aperture consideration
Cladding n2
90 - ΦC
ΦC
Air n0 = 1
αmax
Core n1
From Snell’s Law :
n0Sinαmax = n1Sin(90 - ΦC)
n1Sin ΦC = n2Sin90
Numerical Aperture Calculation
n0Sinαmax = n1Sin(90 - ΦC)
n1Sin ΦC = n2Sin90
What is Acceptance Angle?
NA determines the light
gathering
capabilities of the fibre
Therefore,
nosinαmax = NA
Fibre accepting angle:
Question 1
A silica optical fibre with a core diameter large enough
to be considered by ray theory analysis has a core
refractive index of 1.50 and a cladding refractive index
of 1.47
Determine:
(a) the critical angle at the core cladding interface;
(b) the NA for the fibre;
(c) the acceptance angle in air for the fibre
Solution:
(a) The critical angle at the core-cladding interface
(b) The numerical aperture is:
(c) The acceptance angle in air θa is:
E.g. The speed of light in water V(l) is 2.24 x 10 8 ms -1
and the speed of light in a vacuum V(V) is 2.99 x 10 8 ms 1. What is the Refractive index of Water(n )?
w
V(l) = 2.24 x 10 8 ms -1
V(V) = 2.99 x 10 8 ms -1
Refractive index of water = V(V)
V(l)
= 2.99 x 10 8 ms -1
2.24 x 10 8 ms -1
= 1.33
So The Refractive Index of water is 1.33 (No units).
What is Snell’s Law?
• This describes the bending of light rays when it
travels from one medium to another.
Air
Glass
Water
Air
Snell's law states that the ratio of the sines (Sin) of the angles of
incidence and refraction is equivalent to the ratio of velocities in the
two media, or equivalent to the opposite ratio of the indices of
refraction.
Sin Ө 1
n2
Sin Ө 1
=
Sin Ө 2
n2
=
n1
Sin Ө 2
n1
n 1 Sin Ө 1 = n 2 Sin Ө 2
PO - Ray of Incidence
medium 1
OQ - Ray of Refraction
medium 2
Ө 1 - Angle of Incidence
Ө 2 - Angle of Refraction
n 1 - RI for
n 2 - RI for
TOTAL INTERNAL REFLECTION
n 1 Sin Ө 1 = n 2 Sin Ө 2
With the increase of the angle of
incidence, the angle of
refraction increases accordingly.
When reaches φ2 90°, there is no
refraction and φ1 reaches a
critical angle (φc )
Beyond the critical angle, light ray
becomes
totally
internally
reflected
EQUATIONS ASSOCIATED WITH RAY
PROPAGATION
• CRITICAL ANGLE
• NUMERICAL APERTURE
LIGHT GATHERING CAPACITY
An optical fibre will pick up light from any source. However collecting
light for small core fibres will be a important step towards
communication fibres. This means collecting light from one source and
transferring that light to the optical fibre. This demands COUPLING light
from core to the fibre, in a efficient way. Larger light sources are
generally easy to align with fibres, but their lower intensity generally
delivers less light. Transferring light between fibres requires careful
alignment and tight tolerance. When two fibres’ are permanently joined
are called SPLICING. Temporary joints made by two fibres are called as
CONNECTORS. Special device named COUPLERS are needed to join 3 or
more fibres. Losses in transferring signals in copper can be neglected
but it is not so with fibres. Hence it should account for the losses
deriving from coupling , from connectors, splices and the efficiency of
the light source into the fibre.
NUMERICAL APPERATURE
• LIGHT GATHERING CAPACITY OF A OPTICAL
FIBRE CABLE IS DEFINED AS THE NUMERICAL
APPERTURE
• THE ACCEPTANCE ANGLE IS THE ANGLE
WHERE THE OF SOURCE ISINTRODUCED TO
THE FIBRE
• ACCEPTANCE ANGLE
Optical Fibre
• An optical fibre consists of two parts  the core and the
cladding
• The core is a narrow cylindrical strand of glass and the cladding
is a tubular jacket surrounding it
• The core has a (slightly) higher refractive index than the cladding
Therefore, total Reflection of light

ncore > ncladding
OPTICAL FIBRE FREQUENCIES
• Question 1: A silica optical fibre with a core
diameter large enough to be considered by ray
theory analysis has a core refractive index of
1.50 and a cladding refractive index of 1.47
• Determine:
(a) The critical angle at the core cladding
interface;
(b) The NA for the fibre
(c) The acceptance angle in air for the fibre
This means the
angle over which
the light rays
entering the fibre
, to be
guided along it’s
core. Acceptance
angle is
measured in air
outside the fibre
,it defers from
confinement
angle of the
MODES OF FIBRE
• There are 2 main type of modes in optical
Fibre
• SINGLE MODE
STEP INDEX
• MULTIMODE
STEP INDEX
GRADED INDEX
TYPES OF FIBRE
Elements of Optical Transmission
System
Optical Fibre Transmission System
Major components
1. Modulator
2. Light source
3. Connectors (Couplers)
4. Optical glass Fibre
5. Light sensor/Detector
6. Optical amplifiers/Repeaters
7. Optical fibre joints (splices)
LIMITATIONS TO TRANSMISSION
ATTENUATION
Example
• To calculate the ratio of 1 kW (one kilowatt, or 1000 watts) to 1 W in
decibels, use the formula
• Similarly for amplitude, current or voltage (power is proportional to the
square of the above 3 quantities. )
Laser Output Power, Receiver
Sensitivity and dBm
Example 1
Answer (Example 1)
Transmitter
•
•
•
•
•
8 Connectors
Receiver
Connector loss= 8*1dB= 8dB
Cable loss= (4*100)/1000=0.4dB
System margin = 5dB
Sensitivity= -30 dB
Transmitter Power = connector loss+cable
loss+system margin+sensitivity
• Therefore, 8 + 0.4 + 5 – 30 = -16.6dB
Example 2
Answer (Example 2)
Transmitter
•
•
•
•
•
2 Connectors
Receiver
Connector loss= 2*1.5dB = 3 dB
Cable loss= 0.4dB * 50 = 20 dB
System margin = 8 dB
Sensitivity= -34 dB
Transmitter Power = connector loss+ cable loss + system
margin + sensitivity
• = 3+20+8-34= -3 dB
• No: of splices= 3/ 0.15 = 20 splices
Answer (Example 2)
Transmitter
•
•
•
•
•
2 Connectors
Receiver
Connector loss= 2*1.5dB = 3 dB
Cable loss= 0.4dB * 50 = 20 dB
System margin = 8 dB
Sensitivity= -34 dB
Transmitter Power = connector loss+ cable loss + system
margin + sensitivity
• = 3+20+8-34= -3 dB
• No: of splices= 3/ 0.15 = 20 splices
Question
If the System Margin is -10dB, calculate the receiver
sensitivity.
Give your overall observation?
Answer
Question on Practical system – 1
Long haul telephone optical fibre system consist with the
following components
Part One Question Contd
Calculate the Power Budget
and estimate whether the
system will Function?
Part 1
Answer
Part Two Question – Use of Optical Amplifiers
The Previous system has been re engineered with two optical
amplifiers as detail out below
Verify the Power budget for correct
operation?
Part Two Answer – Use of Optical Amplifiers
Part Two Answer Contd – Use of Optical Amplifiers
Part Example
Two Answer
Contd– Use of Optical Amplifiers
3 (f)
Part Three Questions – Onsite Re engineering Problems
Due to site and optical amplifier operational problem,
the above system was again re engineered as follows
By calculating the Power Budget verify
whether the system can be operational or
not?
Part Three Answer – Onsite Re engineering Problems
Then the Calculation for segments 2 and 3 are given below;
Part Three Answer Contd– Onsite Re engineering Problems
DISPERSION
• Data carried in an optical fibre consists of pulses of light energy
composed of a large number of frequencies travelling at a
given rate.
• There is a limit to the highest data rate (frequency) that can be
sent down a fibre and be expected to emerge intact at the
output.
• This is because of a phenomenon known as Dispersion (pulse
spreading), which limits the "Bandwidth” of the fibre.
Consequences of Dispersion
• Frequency Limitation
• Distance : A given length of fibre, has a
maximum frequency(bandwidth) which can be
sent along it. To increase the bandwidth for
the same type of fibre one needs to decrease
the length of the fibre.
TYPES OF DISPERSION
DISPERSION
CHROMATIC
DISPERSION
MATERIAL
DISPERSION
WAVEGUIDE
DISPERSION
MODAL
DISPERSION
CHROMATIC DISPERSION
• It is a result of group velocity being a function of
wavelength. In any given mode different spectral
components of a pulse travelling through the
fibre at different speed.
• It depends on the light source spectral
characteristics.
• May occur in all fibre, but is the dominant in
single mode fibre
CHROSMATIC DISPERSION
Material dispersion different wavelengths => different speeds
Waveguide dispersion –
different wavelengths => different angles
MATERIAL DISPERSION
• Refractive index of silica is frequency dependent.
Thus different frequency (wavelength) components
travel at different speed
M is the material
dispersion parameter. It
characterizes the amount
of pulse broadening by
material dispersion per
unit length of fibre and
per unit of spectral width.
WAVEGUIDE DISPERSION
• This results from variation of the group velocity with
wavelength for a particular mode. Depends on the size
of the fibre.
• The angle between the ray and the fibre axis varying
with wavelength which subsequently leads to a
variation in the transmission times for the rays, and
hence dispersion.
• This can usually be ignored in multimode fibres, since it
is very small compared with material dispersion. It is
significant in monomode fibres.
• Changing the design of the core-cladding interface can
alter waveguide dispersion.
MODAL DISPERSION - SIMMF
• Lower order modes travel travelling almost
parallel to the centre line of the fibre cover the
shortest distance, thus reaching the end of fibre
sooner.
• The higher order modes (more zig-zag rays) take
a longer route as they pass along the fibre and so
reach the end of the fibre later.
• Mainly in multimode fibres
The time taken for ray 1 to propagate a length of fibre L
gives the minimum delay time:
The time taken for the ray to propagate a length of fibre L
gives the maximum delay time:
Since
The delay difference :
MODAL DISPERSION - GIMMF
• The rays follow smooth curves rather than the
zig-zags of step-index fibres
Ray paths in a gradedindex fibre
(a) a central ray;
(b) a meridional ray
(c) a helical ray avoiding
the centre
MODAL DISPERSION - GIMMF
• The intermodal dispersion is smaller than in
step-index fibres.
• A helical ray, for example, although traversing
a much longer path than the central ray, does
so in a region where the refractive index is less
and hence the velocity greater
• To a certain extent the effects of these two
factors can be made to cancel out, resulting in
very similar propagation velocities down the
fibres for the two types of ray
EQUATIONS FOR SINGLE MODE
• Number of modes
• Diameter
• Cut off wavelength
• For single-mode transmission: λ > λc
• If λ < λc, two or more modes propagate
(multimode fibers)
EQUATIONS FOR SINGLE MODE
• Number of modes
• Diameter
• Cut off wavelength
• For single-mode transmission: λ > λc
• If λ < λc, two or more modes propagate
(multimode fibers)
Wave division multiplexing
• 1. Concepts
• 2.How to multiply the capacity in a given
optical fibre core by adding electronics at the
terminal equipement without installation of
new optical fibre systems
• Hence cost saving is evident in WDM or dense
WDM (DWDM)
• Let’s study !!
Attenuation in Fibre
optical fibre behaves differently for different wavelength of light. The following diagram
shows that. The three windows of wavelengths where the attenuation is lower is given
below. Hence these 3 windows are mostly used for practical purposes.
1. General Observation on Attenuation
and the Present Day Technology
• Attenuation is low between 1500nm-1700nm in wavelength.
• This gives rise to operate 24Tbps speed
• How?
C=fλ where C=3*108
• And f1-f2=[c/(1500nm)]-[c/1700nm]=24Tbps
• The present day technology goes up to 10Gbps or 40Gbps.
• STM1  STM4  STM16  STM64…… STM256
155.52Mbps
6.4ns
620Mbps
2.5Gbps
10Gbps
40Gbps
1.6ns
400ps
100ps
25ps
Present day technology adapting to
the optical fibre
The following 2 major factors play a vital role in designing the maximum
capacity of an optical fibre
• How far the digital multiplexing can be achieved
• As at present , 488ns micro information of a bit pertaining to 2Mbps PCM stream will be
reduced to 25ps when it goes through STM64 (10Gbps). If the technology improves to shrink
less than 25ps , then the number of bits in the higher order PCM will be more than 10Gbps.
•To transmit 10Gbps, the optical fibre requires a bandwidth of around
0.078ns = 78ps ( for 1 wavelength)
•If the available bandwidth in the optical fibre is 200ns , the number of
wavelengths that can be produced is around 2400 , which will result in
producing a total of 24Tbps.
•Hence both Time Division Multiplexing and Dense Wave Division
Multiplexing can further improve the traffic carrying capacity of an optical
fibre up to a total of 24Tbps.
Optical Fibre
Optical Fibre
Overview of WDM
Traditional Digital Fiber Optic Transport
Single Pair of Fibers
Digital Transceiver
Digital Transceiver
Single Pair of Fibers
Digital Transceiver
Digital Transceiver
Single Pair of Fibers
Digital Transceiver
Digital Transceiver
Single Pair of Fibers
Digital Transceiver
Digital Transceiver
Digital Fiber Optic Transport using WDM
WDM MUX
WDM MUX
Digital Transceiver
Digital Transceiver
Digital Transceiver
Single Pair of Fibers
Digital Transceiver
Digital Transceiver
Digital Transceiver
Digital Transceiver
Digital Transceiver
96
Future Scenarios
Theoretical Maximum of an Optical Fibre Cable
488ns
100 ps
Transponders
λ1
1
10Gbps
2
λ2
2399
λ2399
TDM
2Mbps
2400
Optical Fibre
Only 1 core is needed
λ2400
Number of wavelengths = ( 24 * 103 Gb ) / 10 Gb
= 2400 wavelengths
Optical tools for maintanance
• OTDR
• Splicing machine
Optical Time Domain Reflectometry
Principle (OTDR)
Fusion splicing
• It is the process of fusing or welding two fibers together usually by an
electric arc. Fusion splicing is the most widely used method of splicing as it
provides for the lowest loss and least reflectance, as well as providing the
strongest and most reliable joint between two fibers.
• Virtually all singlemode splices are fusion.
• Fusion splicing may be done one fiber at a time or a complete fiber ribbon
from ribbon cable at one time. First we'll look at single fiber splicing and
then ribbon splicing.