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Dar es Salaam institute of Technology (DIT)

ETT 06208/ETD 602
Communication Systems
Ally, J
jumannea@gmail
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With optical fibers, electromagnetic waves are guided
through a media composed of a transparent material
without using electrical current flow
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DIT

An optical communication systems
An optical communication systems is one that uses light as
the carrier of information
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Optical fiber cables have, for all practical purposes, an
infinite bandwidth
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Light frequencies used in optical fiber communication
systems are between 1x1014Hz and 4x1014Hz
(100,000GHz to 400,000GHz)
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DIT

Transmission Media

Twisted Pair

Coaxial Cable

­10 megabit/sec (Mb/s)
1­10 kilometers

­100 megabit/sec
1­10 kilometers
Optical fiber

Digital Microwave Radio
­100 megabit/sec
10­100 kilometers

DIT

Fiber Optic
Transmission Systems
Multimode: ­100 megabit/sec
Singlemode: 10,000+ megabit/sec
10­100­10,000 kilometers

Optical Fiber Cables

DIT

Optical Fibers
Fibers of glass
usually 120 micrometers in diameter
Used to carry signals in the form of light over
distances up to 50 km
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DIT

Parts of Optical Fibers
Core–thin glass center of the fiber where light travels
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Buffer Coating–plastic coating that protects the fiber
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DIT

Disadvantages of Optical Fiber
Interfacing costs
Lower tensile strength
Needs more expensive optical transmitters and
receivers
More difficult and expensive to splice than the
metallic wires
More qualified and experienced persons are
required for maintenance and handling
Optical fiber cables are more susceptible to losses
introduced by bending the cable
Need specialized tools, equipment and training
DIT

Areas of Application
Telecommunications
Local Area Networks
Cable TV
CCTV
Optical Fiber Sensors
DIT

Optical Fiber Architecture

DIT

Transmitter (from E to O)
Transmitter use Laser / LED as signal source
Optical fiber transmission is in the infra­red regions

­ Wavelength longer than visible light around 0
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3dB/km

Attenuated by 7% /km

Diameter: (Example) 2
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6dB/km

Attenuated by 99
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4 mm, Weight: 1,110 kg/km

DIT

Long-distance and Capacity (2/3)
Capacity (Bandwidth): 800nm to 1700nm at attenuation < 3dB/km
Frequency(Hz):f, Light speed (m/sec) :C0, Wavelength:λ (m)
λ

f=

C0

, (C0= 3
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00x108

λ= 800nm

800x10­9
3
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5 microns in diameter) and transmit infra-red
light from LED
MMF

SMF

Clad

Core

DIT

Fiber Types(2)
Multi-mode fibers are further categorized as:
Multi­mode step­index fiber
Discussed in previous slide, has constant refractive index
throughout the core
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This theory is known as Planck’s Law, which states, “When
visible light or high frequency electromagnetic radiation
illuminates a metallic surface, electrons are emitted
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Planck’s Law is expressed mathematically as Ep = hf
where Ep = energy of the photon
h = Planck’s constant = 6
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Refractive index is a ratio of the speed of light in a material
to the speed of light in free space, it does not have any
units
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The angle of refraction can be
larger or smaller than the angle of
incidence, depending on the
refractive indexes of the two
materials
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If the angle of refraction is 900 or greater, the light ray is not allowed to
penetrate the less dense material
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Critical angle represent mathematically as

n2
sin θ 1 =
sin θ 2
n1
with θ2=900, θ1 becomes the
Critical angle (θc), and

n2
n2
−1 n2 
sinθc = (1) ⇒sinθc = ⇒θc = sin  
n 
n1
n1
 1
DIT

Example
A light ray is traveling in a transparent material of
refractive index 1
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46
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Solution:
Using the formula for the critical angle just derived:
Put in the values of the refractive indices:
Divide the two numbers:
So:
DIT

How Does Optical Fiber Transmit Light?
Using principle of Total Internal Reflection
Fiber Optics Relay Systems has
-Transmitter
-Optical Fiber
-Optical Regenerator
-Optical Receiver
DIT

Total Internal Reflection
It is an optical phenomena that occurs when a ray of light strikes a
medium boundary at an angle larger than the critical angle with
respect to the normal to the surface
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However, if the light approaches the boundary at an angle greater
than the critical angle, the light is actually reflected from the boundary
region back into the first material
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This effect is called total internal reflection (TIR)
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DIT

Total Internal Reflection(2)

DIT

Testing of Optical Fiber
Tensile Strength
Refractive Index Profile
Fiber Geometry
Information Carrying Capacity
Operating temperature/humidity range
Ability to conduct light under water
Attenuation
DIT

Optical Fiber System Link Budget
As with any communication system, optical fiber systems
consists of a source and a destination that are separated
by numerous components and devices that introduce
various amount of loss or gain to the signal as it
propagates through the system
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A repeaterless system consists of a light source, such as
an LED or ILD, and a light detector, such as an APD
connected by optical fiber and connectors
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DIT

Optical Fiber Communication Systems
Without repeater

With repeater

DIT

Main Elements in Photonic Layer
λ1
λ2 ･･･

λn

WDM Terminal

Σλ
Wideband Optical Amplifier

Σλ
λ1
･･･λ2
λn
Σλ

Σλ

Optical Add/Drop Multiplexer
(OADM)

Σλ

Σλ

λi

λj

Wavelength MUX/DEMUX

Simultaneous amplification of
WDM signal
Signal Add/Drop
Transmission capacity control

λi

Optical Cross­Connect
(OXC)

Network restoration
Network management

λi
λj

All optical regeneration
Wavelength conversion

λi

Optical Processing

λi
DIT

Basic Network Configurations
(Two­Fiber) Ring Network

(Full) Mesh Network

Star Network

Tree Network

Main
Main

DIT

Main

Thanks!

Technology changes but communication lasts

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