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UNIT 1
OVERVIEW OF OPTICAL FIBRE
COMMUNICATION
CONTENTS:
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Basics of Communication
Electromagnetic Spectrum
Elements of Optical Fiber Communication System
Advantages of OFC.
Basics of Communication
• Communication:
Transfer of information from one point to another over a distance
• Modulation :
Long distance communication is achieved by a process called modulation.
• Information is superimposed on an electromagnetic wave called as CARRIER for the
information signal.
• MODULATING Signal : Low frequency signal
• CARRIER SIGNAL : High frequency signal.
Selection of Electromagnetic carrier wave
• Radio Frequency spectrum (3kHz to 1 GHz)
• Microwave spectrum (1 GHz to 100 GHz)
• Optical frequency spectrum (THZ )
Radio Frequency Wave
• Electromagnetic waves ranging in frequencies between 3 KHz and 1 GHz are normally
called radio waves.
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Radio waves are normally “omni directional.”
When an antenna transmits radio waves, they are propagated in all directions.
This means that the sending and receiving antennas do not have to be aligned.
The Omni directional characteristics of radio waves make them useful for multicasting, in
which there is one sender but many receivers.
• Our AM and FM radio stations, cordless phones and televisions are examples of
multicasting.
Disadvantages:
• Radio waves transmitted by one antenna are susceptible to interference by
another antenna that is sending signals of the same frequency or band.
Microwave:
Continued……
• Microwaves are especially suitable for this use
• more easily focused into narrower beams than radio waves,
• their comparatively higher frequencies allow broad bandwidth
• high data transmission rates,
• Antenna sizes are smaller than at lower frequencies because antenna size
is inversely proportional to transmitted frequency.
Continued…..
• Microwaves are used in
• TV, and telephone communications are transmitted long distances by
microwaves between ground stations and communications satellites.
• Microwaves are used radar technology.
• Microwaves are used in Satellite communication
• Microwaves are also employed in microwave ovens
Limitations of Microwave:
• Above 40 GHz
• the atmosphere becomes opaque to microwaves
• Due to absorption from water vapor oxygen.
• Above 100 GHz, the absorption of electromagnetic radiation by Earth's
atmosphere is so great that it is in effect opaque.
• The atmosphere becomes transparent again in the so-called infrared and
optical window frequency ranges.
Transmission Lines:
• Depending on frequency and applications:
Different cables are used:
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Two wire line : Flat cable
Coaxial cables
Microwave waveguides
Optical Fibers
Different Transmission Lines and its
Limitations:
• Twisted Pair Cable:
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Limited Frequency Spectrum (1MHz)
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Limited data rate
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100 meter cable carry 100Mbps
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5.5km cable carry 2Mbps or less
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Short distance between repeaters
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High error rate
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Susceptible to signal interferences such as EMI.
• Advantages:
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High availability
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Low Cost of Installation
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Low cost of Add-ons , local moves.
Coaxial Cables:
• Upper frequency limit : 4GHz
• Higher the frequency: higher are the losses
• Data rate :
• Coaxial cable of 185m gives a max data rate of 100Mbps.
Requirement of Cables:
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Long distance transmission needs low loss transmission line.
Data rate should be high
Should be compact
Light weight
Modern optical fiber can carry information at around 14Tb/s over 160 km
fiber.
Optical Fiber Communication:
• It is a method of transmitting information from one place to another by
sending light through an optical fiber.
• The light forms an electromagnetic carrier wave that is modulated to carry
information.
• Modulating Signal : Light signal
Transmission medium:
• Optical Fiber
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It is a glass filament which carries light.
It is a cylindrical waveguide that operate at
optical frequency.
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It confines electromagnetic energy in the
form of light.
Electromagnetic Spectrum:
Long electrical Oscillations
Radio Waves
Microwaves
Light Waves
X-Rays
Electromagnetic Spectrum:
Optical Communication uses
infrared and visible light frequency region
Frequency Spectrum
for Optical Fiber Communication:
• Light is the form of electromagnetic Wave just as radio waves, but
with high frequency and shorter wavelength
Frequency Spectrum
for Optical Fiber Communication:
Block diagram of OFC system:
• Basic Communication system:
• Transmitter
• Channel
• Receiver
The process of communicating using fiber-optics involve
the following basic steps:
• Creating the optical signal using a transmitter,
• relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak .
Channel
• and receiving the optical signal and converting it into an electrical signal. Receiver.
Block diagram of Optical fiber Communication
System:
• Information Source
• Message source
• Transducer
• Transducer:
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To convert non electrical signal to electrical signal
Microphone converts sound waves into electrical signals
Camera : Images into electrical signal.
Computer: data already in electrical form
• Optical sources
• The fundamental function of optical source in optical fiber communications
is to convert electrical energy in the form of current into optical energy.
• The main optical sources currently used in optical fiber communications are
lasers and light emitting diodes (LEDs).
Optical sources:
• In FOC system :
• Carrier signal is light signal.
• Information to be transmitted is superimposed on light to change the irradiance of the
optical source as a function of time.
• Such a process of varying light level in accordance with the information signal is called :
• INTENSITY MODULATION
Light Sources:
• LED:
• LED is a non monochromatic , incoherent light source with a larger spectral width as
compared to Lasers and is suitable for short distance communication.
• Speed of operation is slow.
• Lasers:
• Are more desirable optical source
• It provides highly directional, monochromatic light beam.
• Speed of operation is high ,Lifetime is less and costly as compared to LED
• Used for Long distance communication
• Information Channel:
• Path between the transmitter and receiver.
• Is a Optical Fiber made of Glass / plastic which carries information in form of light.
• Light propagation is modeled as rays and hence light transmission through fiber is analyzed
by “RAY THEORY”
• Through channel different propagation effects affect the transmission rate and quality
• Dispersion
• Attenuation
• Loss
• Optical detectors
• The information superimposed on light has to be separated in order to retrieve the original
information.
• This process of recovering information from received light is called DEMODULATION
• The optical detector performs the opposite function from the source: It converts optical
energy to electrical energy.
• The commonly used detector is the photodiode, photo transistor which produces current in
response to incident light.
• PIN (P – Intrinsic – N diode)
• APD (Avalanche Photodiode)
Photodiodes:
• Photodiode is a light-sensitive device that converts the received photons into
electrons.
• The output current of APDs are much higher than PIN as a single photon
generates hundred or thousands of electrons in APD.
• APD are highly sensitive as compared to photodiode.
Block Diagram of Optical Fiber
Communication:
Drive Circuit:
• To match the voltage and current according to the requirements of LED/
Laser devices.
• To prevent the drive current from exceeding the maximum rate and further
affecting the reliability of the LED / Laser
• To meet expected brilliance requirements and ensure the color and brilliance
homogeneity of each LED.
• The driver circuit should keep low power consumption so that the LED
system efficiency can remain at a high level.
LED Driving Circuit :
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Ib needed is low
Ic = βIb
Transistors do have a limited amount of current handling ability
Which can be improved by coupling together as a Darlington pair (often
available in a single package).
• Also higher powered switches such as mosfets, can be driven for higher
power requirements
Channel Couplers:
• Couplers are needed since the source emits light over large angular extent and fiber
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can capture light within a limited angle.
Used to feed (FOCUS) the light signal into the channel.
Example:
In radio / TV broadcasting : coupler is antenna which couples the signal to
atmosphere.
In line communication: couplers are connectors.
In Optical system couplers are lenses used to colliminate the light emmited from the
source and direct it towards cable or photodetector.
Regenerators/ Repeaters:
• Used to boost the power level of weak signals.
• For weak signal repeaters/amplifiers are needed after every hundred or thousands of km to provide sufficient
power to the receiver.
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Repeater amplify signal so that if the signal level falls below a threshold level due to losses , attenuation, distortion.
Repeater detects the distorted signal, amplifies and reshapes the distorted signal and retransmits,
Use of repeater improves the data rate.
Repeater consists of
• Photodiode (APD/PIN diode)
• Electronic pulse generator
• Light source (LED/LASER)
• Electrical receiver / Signal Processor:
• Amplifier : To amplify the detected signal.
• Filter: to filter out unwanted signal and transmit only the information signal.
• Destination:
• Information retrieved in electrical form must be converted into suitable form to be heard / seen
/ processed
• Transducer:
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Loudspeaker for audio message.
CRT for pictures.
Computer or other machines connected to OF system
Advantages of Optical Fiber:
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Greater Bandwidth
Greater Information carrying capacity
Immunity to cross talk
Immunity to static interference
Less Attenuation
Greater Repeater Spacing
Environmental Immunity
Reliable and Durable
Small size and light weight
Economical
Advantages of Optical Fiber:
• Greater Bandwidth and hence greater information capacity:
• At high frequency of an optical fiber, data rates of 2Gbps aver 10’s of km is
achieved.
• Twisted pair: few Mbps over 1km
• Coax: hundred Mbps over 1km.
Immunity to cross talk:
• OF are non metallic conductors of electric current.
• OF is not surrounded by magnetic field which is the prime reason to cross talk in
metallic conductors which are located physically close to each other.
• Since there is no cross talk , it is difficult to tap the information.
• Information is more secure.
• Magnetic field produced by a current carrying wire
Immunity to static Interference:
• Since OF are non conductors of electric current.
• Immune to Electromagnetic Interference (EMI) that is radiations in space.
• OF provides low Attenuation:
• Attenuation is markedly low in OF than compared to Twisted wire, coaxial
cables.
• Transmission loss is less.
• Number of repeaters needed are less.
• Greater Repeater Spacing:
• Attenuation is less hence repeater spacing is more
• Cost and complexity reduced
• Environmental Immunity:
• OF are resistant to environmental extremes (whether variations).
• They operate over larger temperature range.
• Less affected by corrosive liquids, gases.
• Reliable and Durable:
• OF have high tolerance to temperature , environmental conditions.
• OF cables last longer and are more reliable than metallic cables.
• Smaller Size and lighter weight:
• Thinner than Coaxial cable / twisted pair.
• Occupy less space.
• Economical:
• Raw material available in plenty.
• Cost is approximately same as metallic cable.
• OF have less loss, less no. of repeaters, low installation and system cost and
improved reliability.
Disadvantages of Optical Fiber:
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A small bend on optical fiber causes loss.
Alignment between fibers , connectors & core has to be precise.
The cost of fusion splicing is high.
The fault localization is some what difficult.
Clean environment is required during splicing. A small dust particle
can create high attenuation.