Fiber Optics Introduction

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Fiber Optics Introduction


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Introduction The use of light to send messages is not new. The idea of using glass fiber to carry an

optical communications signal originatedwith Alexander Graham Bell.

However this idea had to wait some 80 yearsfor better glasses and low-cost electronics forit to become useful in practical situations.

Fiber-optic communications is based on theprinciple that light in a glass medium cancarry more information over longer distancesthan electrical signals can carry in a copper

or coaxial medium.


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Introduction With fewtransmission losses, low interference, and high

bandwidth potential, optical fiber is an almost idealtransmission medium.

Among the tens of thousands of developments andinventions that have contributed to this progress threestand out as milestones:

1. The invention of the LASER (in the late 1950's)

2. The development oflow loss optical fiber (1970's)3. The invention of the optical fiber amplifier (1980's)


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Introduction In initial time, portion ofelectro magnetic

spectrum was used to convey information incommunications systems.

The data is transferred over the communicationchannel by superimposing the information ontoa sinusoidally varying electromagnetic wave,which is known as carrier.

At the destination information is removed fromthe carrier wave and processed as desired.

More the information to be carried, higher thecarrier frequency there by increasing thetransmission bandwidth.


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Electromagnetic Spectrum


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The light injected into the fiber is a very small portion,which falls into visible wavelength from 400nm to700nm and the near infrared wavelength from 700nm

to 1700nm in the electromagnetic spectrum shown inthe figure



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In these regions, there are four special wavelengths to beused for fiber optics transmission for the low losscharacteristics as follows.



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Microwave Letter Band

Designationsf (GHz) Letter Band

Designation

1 2 L band

2 4 S band

4 8 C band

8 12.4 X band12.4 18 Ku band

18 26.5 K band

26.5 40 Ka band


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Electromagnetic Spectrum Windows are the regions in which optical

light travel long distances due to verygood attenuation characteristics.

The visible spectrum (or sometimesoptical spectrum) is the portion of theelectromagnetic spectrum that is visible to(can be detected by) the human eye.

Electromagnetic radiation in this range ofwavelengths is called visible light orsimply light.

A typical human eye will respond to

wavelengths in air from 400 to 700 nm


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Electromagnetic Spectrum An important principle in physics became the

theoretical foundation for optical fiber

communications: light in a glass medium cancarry more information over longer distancesthan electrical or radio frequency (RF) signalscan carry in a copper, coaxial or wireless

medium. It is customary to specify the band of interest

in terms of wavelength in opticalcommunication systems, instead of interms of

frequency as used in radio region.


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Electromagnetic Spectrum Fiber-optic communication is a method of

transmitting information from one place toanother by sending light through an optical fiber.

The light forms an electromagnetic carrier wavethat is modulated to carry information.

First developed in the 1970s, fiber-opticcommunication systems have revolutionized the

telecommunications industry and played a majorrole in Information Age.

Because of its advantages over electricaltransmission, the use ofoptical fiber has largely

replaced copper wire communications in thedeveloped world.


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Fiber Optics

CommunicationThe process of communication using fiber-

optics involves the following basic steps:

Creating the optical signal using atransmitter

Relaying the signal along the fiber andensuring that the signal does not becometoo distorted or weak

Receiving the optical signal and converting

it into an electrical signal


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History The need forreliable long-distance communication

systems has existed since antiquity. Over time, the sophistication of these systems has

gradually improved, from smoke signals totelegraphsand finally to the first coaxial cable, putinto service in 1940.

As these communication systems improved, certainfundamental limitations presented themselves.

Electrical systems were limited by their small repeaterspacing (the distance a signal can propagate beforeattenuation requires the signal to be amplified), andthe bit rate ofmicrowave systems was limited by

their carrier frequency.


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History In the second half of the twentieth century, it was

realized that an optical carrier of informationwould have a significant advantage over the

existing electrical and microwave carrier signals. However, no coherent light source or suitabletransmission medium was available.

After the development oflasers in the 1960ssolved the first problem, development ofhigh-

quality optical fiber was proposed as a solution tothe second. Optical fiber was finally developed in 1970 by

Corning Glass Works with attenuation low enoughfor communication purposes (about 20dB/km).


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History At the same time GaAs (Gallium Arsenide)

semiconductor lasers were developed thatwere compact and therefore suitable for

fiber-optic communication systems. After a period of intensive research from

1975 to 1980, the first commercial fiber-optic communication system was developed,which operated at a wavelength around 0.8

m and used GaAs (Gallium Arsenide)semiconductor lasers.

This first generation system operated at abit rate of 45 Mbit/s with repeater spacing ofup to 10 km.


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The second generation of fiber-opticcommunication was developed forcommercial use in the early 1980s,

operated at 1.3 m wavelength, and usedInGaAsP (Indium Gallium ArsenidePhosphide) semiconductor lasers.

These systems were initially limited by

dispersion. 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 repeaterspacing up to 50 km.


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Third-generation fiber-optic systems operated at 1.55 mwavelength and had loss of about 0.2 dB/km.

They achieved this despite earlier difficulties with pulse-

spreading at that wavelength using conventionalInGaAsP (Indium Gallium Arsenide Phosphide)semiconductor lasers.

Scientists overcame this difficulty by using dispersion-shifted fibers designed to have minimal dispersion at

1.55 m. These developments eventually allowed 3rd generation

systems to operate commercially at 2.5 Gbit/s withrepeater spacing in excess of 100 km.


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The fourth generation of fiber-opticcommunication systems used opticalamplification to reduce the need forrepeaters to increase fiber capacity.

This improvement from 1992 to 2001created a revolution by increasing thesystem capacity by 10 Tb/s.

Recently, bit-rates of up to 14 Tbit/s havebeen reached over a single 160 km lineusing optical amplifiers.


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Technology

Modern fiber-optic communication systems generallyinclude an optical transmitter to convert an electrical signalinto an optical signal to send into the optical fiber, a fiber-

optic cable routed through underground conduits andbuildings, multiple kinds of amplifiers, and an opticalreceiver to recover the signal as an electrical signal.

The information transmitted is typically digital informationgenerated by computers, telephone systems, and cable

television companies.


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Transmitters

The most commonly-used opticaltransmitters are semiconductor devices such

as Light Emitting Diodes (LED) and LaserDiodes.

The difference between LED and laser diodesis that LED produce incoherent light, while

laser diodes produce coherent light. For use in optical communications,

semiconductor optical transmitters must be

designed to be efficient and reliable.


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Transmitters contd..,

Most LED's used for displays are made fromGallium Arsenide Phosphide (GaAsP) andGallium Phosphide (GaP).

However, due to their relatively simple design,LEDs are very useful for low-cost applications.

A semiconductor laser emits light throughstimulated emission rather than spontaneousemission, which results in high output power aswell as other benefits related to the nature ofcoherent light.

The output of a laser is relatively directional,allowing high coupling efficiency (~50 %) into

single-mode fiber.


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Laser

Laser stands for Light Amplification by Stimulated Emission of Radiation.

There are two important groups of words inthis acronym:

light amplification stimulated emission


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Laser

Spontaneous emissionSpontaneous emission is the normal case.

When an electron enters a high energy (excited)stateit is able to stay there relatively for a long time(a few microseconds) before it changes statespontaneously.

When light is emitted spontaneously it phase anddirection will be random but the wavelength will be

determined by the amount of energy that theemitting electron must give up.


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Laser

Stimulated emissionStimulated emission is what happens in theoperation of a laser.

When an electron is in semi-stable (meta stable)

high energy state, it can be stimulated by thepresence of a photon of light to emit its energy inthe form of another photon.

In this case the incident photon must have theright energy (wavelength) within quite small limits.

For spontaneous or stimulated emission to occur,energy must be supplied to boost the electronfrom its low energy state to a higher energy state.


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Principle of laser


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Principles of Laser

An electron within an atom (or a molecule or an ion)starts in a low energy stable state often called theground state.

Energy is supplied from outside and is absorbed bythe atomic structure whereupon the electron entersan excited (higher energy) state.

A photon arrives with an energy close to the sameamount of energy as the electron needs to give up to

reach a stable state. The arriving photon triggers a resonance with the

excited atom. As a result the excited electron leavesits excited state and transitions to a more stable state

giving up the energy difference in the form of aphoton.


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Receivers

The main component of an optical receiver is aphoto detector that converts light into electricitythrough the photoelectric effect.

The light emerging from the end of an optical fiberlink must be detected and converted into electronicpulses for further processing so that the transmittedinformation can be received.

The two types of detectors normally used are; The Avalanche Photodiode (APD) and

The PIN Photodiode (PIN).


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The Fiber Optic Cable

The cable consists of one or more glass fibers, whichact as waveguides for the optical signal.

Fiber optic cable is similar to electrical cable in its

construction, but provides special protection(environmental and mechanical protection) for theoptical fiber within.

For systems requiring transmission over distances of

many kilometers, or where two or more fiber opticcables must be joined together, an optical splice iscommonly used.


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Fiber optic cable consists of . core

cladding

coating strengthening fibers

cable jacket


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Core -This is the physical medium thattransports optical data signals from an attachedlight source to a receiving device.

The core is a single continuous strand of glassor plastic that's measured (in microns) by thesize of its outer diameter.

The larger the core, the more light the cable

can carry. Cladding - This is a thin layer that surrounds

the fiber core and serves as a boundary thatcontains the light waves and enabling data totravel throughout the length of the fibersegment.


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Coating -This is a layer of plastic thatsurrounds the core and cladding to

strengthen the fiber core, help absorbshocks, and provide extra protectionagainst excessive cable bends.

Strengthening fibers -These

components help protect the core againstcrushing forces and excessive tensionduring installation.

Cable jacket -This is the outer layer of

any cable


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Advantages

Optical fiber communication has many well-knownadvantages:

Weight and Size

Fibre cable is significantly smaller and lighter thanelectrical cables to do the same job.

A fiber optic cable, even one that contains manyfibers, is usually much smaller and lighter inweight than a wire or coaxial cable with similar

information carrying capacity


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Information Capacity

The data rate of systems in use in 1998 is

generally 150 or 620 Mbps on a single(unidirectional) fibre.

The usual rate for new systems is 2.4Gbps or even 10 Gbps. (also Tbps is

available) This is very high in digital transmission

terms.


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No Electromagnetic Interference

Because the connection is not electrical, you canneither pick up nor create electrical interference

(the major source of noise).

The fiber is totally immune to virtually all kinds ofinterference, including lightning, and will notconduct electricity. It can therefore come in

direct contact with high voltage electricalequipment and power lines.


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Distances between Regenerators As a signal travels along a communication line

it loses strength (is attenuated)

The traditional way to regenerate the signal,restoring its power using a repeater. In long-line optical transmission cables now in

use by the telephone companies, the repeaterspacing is typically 40 kilometres.

This compares with 12 km for the previouscoaxial cable electrical technology. The number of required repeaters and their

spacing is a major factor in system cost.


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Better Security

It is possible to tap fibre optical cable.

But it is very difficult to do and theadditional loss caused by the tap isrelatively easy to detect.

There is an interruption to service while

the tap is inserted and this can alertoperational staff to the situation.


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No Corrosion:

As the basic fiber is made of glass, it will

not corrode and is unaffected by mostchemicals.

It can be buried directly in most kinds ofsoil or exposed to most corrosive

atmospheres in chemical plants withoutsignificant concern.


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Safety:

Since the only carrier in the fiber is light,

there is no possibility of a spark from abroken fiber.

Even in the most explosiveof atmospheres, there is no fire hazard,

and no danger of electrical shock topersonnel repairing broken fibers.

Fiber Optics Introduction

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