This includes approaches to unlocking the transmission , Degree thesis for Communication. Anglia Ruskin University
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abdelghany-nasef

This includes approaches to unlocking the transmission , Degree thesis for Communication. Anglia Ruskin University

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optical comm The work in optical transmission within the optical networks group involves understanding the fibre channel transmission limits and developing techniques to approach these limits. This includes approaches ...
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Scientech 2138A AT Manual

Optical Fiber Communication Scientech 2501A

Product Tutorials Ver. 1.1

Designed & Manufactured by- An ISO 9001:2008 company Scientech Technologies Pvt. Ltd. 94, Electronic Complex, Pardesipura, Indore - 452 010 India,

+ 91-731 4211100, : info@scientech.bz , : www.ScientechWorld.com

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Optical Fiber Communication Scientech 2501A

Table of Contents Safety Instructions 4 Introduction 5Features 6Technical Specifications 7Theory 8Optical Fiber Communication System 9Advantages of Fiber Optic System 17Recommended Testing Instruments for Experiments 44Experiments

Experiment 1 45 Study of 650 nm Fiber Optic Analog link.

Experiment 2 47 Study of 650 nm Fiber Optic Digital Link.

Experiment 3 49 To obtain Intensity Modulation of the Analog Signal & Demodulation

Experiment 4 51 To obtain Intensity Modulation of the Digital Signal & Demodulation

Experiment 5 53 Study of Frequency Modulation (FM)

Experiment 6 55 Study of Pulse Width Modulation

Experiment 7 57 Measurement of Propagation or Attenuation Loss in the optical fiber

Experiment 8 59 Study of Bending Loss

Experiment 9 61 Measurement of Optical Power using optical power meter

Experiment 10 63 Measurement of Propagation Loss in optical fiber using POM

Experiment 11 65 Measurement of Numerical Aperture (NA) of optical fiber

Experiment 12 67 Study of Characteristics of E-O converter using optical power meter

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Experiment 13 69 Study of Characteristics of Fiber Optic Communication Link

Experiment 14 ` 71 Study of Voice Communication through fiber Optic cable using Amplitude Modulation

Experiment 15 73 Demonstration of Voice Transmission through optical fiber using FM

Experiment 16 75 Study of Voice Transmission through optical fiber using PWM

Experiment 17 77 Study of the effects of Switched Fault Number 1 & 8 on Amplitude Modulation System

Experiment 18 79 Study of the effects of Switched Fault Number 4, 5 & 7 in FM System

Experiment 19 81 Study of the effects of Switched Fault Number 2, 3 & 6 on Pulse Width Modulation System

Experiment 20 83 Determination of Bit Rate supported by the fiber optic link

Experiment 21 85 Determination of Sensitivity of the fiber optic link

Experiment 22 87 Determination of Power Margin (Power Budget)

Experiment 23 89 V-I characteristics of Photo LED

Experiment 24 91 V-I characteristics of Photo Detector

Experiment 25 93 To Measure Bit Error Rate

Experiment 26 96 Study and Observation of Eye Pattern

10. Frequently Asked Questions 101

Glossary of Fiber Optic terms 109 Warranty 113

List of Accessories 113

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Safety Instructions Read the following safety instructions carefully before operating the instrument. To avoid any personal injury or damage to the instrument or any product connected to it.

Do not operate the instrument if suspect any damage to it. The instrument should be serviced by qualified personnel only. For your safety: Use proper Mains cord : Use only the mains cord designed for this instrument.

Ensure that the mains cord is suitable for your country.

Ground the Instrument : This instrument is grounded through the protective earth conductor of the mains cord. To avoid electric shock the grounding conductor must be connected to the earth ground. Before making connections to the input terminals, ensure that the instrument is properly grounded.

Observe Terminal Ratings : To avoid fire or shock hazards, observe all ratings and marks on the instrument.

Use only the proper Fuse : Use the fuse type and rating specified for this instrument.

Use in proper Atmosphere : Please refer to operating conditions given in the manual.

Do not operate in wet / damp conditions.

Do not operate in an explosive atmosphere.

Keep the product dust free, clean and dry.

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Introduction Scientech TechBooks are compact and user friendly learning platforms to provide a modern, portable, comprehensive and practical way to learn Technology. Each TechBook is provided with detailed Multimedia learning material which covers basic theory, step by step procedure to conduct the experiment and other useful information.

Scientech 2501A Optical Fiber Communication TechBook demonstrate simplex method of transmitting information from one place to another by sending pulses of light through an Optical fiber. The TechBook demonstrates the properties of Simplex Analog and Digital Transreceiver, characteristics of Fiber Optics cable, Modulation / Demodulation techniques, Bit Error Rate measurement and observation of Eye Pattern. A large number of experiments are included in the workbook and many more can be performed using Scientech 2501A.

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Features ● Simplex Analog and Digital Trans-receiver ● 660 nm channel with Transmitter & Receiver

● AM-FM-PWM modulation / demodulation ● On board Function Generator

● On board Clock & Data Generator ● On board Bit Error Counter

● Crystal controlled Clock ● Functional blocks indicated on-board mimic

● Input-output & test points provided on board ● On board voice link

● Built in DC Power Supply ● Numerical Aperture measurement jig and mandrel for bending loss measurement

● Switched faults on Transmitter & Receiver

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Technical Specifications Transmitter : 1 no., Fiber Optic LED, Wavelength 660 nm Receiver : 1 no., Fiber Optic Photo detector Modulation Techniques : AM, FM, PWM Drivers : 1 no. with Analog & Digital modes Clock : Crystal controlled Clock 4.096 MHz PLL Detector : 1 no. AC Amplifier : 1 no. Comparator : 1 no. Filter : 1 no. 4th order Butterworth, 3.4 KHz cut-off

frequency Analog Band Width : 350 KHz Digital Band Width : 2.5 MHz Function Generator : 1 KHz Sine wave (Amplitude adjustable) 1 KHz Square wave (TTL) Clock Generator : 64 KHz/128 KHz/256 KHz (TTL) Data Generator : 15 Bit Noise Generator : Variable level Bit Error Counter : 4 digits, 7 segment display Voice Link : F. O. voice link using microphone & speaker Switched Faults : 8 nos Fiber Optic Cable : Standard SMA Connector type Cable Type : Step indexed multimode PMMA plastic cable Core Refractive Index : 1.492 Clad Refractive Index : 1.406 Numerical Aperture : Better than 0.5 Acceptance Angle : Better than 60 deg. Fiber Diameter : 1000 microns Outer Diameter : 2.2 mm Fiber Length : 0.5 m & 1 m Test Points : 34 nos. Inter connections : 2 mm sockets Dimensions (mm) : W 326 × D 252 × H 52 Weight : 1 Kg approximately Operating conditions : 0-400 C, 80% RH Power Supply : 110-220 V, ±10%, 50/60 Hz Power Consumption : 3 VA approximately Product Tutorials : Online (Theory, procedure, reference results, etc).

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Theory Fiber optic communication is a method of transmitting information from one place another by sending pulses of light through an optical fiber. The light forms electromagnetic carrier wave that is modulated to carry information. First developed in the 1970s, fiber-optic communication systems have revolutionized the telecommunications industry and have played a major role in the advent of the Information Age. Because of its advantages over electrical transmission, optical fibers have largely replaced copper wire communications in core networks in the developed world.

Optical Fiber The process of communicating using fiber-optics involves the following basic steps: Creating the optical signal involving the use of a transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, receiving the optical signal, and converting it into an electrical signal. Communication may be broadly defined as the transfer of information from one point to another. Before Fiber Optics came along, the primary means of real time data communication was electrical in nature. It was accomplished by using copper wire or by modulating information on to an electromagnetic wave that acts as a carrier for the information signal. All these methods have one problem in common the communication had to be over a straight-line path. While, in Fiber Optic Communication, the optical wave propagates inside the fiber and acquires the shape of the fiber. Fiber Optics provides an alternative means of sending information over significant distances using light energy. Light as utilized for communication has major advantages because it can be modulated at significant higher frequencies than electrical signals. That is till 1870, when an Irish physicist John Tyndall carried out a simple experiment. He filled a container with water and shone light into it. In the dark room he pulled the bung from the opposite end of the container. The light shone out in the direction of the curved path of the water. The light was guided and a new science was born called Fiber Optics. This was achieved due to the refraction property of the light, which made it possible to get the light reflected inside the optical fiber with certain approaching angles within desired threshold and continuing the process within the cable till the optical wave reached the other end and thus the light propagated inside the optical fiber.

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Introduction to Optical Fibre: n optical fiber is a thin, flexible, transparent fiber that acts as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communication. Fibers are used instead of metal wires because signals travel along them with less loss and are also immune to electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications, including sensors and fiber LASERS.

Optical Fibre Communication System

Optical Fiber Communication System

Understanding how fibre optics are made and function for uses in everyday life is an intriguing work of art combined with science. Fibre optics has been fabricated from materials that transmit light and are made from a bundle of very thin glass or plastic fibres enclosed in a tube. One end is at a source of light and the other end is a camera lens, used to channel light and images around the bends and corners. Fibre optics has a highly transparent core of glass, or plastic encircled by a covering called "cladding". Light is stimulated through a source on one end of the fibre optic and as the light travels through the tube, the cladding is there to keep it all inside. A bundle of fibre optics may be bent or twisted without distorting the image, as the cladding is designed to reflect these lighting images from inside the surface. This fibre optic light source can carry light over mass distances, ranging from a few inches to over 100 miles. There are two kinds of fibre optics. The single-mode fibre optic is used for high speed and long distance transmissions because they have extremely tiny cores and they accept light only along the axis of the fibres. Tiny LASERS send light directly into the fibre optic where there are low-loss connectors used to join the fibres within the

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system without substantially degrading the light signal. Then there are multi-mode which have much larger cores and accept light from a variety of angles and can use more types of light sources. Multi-mode fibre optics also uses less expensive connectors, but they cannot be used over long distances as with the single-mode fibre optics. Fibre optics has a large variety of uses. Most common and widely used in communication systems, fibre optic communication systems have a variety of features that make it superior to the systems that use the traditional copper cables. The uses of fiber optics with these systems use a larger information-carrying capacity where they are not hassled with electrical interference and require fewer amplifiers then the copper cable systems. Fibre optic communication systems are installed in large networks of fibre optic bundles all around the world and even under the oceans. Many fibre optic testers are available to provide you with the best fibre optic equipment. Optical Sources LASER and LED In fibre optic communication systems, LASERS are used to transmit messages in numeric code by flashing on and off at high speeds. This code can constitute a voice or an electronic file containing, text, numbers, or illustrations, all by using fibre optics. The light from many LASERS are added together onto a single fibre optic enabling thousands of currents of data to pass through a single fibre optic cable at one time. This data will travel through the fibre optics and into interpreting devices to convert the messages back into the form of its original signals. Industries also use fibre optics to measure temperatures, pressure, acceleration and voltage, among an assortment of other uses.

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Here, the information source provides an amplified electrical signal to a transmitter comprising an electrical stage, which drives an optical source to give conversion, may be either a semiconductor, LASER (Light Amplification by Stimulated Emission of Radiation) or LED. The transmission medium consists of optical source, which provides an electrical to optical conversion, an optical fibre cable used for transmission of signal and the receiver, consists of an optical detector, which drives a further electrical stage and hence provides demodulation of optical carrier. This electrical signal is amplified and applied to the destination. e.g. Speaker. Photo diodes (P-I-N or Avalanche) and in some instances photo transistors and photo conductors are utilized for detection of optical signal and optical to electrical conversion. The optical carrier may be modulated using an analog or digital information signal. Analog modulation involves the variation of light emitted from the optical source in continuous manner. In digital modulation however, discrete changes in the light intensity are obtained (i.e. ‘On-Off’ pulses) although often simpler to implement, analog modulation with an optical fibre communication system is less efficient, requiring a far higher signal to noise ratio (SNR) at the receiver than digital modulation. Also, linearity needed for analog modulation is not provided by semiconductor optical sources especially at high modulation frequencies. Principle of operation of Optical Fibre: The principle of operation of optical fibre lies in the behaviour of light. It is a widely held view that light always travels in straight line and at constant speed. Of course, the light propagates in straight lines, but when it is reflected inside the optical fibre million and trillion times by the clad, each movement comprising of a straight line and consequently because of such reflections, it acquires the shape of the optical fibre. So effectively, it is said to have been travelling along the fibre. It changes its direction only if there is a change in the dielectric medium as also illustrated by the Tyndall’s experiment. To understand the propagation of light within an optical fibre it is necessary to take into account refractive index of the dielectric medium. Refractive index of a medium is defined as the ratio of velocity of light in vacuum to velocity of light in medium.

Velocity of light in vaccumRefractive index = Velocity of light in medium

Since, the velocity of light in any solid, transparent material is less than in vacuum the refractive index of such material is always greater than 1.0. A ray of light travels slowly in an optically dense medium than one that is less dense. Now, the direction that the light approaches the boundary between the two materials is very important. When a ray is incident on the interface between two dielectrics of differing refractive indices, refraction occurs. The light is refracted and also partly reflected internally in the same medium; which is referred as Partial Internal Reflection. It may be observed that the ray approaching the interface is propagating in a dielectric of refractive index n1 and is at an angle φ1 to the normal at the surface of the interface. If the dielectric on the other side of interface has a refractive index n2 which is less than n1, then the refraction is such that the ray path in this lower index medium is at angle φ2 to the normal where φ2 is greater than φ1.

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The angle of incidence φ1and refraction φ2 are related to each other and to refractive indices of dielectrics by Snell's law of refraction which states that:

1 1 2 2n sin n sinφ φ=

1

2

2

1

sin sin

n n

= φ φ

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It is this change in refractive indices which causes the change in the path of the incident ray as evident from the Snell’s law. Larger the change in the refractive indices larger change in the direction of the incident ray. The sine of the angles will be in the ratio of their refractive indices. As the angle of incident ray increases, the angle of refraction also increases even faster and when the angle of refraction becomes 90° thereafter, if the angle of incidence is increased a condition is arrived where the incident ray is totally reflected in the same medium from where it has emerged; this is referred as the total internal reflection.

Total Internal Reflection: Since, the angle of refraction is always greater than the angle of incidence, when the incident medium is denser than the refraction medium. Thus, the angle of refraction is 90° and the refracted ray emerges parallel to the interface between the dielectrics. This is the limiting case of refraction and this angle of incidence is known as critical angle φc. The value of critical angle is given by:

2

1 c

= 90 =

φ φ φ

°

Substituting this in the equation for Snell’s law gives

1 c 2

2 c

1

n sin = n sin 90 n sin = n

φ

φ

°

At angles of incidence greater than the critical angle the light is reflected back into the originating dielectric medium. This behaviour of light is termed as Total Internal Reflection.Here, Angle of Incidence = Angle of Reflection

This is the mechanism by which light may be considered to propagate down an optical fiber with low loss. Shown in next figure below illustrates the transmission of a light ray in an optical fiber via a series of total internal reflection at the interface of the silica core and slightly lower refractive index silica cladding.

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The light ray shown in next figure is known as meridian ray as it passes through the axis of the fiber core. It is generally used when illustrating the fundamental transmission properties of optical fiber.

Acceptance Angle: Since, only rays with an angle greater than critical angle at the core cladding interface are transmitted by total internal reflection, it is clear that not all rays entering the fiber core will continue to propagate down the length.

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Shown in next figure illustrates two incident rays I and B. It may be observed that ray ‘I’ enters the fiber core at an angle θi less than θα (conical half angle for the fiber explained herein under) to the fiber axis and is refracted at the air- core interface before transmission to the core- cladding interface at an angle φ more than the critical angle φc. This ray is totally internally reflected and propagated along the fiber. While incident ray ‘B’ is incident into the fiber core at an angle θb greater than θa and will be transmitted to the core- cladding interface at an angle less thanφc and will not be totally internally reflected instead will be refracted into cladding and eventually lost by the radiation. Thus, for rays to be transmitted by total internal reflection within the fiber core they must be incident on the fiber core within an acceptance cone defined by conical half angle θa. Hence, θa is the maximum angle to the axis at which light may enter the fiber in order to be propagated and is referred to as the acceptance angle for the fiber?

Here < <

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Numerical Aperture: It gives the relationship between the acceptance angle and the refractive indices of the three media involved viz. the core, the cladding and air.

In the next figure above φi corresponds to θi; when φi approaches φc;θi approaches θa By Snell's law of refraction:-

0 1 ) c

1

2 1/ 2 1

1

sin sin(90 {where is the critical angle} cos (1 sin )

(1

a c

c

c

n n n n

nn

θ φ φ φ

φ

= ° − =

= −

= − 2

1/ 22 2 2 1 1

2 2 1 2

) {assin }

c n

n n n n

φ =

= −

This term is referred as numerical aperture of the Wave Guide – Optical Fiber.

2 2 1/ 2 2 2 0 1 2 1 2

1 2 1 2

Numerical Aperture sin ( ) ( )

( )( )

an n n n n

n n n n

θ= = − = −

= − +

Where, n0 = Refractive index of air

n1 = Refractive index of core n2 = Refractive index of cladding The Numerical Aperture is a very useful measure of light collecting ability of a fiber. It directly relates to the refractive indices of the core and cladding. As we observe from the above equation, greater the absolute value of the indices of core and cladding, greater the numerical aperture; similarly, greater the difference between the refractive indices greater the numerical aperture. In accordance to the requirement of the numerical aperture, the material for the core and cladding is chosen, keeping in view the other parameters and requirements for transmission.

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Advantages of Fibre Optic System

• Enormous Potential Band Width (BW) : The information carrying capacity of a transmission system is directly proportional to the carrier frequency of the transmitted signals.The optical carrier frequency in the range 1013 to 1016 Hz. (generally near infrared around 1014 or 1015 Hz) yields a far greater potential transmission B.W. than metallic cable system. (i.e. coaxial cable Bandwidth up to 500 MHz) or even milli meter wave radio system, (i.e. system currently operating with modulation Bandwidth of 700 MHz) . Thus the optical fibres have enormous transmission bandwidths and high data rate. Using wavelength division multiplexing operation, the data rate or information carrying capacity of optical fibers is enhanced to many orders of magnitude.At present the Bandwidth available to fiber system is not fully utilized by modulation at several GHz over hundred km. and hundreds of MHz over 300 Km with intervening electronics (repeaters) is possible. Therefore, the information carrying capacity of optical fiber system has proved far superior to best copper cable available, by comparison losses in coaxial cable systems restrict. A much-enhanced Bandwidth utilization for an optical fiber can be achieved by transmitting several optical signals each at different centre wavelengths in parallel on the same fiber. This wavelength division multiplexed operation particularly with dense packing of the optical wavelength (or fine frequency spacing) offers potential information carrying capacity.

• Small size and weight : Optical fibres have very small diameter in the ranges from 10 micrometers to 50 micrometers. The space occupied by the fiber cable is negligibly small compared to conventional electrical cables.Hence, when they are covered with protective coatings they are far smaller & lighter. This is a tremendous boon towards the alleviation of duct congestion in cities and allowing expansion of signal transmission in mobiles e.g. aircrafts, ships etc.

• Electrical Isolation : Optical fibres are fabricated from glass or plastic polymers, they are electrical insulators therefore they do not exhibit earth loop, interference problems, electromagnetic wave or any high current lightening. This property makes them suitable for communication in electrically hazardous environment as fiber create no arcing or spark hazard at abrasions or short circuit & usually fiber do not contain sufficient energy to ignite vapours or gases.It is also suitable in explosive environment.

• Immunity to Interference and Cross talk : Optical fibers form a dielectric wave-guide and therefore are free from Electro Magnetic Interference (E.M.I), Radio Frequency Interference (R.F.I) or switching transients. It is not susceptible to lightening striker if used overhead rather than underground. Moreover it is easy to ensure that there is no optical interference between fibers. Since optical interference among different fibres is not possible, cross talk is negligible even many fibres are cabled together.

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• Signal Security : The light from optical fibers does not radiate significantly and therefore they provide a high degree of signal security. Unlike in copper cables, a transmitted signal cannot be drawn from a fiber without tampering it. Thus, the optical fiber communication provides 100% signal security. A transmitted optical signal cannot be obtained from a fiber in a non-invasive manner (i.e. without drawing optical power form the fiber). In theory, any attempt to acquire a message signal transmitted optically may be detected. This feature is obviously attractive for military & banking.

• Low transmission loss: Due to the usage of ultra low loss fibres and the erbium doped silica fibres as optical amplifiers ,Optical fibers results in low attenuation or transmission loss in comparison with the best copper conductor. It facilitates the implementation of communication links with extremely wide repeater spacing thus reducing both system cost and complexity. This quality along with already proven modulation B W capability of fiber cable, it is used in long haul telecommunication applications.Hence for long distance communication fibres of 0.002 dB/km are used. Thus the repeater spacing is more than 100 km.

• Potential Low Cost : The glass that generally provides optical fiber transmission medium is made from sand not a scarce resource. In comparison with copper conductors, optical fiber offers low cost line communication. This is because many miles of optical cable are easier and less expensive to install than the same amount of copper wire or cable.

• Thinner: Fiber optics is thinner than copper wire cables, so they will fit in smaller, more crowded places. This is important for underground cable systems, like in cities, where space needs to be shared with sewer pipes, power wires, and subway systems.

• Non-flammable Since fiber optics send light instead of electricity, fiber optics are non- flammable. This means there is not a fire hazard. Fiber optics also do not cause electric shocks, because they do not carry electricity.

• Ruggedness and flexibility The fibre cable can be easily bend or twisted without damaging it. Further the fiber cables are superior than the copper cables in terms of handling, installation, storage, transportation, maintenance, strength and durability.

• Low cost and availability Since the fibres are made of silica which is available in abundance. Hence, there is no shortage of material and optical fibers offer the potential for low cost communication.

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• Reliability The optical fibres are made from silicon glass which does not undergo any chemical reaction or corrosion. Its quality is not affected by external radiation. Further due to its negligible attenuation and dispersion, optical fiber communication has high reliability. All the above factors also tend to reduce the expenditure on its maintenance.

The disadvantages of optical fibres are:

• Price - Even though the raw material for making optical fibres, sand, is abundant and cheap, optical fibres are still more expensive per metre than copper. Although, one fibre can carry many more signals than a single copper cable and the large transmission distances mean that fewer expensive repeaters are required.

• Fragility - Optical fibres are more fragile than electrical wires.

• Affected by chemicals - The glass can be affected by various chemicals including hydrogen gas (a problem in underwater cables.)

• Opaqueness - Despite extensive military use it is known that most fibres become opaque when exposed to radiation.

• Requires special skills - Optical fibres cannot be joined together as a easily as copper cable and requires additional training of personnel and expensive precision splicing and measurement equipment.

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The Optic Fibre: The simplest fibre optic cable consists of two concentric layers of transparent materials. The inner portion the core transports the light, the outer covering (the cladding) must have a lower refractive index than the core so the two of them are made up of different materials.

To provide mechanical protection for the cladding an additional plastic layer; called Primary Buffer is added. Some constructions of optic fibre have additional layers of buffers that are then referred to as Secondary Buffers. It is very important to note that the whole fibre-Core, Cladding & Primary Buffer is solid and the light is confined to the core by the Total Internal Reflection due to the difference in the refractive index of the core as compared to that of cladding.

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Single Mode versus Multi Mode: As we have already seen that there are particular angles of propagation defined by cone of acceptance, which can be transmitted down the optic fiber. At these angles, the electromagnetic wave that the light can set up a number of completes patterns across the fiber. The number of complete patterns called Modes depends on the dimensions of the optic fiber core. There are essentially two different types of fiber optic transmission schemes in use viz.} Single Mode } Multi Mode

Single Mode: As the name suggests the single mode cable is able to propagate only one mode (Electromagnetic wave). This is used in long distance and/or, high-speed communication. It is beneficial over long distances since it completely eliminates a problem known as inter modal Dispersion associated with Multimode cables. All our long distance telephone conversations are now carried by single mode optic fiber system over at least some part of the route.

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Multi Mode: The term multimode means that the diameter of the fiber optic core is large enough to propagate more than one mode (Electro Magnetic Wave).

Because of the multiple modes the pulse that is transmitted down the fiber tends to become stretched over distance this is referred to as dispersion & has the effect of reducing the available bandwidth. These are typically used in applications such as LAN (Local Area Networks) & FDDI (Fiber Distributed Area Interface)

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Optical Fiber Index Profile Index profile is the refractive index distribution across the core and the cladding of a fiber. Some optical fiber has a step index profile, in which the core has one uniformly distributed index and the cladding has a lower uniformly distributed index. Other optical fiber has a graded index profile, in which refractive index varies gradually as a function of radial distance from the fiber center. Graded-index profiles include power- law index profiles and parabolic index profiles.

Step Index And Graded Index Fibers: The first type of fiber optic cable put to use was called step index. In this design, the cladding has a different index of refraction than the core. The light bounces off the side and is reflected back into the fiber core. The problem with this design is that the reflected light must travel a slightly longer distance, than that which travels down the centre of the fiber, thus limiting the maximum transmission rate. This design was improved with the use of Graded index fiber. In this design, the index of refraction decreases in proportion to the distance away from the centre of the fiber core. The light moves more quickly in the outer portion thus compensating for the additional distance. The change in index has the effect of bending. The light reflects back towards the core. This change increases the transmission capacity by a reasonable factor. In the newest single mode design, the diameter of the fiber core is so small that all the light travels in a straight line. Even the latest fiber optic facility in use today uses less than 5% of the maximum theoretical capacity of a single mode fiber.

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Some of the optical fibers in use are:

• Multimode step index fibers.

• Multimode graded index fibers.

• Single mode step index fibers.

• Plastic - clad fibers.

• All plastic fibers. Dimensions of fiber optic cables are written as a ratio e.g. a cable with cladding diameter of 125 microns and fiber core diameter of 62.5 or 50 microns will be referred to as 62.5 /125 or 50 / 125 fibers. That is if the diameter of the core is Dcr and the diameter of the clad is Dcd both in microns (1 micron = 10- 6 meters), then the dimensions of the fibre optic cable will be denoted as Dcr/ Dcd.

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Choice of Operating Frequency: Once we had the LASER and the new optic fiber available, every thing was in place for a significant upsurge in communications. This resulted in two driving forces: one towards the ability to send more data faster and secondly to send the data to greater distances without being re-amplified.

More Data Faster: As the transmission rate of data is increased, the required bandwidth increases and this can be best accommodated by increasing the carrier frequency. This premise has stood us in good stead over many years. The speech and poor quality music transmissions on the medium frequency, AM radio, gives way to the higher frequency of FM radios which accommodate the increased bandwidth necessary for improved music quality. When television required even higher data rates, we responded by moving to even higher frequencies. These previous experience rather suggested that the light used for fiber optic communications should be of the highest frequency possible. But there was a surprise in store!

Lower Frequencies Mean Lower Losses: The first experiments used visible light of different colours (frequencies). As the losses were measured, we found that the higher frequencies caused more losses.

The losses actually increased by the 4th power of the frequency. This means that a tripling of the frequency would result in the losses increasing by 34 or 81 times. We therefore have two conflicting influences:

High frequency = High Data Rates

Low frequency = Long Ranges

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