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CONTENTS CHAPTER PAGE NO.

1. Abstract………………………………………………………………………………. 3

2. Introduction…………………………………………………………………………. 4

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3. Physics of optical fibers………………………………………………………… 5

4. Advantage of fiber optic system…………………………………………………. 8

5. Fiber optic Transmission system ………………………………………………. 9

6. How are optical fibers made………………………………………………………. 16

7. Applications…………………………………………………………………………….

8. How are major under sea cables laid in the ocean? ………………………….

9. Conclusion……………………………………………………………………………..

10. Bibliography………………………………………………………………………….

11. Appendix………………………………………………………………………………

1. Abstract

Optical Fiber communications form a crucial role in exchanging information globally. The development of optical

fibers made a revolution starting from a small fiber to a cable of wires connecting countries continently. This

seminar first instigates the fundamentals of physics involved in the optical fiber. Secondly, explains about the

heart of the seminar “Optical Fibers communication system” and its related components. Special care is taken to

elaborate its usage in Switching and access networks. The benefits drawn through this communication is briefed

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to emphasize its impact. Finally, it deals with the manufacture of optical fibers and an idea prompted about the

undersea cable system.

2. INTRODUCTION

The 21st century, the era of ‘Information technology’, this is the outcome of many brilliant inventions and

discoveries. Progressing from the copper wire of a century ago to today’s fiber optic cable , our increasing ability

to transmit more information, more quickly and over longer distances has expanded the boundaries of our

technological development in all areas.

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Fiber optic communication has revolutionized the telecommunications industry. It has also made its presence

widely felt within the data networking community as well. Using fiber optic cable, optical communications have

enabled telecommunications links to be made over much greater distances and with much lower levels of loss in

the transmission medium and possibly most important of all, fiber optical communications has enabled much

higher data rates to be accommodated.

As a result of these advantages, fiber optic communications systems are widely employed for applications

ranging from major telecommunications backbone infrastructure to Ethernet systems, broadband distribution,

and general data networking.

3. PHYSICS OF OPTICAL FIBERS

Fiber Optics is the technique of transmitting light through transparent, flexible fibers of glass or plastic.

The fibers, called optical fibers, can channel light over a curved path. Bundles of parallel fibers can be used to

illuminate and observe hard-to-reach places. Optical fibers of very pure glass are able to carry light over long

distances ranging from a few inches or centimeters to more than 100 miles (160 km) with a little dimming.

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Cables containing such fibers are used in certain types of communications systems. Some individual fibers are

thinner than human hair and measure less than 0.00015 inch (0.004 mm) in diameter.

3.1 Optical Fiber cable construction:

If you look closely at a basic single optical f iber, you will see that it has the following parts:

Core - Thin glass center of the fiber where the light travels

Cladding - Outer optical material surrounding the core that reflects the light back into the core

Buffer coating - Plastic coating that protects the fiber from damage and moisture

Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The bundles are

protected by the cable's outer covering, called a jacket.

Basic optical fiber is ideal for most inter-building applications where extreme ruggedness is not required. In

addition to the “basic” variety, it is also available for just about any application, including direct buried, armored,

rodent resistant cable with steel outer jacket, and UL approved plenum grade cable. Color-coded, multi-fiber

cable is also available.

3.2 Principle operation:

Fiber optics is based on the optical phenomenon known as total internal reflection [A-1]. This causes the fiber to

act as a waveguide. With the simplest form of optical fiber, light entering one end of the fiber strikes the

boundary of the fiber and is reflected inward. The light travels through the fiber in a succession of zigzag

reflections until it exits from the other end of the fiber. Other forms of optical fibers are designed in such a way

that the zigzagging of the light is greatly reduced or virtually eliminated.

3.3 Optical fiber types

Optical fibers can also be split into single mode fiber, and multimode fiber. Mention of both single mode fiber and

multi-mode fiber is often seen in the literature.

3.3.1 Multimode fiber:

a) Step Index: Core and cladding materials have uniform but different refractive index.

b) Graded Index: Core material has variable index as a function of the radial distance from the center.

This form of fiber has a greater diameter than single mode fiber, being typically around 50 microns in

diameter, and this makes them easier to manufacture than the single mode fibers. It can capture light

from the light source and pass it to the receiver with a high level of efficiency. As a result it can be used

with low cost light emitting diodes

It also suffers from multi-mode modal dispersion and this severely limits the usable bandwidth. As a result

it has not been widely used since the mid 1980s. Single mode fiber cable is the preferred type.

WDM is not normally used on multi-mode fiber

Driven by LEDs and most commonly used for short and medium length point-to-point transmission systems.

Multimode fiber gives you high bandwidth at high speeds (10 to 100MBS - Gigabit to 275m to 2km) over

medium distances. , in long cable runs (greater than 3000 feet [914.4 meters), multiple paths of light can

cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so

designers now call for single mode fiber in new applications using Gigabit and beyond.

3.3.2 Single mode fiber: (Mono/Uni-mode optical fiber, Single-mode optical waveguide)

Typically single mode fiber core are around eight to ten microns in diameter, much smaller than a hair. The

core diameter is almost equal to the wavelength of the emitted light. So that it propagates along a single

path.

No multi-modal dispersion means that it has a much wider bandwidth.

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WDM is normally employed in single mode fiber.

The main limitation to the bandwidth: Chromatic dispersion where different colors, i.e. Wavelengths

propagate at different speeds. Chromatic dispersion of the optical fiber cable occurs within the centre of

the fiber itself.

Note: It is found that it is negative for short wavelengths and changes to become positive at longer

wavelengths. As a result there is a wavelength for single mode fiber where the dispersions are zero. This

generally occurs at a wavelength of around 1310 nm and this is the reason why this wavelength is widely

used. (Obtained by solutions to Maxwell’s Equations and Bessel Functions)

The disadvantage of single mode fiber is that it requires high tolerance to be manufactured and this

increases its cost. Against this the fact that it offers superior performance, especially for long runs means

that much development of single mode fiber has been undertaken to reduce the costs.

Driven by a laser diode and is most often used for long distance telecommunications purposes.

3.4 Attenuation of Optical Fibres

The loss of power in light in an optical fiber is measured in decibels (dB). Fiber optic cable specifications express

cable loss as attenuation per 1-km length as dB/km. This value is multiplied by the total length of the optical

fiber in kilometers to determine the fiber's total loss in dB.

Optical fiber light loss is caused by a number of factors that can be categorized into extrinsic and intrinsic

losses:

3.4.1 Extrinsic

i) Bending loss: Bend loss occurs at fiber cable bends that are tighter than the cable's minimum bend radius.

Bending loss can also occur on a smaller scale from such factors as:

Sharp curves of the fiber core.

Displacements of a few millimeters or less, caused by buffer or jacket imperfections.

Poor installation practice

This light power loss, called microbending, can add up to a significant amount over a long distance

ii) Splice and connector loss: Splice loss occurs at all splice locations. Mechanical splices usually have the

highest loss, commonly ranging from 0.2 to over 1.0 dB, depending on the type of splice. Fusion splices have

lower losses, usually less than 0.1 dB. A loss of 0.05 dB or less is usually achieved with good equipment and

an experienced splicing crew. High loss can be attributed to a number of factors, including:

Poor cleave.

Misalignment of fiber cores.

An air gap. Contamination.

Index-of-refraction mismatch.

Core diameter mismatch

Losses at fiber optic connectors commonly range from 0.25 to over 1.5 dB and depend greatly on the type of

connector used. Other factors that contribute to the connection loss include:

Dirt or contaminants on the connector (very common).

Improper connector installation.

A damaged connector face.

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Poor scribe (cleave).

Mismatched fiber cores.

Misaligned fiber cores.

Index-of-refraction mismatch

3.4.2 Intrinsic

i) Loss inherent to fiber: Light loss in a fiber that cannot be eliminated during the fabrication process is due to

impurities in the glass and the absorption of light at the molecular level. Loss of light due to variations in

optical density, composition, and molecular structure is called Rayleigh scattering. Rays of light encountering

these variations and impurities are scattered in many directions and lost.

The absorption of light at the molecular level in a fiber is mainly due to contaminants in glass such as water

molecules (OH-). The ingress of OUT molecules into an optical fiber is one of the main factors contributing to the

fiber's increased attenuation in aging. Silica glass's (Si02) molecular resonance absorption also contributes to

some light loss.

ii) Loss resulting from fiber fabrication: Irregularities during the manufacturing process can result in the loss

of light rays. For example, a 0.1 percent change in the core diameter can result in a 10-dB loss per kilometer.

Precision tolerance must be maintained throughout the manufacturing of the fiber to minimize losses.

Figure 1 shows the net attenuation of a silica glass fiber and the three fiber operating windows at 850, 1310,

and 1550 nm. For long-distance transmissions, 1310- or 1550-nm windows are used. The 1550-nm window has

slightly less attenuation than 1310 nm. The 850-nm communication is common in shorter-distance, lower-cost

installations.

4. Advantages of Fiber Optics System

Fiber optic transmission systems – a fiber optic transmitter and receiver, connected by fiber optic cable – offer a

wide range of benefits not offered by traditional copper wire or coaxial cable. These include:

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1. Less expensive - Several miles of optical cable can be made cheaper than equivalent lengths of copper wire.

This saves your provider (cable TV, Internet, etc) and your money.

2. Thinner – (Light Weight) A fiber optic cable, even one that contains many fibers, is usually much smaller

and lighter in weight than a wire or coaxial cable with similar information carrying capacity. It is easier to

handle and install, and uses less duct space. (It can frequently be installed without ducts.)

3. Higher carrying capacity- This has the ability to carry much more information and deliver it with greater

fidelity than either copper wire or coaxial cable. This allows more phone lines to go over the same cable or

more channels to come through the cable into your cable TV box.

4. Less signal degradation - The loss of signal in optical fiber is less than in copper wire. The fiber is totally

immune to virtually all kinds of interference, including lightning, and will not conduct electricity. It can

therefore come in direct contact with high voltage electrical equipment and power lines. It will also not create

ground loops of any kind.

5. Light signals - Unlike electrical signals in copper wires, light signals from one fiber do not interfere with

those of other fibers in the same cable. This means clearer phone conversations or TV reception.

6. Low power - Because signals in optical fibers degrade less, lower-power transmitters can be used instead of

the high-voltage electrical transmitters needed for copper wires. Again, this saves your provider and your

money.

7. Digital signals - Fiber optic cable can support much higher data rates, and at greater distances, than coaxial

cable, making it ideal for transmission of serial digital data (information) in computer networks.

8. Non-flammable - Because no electricity is passed through optical fibers, there is no fire hazard. Moreover,

the only carrier in the fiber is light; there is no possibility of a spark from a broken fiber. Even in the most

explosive of atmospheres, there is no fire hazard, and no danger of electrical shock to personnel repairing

broken fibers.

9. No corrosion: As the basic fiber is made of glass, it will not corrode and is unaffected by most chemicals. It

can be buried directly in most kinds of soil or exposed to most corrosive atmospheres in chemical plants

without significant concern.

10. Secure: Fiber optic cable is ideal for secure communications systems because it is very difficult to tap but

very easy to monitor. In addition, there is absolutely no electrical radiation from a fiber.

In Compendium:

To give perspective to the incredible capacity that fibers are moving towards, a 10Gbps signal has the ability to

transmit any of the following per second:

1000 books

1,30,000 voice channels

16 HDTV channels or 100 HDTV channels using compression techniques. (An HDTV requires a much higher

bandwidth than today’s standard television)

Because of these advantages, you see fiber optics in many industries, most notably telecommunications and

computer networks. For example, if you telephone Europe from the United States (or vice versa) and the signal is

bounced off a communications satellite, you often hear an echo on the line. But with transatlantic fiber-optic

cables, you have a direct connection with no echoes.

How are fiber optic cables able to provide all of these advantages? This seminar will provide an overview of f iber

optic technology – with sections devoted to each of the three system components – transmitters, receivers, and the

fiber cable itself. An appreciation of the underlying technology will provide a useful framework for understanding

the reasons behind its many benefits.

5. Fiber Optic Transmission System:

A modern alternative to sending (binary) digital information via electric voltage signals is to use optical (light)

signals. Electrical signals from digital circuits (high/low voltages) may be converted into discrete optical signals

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(light or no light) with LEDs or solid-state lasers. Likewise, light signals can be translated back into electrical

form through the use of photodiodes or phototransistors for introduction into the inputs of gate circuits.

Transmitting digital information in optical form may be done in open air, simply by aiming a laser at a photo

detector at a remote distance, but interference with the beam in the form of temperature inversion layers, dust,

rain, fog, and other obstructions can present significant engineering problems:

One way to avoid the problems of open-air optical data transmission is to send the light pulses down an ultra-

pure glass fiber. Glass fibers will "conduct" a beam of light much as a copper wire will conduct electrons, with

the advantage of completely avoiding all the associated problems of inductance, capacitance, and external

interference plaguing electrical signals. Optical fibers keep the light beam contained within the fiber core by a

phenomenon known as total internal reflectance.

Recollect: An optical fiber is composed of two layers of ultra-pure glass, each layer made of glass with a slightly

different refractive index, or capacity to "bend" light. With one type of glass concentrically layered around a

central glass core, light introduced into the central core cannot escape outside the fiber, but is confined to travel

within the core:

Elements of Optical transmission:

Electrical-to Optical transducers

Optical media

Optical-to-Electrical Transducer

Digital signal processing, repeaters and clock recovery.

5.1 Fiber optic transmitter (Electrical to optical transducers)

In order that data can be carried along a fiber optic cable, it is necessary to have a light source or optical

transmitter. This fiber optic transmitter is one of the key elements of any fiber optic communications system and

the choice of the correct one will depend upon the particular application that is envisaged.

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How is an electrical AV signal converted into an optical AV signal?

An electrical AV signal is converted into an optical AV signal using an optical transmitter or an electrical-to-

optical converter. An optical transmitter uses a laser diode as the light source, varying the intensity of the laser

light in accordance with the electrical signal. For an analog signal, the intensity of the light source varies with

the voltage or current of the electrical signal. For digital signals, the light intensity is high or low, which

represents logical ones or zeros.

5.2 Optical Transmitter Specifications:

Power level: It is obvious that the fiber optic transmitter should have a sufficiently high level of light output for the

light to be transmitted along the fiber optic cable to the far end. Some fiber optic cable lengths may only be a few

meters or tens of meters long, whereas others may extend for many kilometers. In the case of the long lengths, the

power of the fiber optic transmitter is of great importance.

Type of light: Light can be split into two categories, namely coherent and incoherent light. Essentially, coherent light

has a single frequency, whereas incoherent light contains a wide variety of light packets all containing different

frequencies, i.e. there is no single frequency present. While some emitters may appear to emit a single color, they can

still be incoherent because the light output is centered on a given frequency or wavelength.

The frequency or wavelength of the light: Often fiber optic systems will operate around a given wavelength.

Typically the wavelength of operation is given.

The rate at which the transmitter can be modulated: this affects the data rate for the overall transmission. In

some instances low rate systems may only need to carry data at a rate of a few Mbps, whereas main

telecommunications links need to transmit data at many Gaps.

Types of Fiber optic transmitter summary:

In view of the different characteristics those LEDs and laser diode fiber optic transmitter’s posses they are used in

different applications. The table below summarizes some of the chief characteristics of the two devices.

CHARACTERISTIC LED LASER

DIODE

Cost Low High

Data rate Low High

Distance Short Long

Fiber type Multimode fiber Multimode and single mode fiber

Lifetime High Low

Temperature sensitivity Minor Significant

5.3 Line coding in Optical Transmission:

Some fiber systems use the line codes that are described for fiber applications

A few line codes are specifically developed for fiber applications like mBnB Line codes

Note that optical sources and detectors are primarily used in nonlinear modes of operation with significant gain

and threshold variations. They are best suited for on or off.

On-off keying is most natural for optical transmission.

5.4 Use of Multiplexers:

The transmission of multiple optical signals (channels) over the same fiber is a simple way to increase the

transmission capacity of the fiber against the fiber dispersion, fiber nonlinearity and speed of electronic

components which limit the bit rate. So, multiplexing techniques are followed to increase the bit rate.

Multiplexing means many signals at a given time.

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Suppose for each channel the bit rate is 100 Gb/s and by accommodating 100 channels through multiplexing

technique the total bit rate through a single fiber can be increased to 10 Tbps (1 Tera =1012). Thus the

information carrying capacity of a fiber is increased by the multiplexing technique. There are three types of

multiplexing techniques:

(i) TDM – time division multiplexing

(ii) FDM – frequency division multiplexing

(iii) WDM – wavelength division multiplexing

TDM and FDM techniques are operated in the electrical domain and are widely used in the conventional radio

wave communication. WDM technique is very useful in the optical domain and by WDM; the bit rate can be

increased beyond 10 Tb/s in the optical fiber communication.

Figure 9 shows the basic principle of WDM technique. Here different wavelengths carrying separate signals are

multiplexed by the multiplexer and then they are transmitted through a single fiber. At the receiver end, the

separate signals at different wavelengths are demultiplexed by the demultiplexer and are given to separate

receivers. From the receiver side also the signals can be transmitted in the same manner through the same fiber.

Thus instead of handling a single channel with single wavelength and limited bit rate (10 Gb/s), the bit rate is

raised to about 10 Tb/s, hence the information capacity of the fiber is increased by WDM technique.

5.5 SONET

SONET (Synchronous Optical Network) is a US standard for the internal operation of telephone company

optical networks. It is closely related to a system called SDH (Synchronous Digital Hierarchy) adopted by

the CCITT (now the ITU-T) as a recommendation for the internal operation of carrier (PTT) optical networks

worldwide.

Note: Despite the name, SONET is not an optical networking system. It is an electronic networking system

designed to use optical link connections.

SONET and SDH are of immense importance for two reasons:

i) They offer vast cost savings in public communications networks by redefining the system of channel

multiplexing. This is achieved through time division multiplexing of user data channels throughout

the network. SONET/SDH offers a significantly better method of doing this.

ii) Management of the cable plant. Within a typical telephone company there are many end-user service

offerings. Each of these is a network in its own rite (including and especially the telephone network).

Each of these networks needs link connections of various speeds connecting nodes (central offices)

at arbitrary points around the country. However the company wants to manage and share its cable

plant as a single entity.

Sonet Protocol Structure:

The basic structure in SONET is a frame of 810 bytes which is sent every 125 µsec. This allows a single byte

within a frame to be part of a 64 kbps digital voice channel. Since the minimum frame size is 810 bytes then the

minimum speed at which SONET will operate is 51.84 megabits per second.

810 bytes ×8000 frames/sec ×8 (bits) = 51.84 megabits/sec. This basic frame is called the Synchronous

Transport Signal level 1 (STS-1), which is an electrical signal. The diagrammatic representation of the frame as a

square is done for ease of understanding. The 810 bytes are transmitted row by row starting from the top left of

the diagram. One frame is transmitted every 125 µsec.

5.6 Fiber optic splicing

This method is similar to connectors. A fiber optic splice is defined by the fact that it gives a permanent or

relatively permanent connection between two fiber optic cables.

There are many occasions when fiber optic splices are needed. One of the most common occurs when a fiber

optic cable that is available is not sufficiently long for the required run. In this case it is possible to splice

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together two cables to make a permanent connection. As fiber optic cables are generally only manufactured in

lengths up to about 5 km, when lengths of 10 km are required, for example, then it is necessary to splice two

lengths together.

Fiber optic splices can be undertaken in two ways:

1) Mechanical splices:

The mechanical splices are normally used when splices need to be made quickly and easily.

Operation:

It is necessary to strip back the outer protective layer on the fiber optic cable, clean it and then perform a

precision cleave or cut. When cleaving (cutting) the fiber optic cable it is necessary to obtain a very clean cut, and

one in which the cut on the fiber is exactly at right angles to the axis of the fiber.

Once cut the ends of the fibers to be spliced are placed into a precision made sleeve. They are accurately aligned

to maximize the level of light transmission and then they are clamped in place. A clear, index matching gel may

sometimes be used to enhance the light transmission across the joint.

Mechanical fiber optic splices can take as little as five minutes to make, although the level of light loss is around

ten percent.

2) Fusion Splices:

This type of connection is made by fusing or melting the two ends together. This type of splice uses an electric

arc to weld two fiber optic cables together and it requires specialized equipment to perform the splice. Once the

fiber optic splice has been made, an estimate of the loss is made by the fiber optic splicer. This is achieved by

directing light through the cladding on one side and measuring the light leaking from the cladding on the other

side of the splice.

The equipment that performs these splices provides computer controlled alignment of the optical fibers and it is

able to achieve very low levels of loss, possibly a quarter of the levels of mechanical splices. However this comes

at a process as fusion welders for fiber optic splices are very expensive.

Mechanical and fusion splices:

The two types of fiber optic splices are used in different applications. The mechanical ones are used for

applications where splices need to be made very quickly and where the expensive equipment for fusion splices

may not be available. Some of the sleeves for mechanical fiber optic splices are advertised as allowing connection

and disconnection. In this way a mechanical splice may be used in applications where the splice may be less

permanent.

Fusion splices offer a lower level of loss and a high degree of permanence. However they require the use of the

expensive fusion splicing equipment. In view of this they tend to be used more for the long high data rate lines

that are installed that are unlikely to be changed once installed.

5.7 Optical Couplers

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5.8 Need for Repeaters:

A light pulse emitted by the LED taking a shorter path through the fiber will arrive at the detector sooner

than light pulses taking longer paths. The result is distortion of the square-wave's rising and falling edges,

called pulse stretching. This problem becomes worse as the overall fiber length is increased:

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However, if the fiber core is made small enough (around 5 microns in diameter), light modes are restricted

to a single pathway with one length. Fiber so designed to permit only a single mode of light is known

as single-mode fiber. Because single-mode fiber escapes the problem of pulse stretching experienced in

long cables, it is the fiber of choice for long-distance (several miles or more) networks. The drawback, of

course, is that with only one mode of light, single-mode fibers do not conduct as much light as multimode

fibers. Over long distances, this exacerbates the need for "repeater" units to boost light power.

5.9 Optical Regenerator:

As mentioned above, some signal loss occurs when the light is transmitted through the fiber, especially over long

distances (more than a half mile, or about 1 km) such as with undersea cables. Therefore, one or more optical

regenerators are spliced along the cable to boost the degraded light signals.

An optical regenerator consists of optical fibers with a special coating (doping). The doped portion is "pumped"

with a laser. When the degraded signal comes into the doped coating, the energy from the laser allows the doped

molecules to become lasers themselves. The doped molecules then emit a new, stronger light signal with the

same characteristics as the incoming weak light signal. Basically, the regenerator is a laser amplifier for the

incoming signal.

5.10 Fiber optical Receiver

Once data has been transmitted across a fiber optic cable, it is necessary for it to be received and converted into

electrical signals so that it can be processed and distributed to its final destination. The fiber optic receiver is the

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essential component in this process as it performs the actual reception of the optical signal and converts it into

electrical pulses. Within the fiber optic receiver, the photo detector is the key element

A variety of semiconductor photo-detectors may be used as fiber optic receivers. They are normally

semiconductor devices, and a form of photo-diode. A variety of diodes may be used in fiber optic receivers,

namely p-n photodiode, a p-i-n photodiode, or an avalanche photodiode. Metal-semiconductor-metal (MSM)

photo detectors are also used in fiber optic receivers on occasions as well.

Overall receiver

Although the photo-detector is the major element in the fiber optic receiver, are the other elements to the whole

unit. Once the light has been received by the fiber optic receiver and converted into electronic pulses, the signals

are processed by the electronics in the receiver. Typically these will include various forms of amplification

including a limiting amplifier. These serve to generate a suitable square wave that can then be processed in any

logic circuitry that may be required.

Once in a suitable digital format the received signal may undergo further signal processing in the form of a clock

recovery, etc. This will undertaken before the data from the fiber optic receiver is passed on.

Diode performance

One of the keys to the performance of the overall fiber optic receiver is the photodiode itself. The response times

of the diodes govern the speed of the data that can be recovered. Although avalanche diodes provide high speed

they are also noisier and require a sufficiently high level of signal to overcome this.

The most common type of diode used is the p-i-n diode. This type of diode gives a greater level of conversion than

a straight p-n diode as the light is converted into carriers in the region at the junction, i.e. between the p and n

regions. The presence of the intrinsic region increases this area and hence the area in which light is converted.

6. How are optical Fibers made?

Now that we know how fiber-optic systems work and why they are useful -- how do they make them? Optical

fibers are made of extremely pure optical glass. We think of a glass window as transparent, but the thicker the

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glass gets, the less transparent it becomes due to impurities in the glass. However, the glass in an optical fiber

has far fewer impurities than window-pane glass. One company's description of the quality of glass is as follows:

If you were on top of an ocean that is miles of solid core optical fiber glass, you could see the bottom clearly.

Making optical fibers requires the following steps:

1. Making a preform glass cylinder

2. Drawing the fibers from the preform

3. Testing the fibers

6.1 Making the Preform Blank

MCVD process for making the preform blank

The glass for the preform is made by a process called modified chemical vapor deposition (MCVD).

In MCVD, oxygen is bubbled through solutions of silicon chloride (SiCl4), germanium chloride (GeCl4) and/or

other chemicals. The precise mixture governs the various physical and optical properties (index of refraction,

coefficient of expansion, melting point, etc.). The gas vapors are then conducted to the inside of a synthetic

silica or quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up and down the outside

of the tube. The extreme heat from the torch causes two things to happen:

Lathe used in preparing the preform blank

The silicon and germanium react with oxygen, forming silicon dioxide (SiO2) and germanium dioxide

(GeO2).

The silicon dioxide and germanium dioxide deposit on the inside of the tube and fuse together to form

glass.

The lathe turns continuously to make an even coating and consistent blank. The purity of the glass is

maintained by using corrosion-resistant plastic in the gas delivery system (valve blocks, pipes, seals) and by

precisely controlling the flow and composition of the mixture. The process of making the preform blank is highly

automated and takes several hours. After the preform blank cools, it is tested for quality control (index of

refraction).

6.2 Drawing Fibers from the Preform Blank

Once the preform blank has been tested, it gets loaded into a fiber drawing tower.

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Diagram of a fiber drawing tower used to draw optical glass fibers from a preform blank

The blank gets lowered into a graphite furnace (3,452 to 3,992 degrees Fahrenheit or 1,900 to 2,200 degrees

Celsius) and the tip gets melted until a molten glob falls down by gravity. As it drops, it cools and forms a thread.

The operator threads the strand through a series of coating cups (buffer coatings) and ultraviolet light curing

ovens onto a tractor-controlled spool. The tractor mechanism slowly pulls the fiber from the heated preform

blank and is precisely controlled by using a laser micrometer to measure the diameter of the fiber and feed the

information back to the tractor mechanism. Fibers are pulled from the blank at a rate of 33 to 66 ft/s (10 to 20

m/s) and the finished product is wound onto the spool. It is not uncommon for spools to contain more than 1.4

miles (2.2 km) of optical fiber.

Finished spool of optical fiber

6.3 Testing the Finished Optical Fiber

The finished optical fiber is tested for the following:

Tensile strength - Must withstand 100,000 lb/in2 or more

Refractive index profile - Determine numerical aperture as well as screen for optical defects

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Fiber geometry - Core diameter, cladding dimensions and coating diameter are uniform

Attenuation - Determine the extent that light signals of various wavelengths degrade over distance

Information carrying capacity (bandwidth) - Number of signals that can be carried at one time (multi-

mode fibers)

Chromatic dispersion - Spread of various wavelengths of light through the core (important for bandwidth)

Operating temperature/humidity range

Temperature dependence of attenuation

Ability to conduct light underwater - Important for undersea cables

Once the fibers have passed the quality control, they are sold to telephone companies, cable companies and

network providers. Many companies are currently replacing their old copper-wire-based systems with new fiber-

optic-based systems to improve speed, capacity and clarity.

7. How are major undersea cables laid in the ocean?

Laying of cables in the oceans of our world is a fascinating business. Real men and women toil long and tedious

hours to make this possible.

Submarine cables are laid down by using specially modified ships (sometimes even purpose built ships) that

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carry the submarine cable on board and slowly lay it out on the seabed as per the charts/plans given by the

cable operator. The ships can carry with them up to 2,000 kilometers length of cable.

Submarine Cable Map

Source: Submarine Cable Map 2014

Depending on the equipment on-board the cable-ship, the type of plough used, the sea conditions and the ocean-

bed where the cable is being laid-down, cable ships can do anywhere from 100-150km of cable laying per day.

Newer ships and ploughs now do about 200 km of cable laying per day.

The ships are commonly referred to as cable-layers or cable-ships.

The cables are specially constructed for submarine operations as they have to endure harsh conditions as well as

pressure.

Cable Dissection

Here is what a typical 3-D cross-sectional cut-out of a submarine cable looks like:

1. Polyethylene

2."Mylar" tapes

3. Stranded metal (steel) wires

4. Aluminum water barrier

5. Polycarbonate

6. Copper or aluminum tube

7. Petroleum jelly

8. Optical fibers

These fiber optic cables carry DWDM laser signals (TCP/IP packets etc.) at a rate of terabytes per second.

They use optical repeaters to strengthen the signal which attenuates over long distances. These are

powered by copper cables shown above.

They have a decade lifespan and costs vary (depending on the length of the cable). Typical costs for

projects are anywhere from US$ 100 Million to $500 Million.

20

Appendix 3 - Do private telecommunications companies own the undersea cables that connect the internet across

continents?

We don't use satellites because they can't carry terabytes of data for less than a billion dollars per

communication line.

In real-life the cable would look like this:

Here is another look...

Depending on where the cable is being laid, it might differ in thickness. Thinner cable systems are used for

shallower ocean depths, whilst thicker cables are used for deep ocean beds, typically up to 20,000 feet. Such

cables are able to withstand pressure from 12,000 lbs/square-inch to 22,000 lbs/square-inch (this is necessary

because of the extreme pressures in the deep ocean beds.

Submarine cable laying process starts from the Landing Station, where a long cable section is attached

(connected) to the landing-point and then extended out to a few miles in the sea. This end is connected to the

cable on the ship and then the ship starts its cable laying process (a simple representation of this process can be

read here: Appendix 4

This is how the cable approaching the landing station looks like (notice the cable laying ship in the

horizon):

21

Depending on the geography of where the cable is laid out, the cable coming in from the ocean to the landing

station might be advertised or not. Most of the time, it is buried as much as it can be and warning signs are

placed so as to inform everyone that a submarine cable is landing ashore. Most of the time cable consortium

companies try to hide the cable as much as they can, so that only those who need to know, are informed of the

exact route of the cable. This would include municipalities, port authorities and shipping companies.

The market for submarine cables is dominated by Europe (UK, Italy, France, and Germany) and a bit by Japan.

US is overall a small player when compared to the others, as US itself did not have much need to expand cables

to other countries, as much as the other countries had a need to connect to the US.

The ships, which are specialized, are almost all owned by the submarine cable consortium or manufacturers.

These ships are stationed at various points along where the cable extends to ensure that in the event of a cable-

cut, the ships can set sail immediately for cable repairs.

A cable laying ship at port

Cable coiled up in the cargo-hold (the coiling of 100s of miles of cable in the cargo hold is a process that

can take 3 to 4 weeks to complete.

22

Another picture of Fiber Optic Cable being carefully wound and rolled into the cargo hold.

Another submarine cable laying ship at port

Cable Landing Install Ship (that connects the Landing Cable to the Ocean Cable)

The portion of the ship from where the cables are lowered into the sea

23

Cable turntable - the turntable is slowly unwinding the cable and lowering it on to the ocean floor.

A cable laying ship at sea. Notice the cable being lowered onto the sea bed, on the right-hand (starboard)

side (white portion) at the rear end of the ship.

Here is a photograph of a repeater being launched into the sea (which is placed every 40-60 kilometers)

to fix and strengthen the fiber-optic signal and to amplify it, etc. These are powered by the copper

cables which are wrapped around the fiber optic cable as shown below.

24

The process also involves a plough. The cable is not simply left to just sit on the ocean bed, but is actually being

fed into a plough, that is laying the cable in the trench.

Here is the process as seen of submarine cable laying (with the plough)

Here is another perspective of the same (you can see how the cable is being fed into the trench)

Here is the cable-plough on shore, being slowly pulled to the ocean

25

Some of these ploughs are hydraulically assisted and most use water jets to easy trench clearing.

Here is another smaller cable plough

A submarine cable diver inspecting a submarine cable.

When cables are damaged, either divers or specialized small submersibles with cameras and lights are sent down

26

to the seabed to investigate where the cuts are. Then, either the divers or robotic arms on the submersible bring

the two ends of the cable to the surface, where they are re-spliced and joined again.

Here is an example of a submarine cable robot that is lowered onto the sea bed to retrieve each end of the broken /

damaged / sliced cable and send it to the surface ship for repairs

Different cable types (by Alcatel)

Work on Submarine Cable between Malta & Sicily Underway

27

NKT Cables Opens Logistics Center in the Port of Rotterdam (Netherlands)

Dhiraagu (The Telecom Company for Maldives) laying submarine cable across Maldives

Submarine Cable ship, with the Cable turntable nearly empty

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8. Bibliography

http://www.radio-electronics.com/info/telecommunications_networks/fiber-fibre-

optics/communications-basics-tutorial.php

http://computer.howstuffworks.com/fiber-optic.htm

http://www.cableorganizer.com/articles/fiber-optics-tutorial/history-production-fiber-

optic.html

http://www.allaboutcircuits.com/vol_4/chpt_14/5.html

http://www.gatewayforindia.com/technology/opticalfiber.htm

http://www.ias.ac.in/pramana/v57/p849/fulltext.pdf

http://www.extron.com/company/article.aspx?id=foddgga#4

http://www.ad-net.com.tw/index.php?id=472

http://www.quora.com/How-are-major-undersea-cables-laid-in-the-ocean

http://www.k-kcs.co.jp/english/s...).

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9. APPENDIX – 1

Total Internal Reflection (TIR)

When light passes from a medium with one index of refraction (m1) to another medium with a lower index of

refraction (m2), it bends or refracts away from an imaginary line perpendicular to the surface (normal line). As

the angle of the beam through m1 becomes greater, (with respect to the normal line) the refracted light through

m2 bends further away from the normal line.

At one particular angle called critical angle (θ) the refracted light will not go into m2, but instead will travel

along the surface between the two media

NOTE: sine (θ) = n2/n1, where n1 and n2 are the indices of refraction [n1>n2].

If the beam through m1 is greater than the critical angle(θ), then the refracted beam will be reflected

entirely back into m1 (called TIR), even though m2 may be transparent!

In physics, the critical angle is described with respect to the normal line. In fiber optics, the critical angle is

described with respect to the parallel axis running down the middle of the fiber. Therefore, the fiber-optic critical

angle = 90º – θ.

In an optical fiber, the light travels through the core (m1, high index of refraction) by constantly reflecting from

the cladding (m2, lower index of refraction) because the angle of the light is always greater than the critical

angle(θ). Light reflects from the cladding no matter what angle the fiber itself gets bent at, even if it's a full circle!

Because the cladding does not absorb any light from the core, the light wave can travel great distances. However,

some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the

signal degrades depends upon the purity of the glass and the wavelength of the transmitted light (for example,

850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km).

Some premium optical fibers show much less signal degradation -- less than 10 percent/km at 1,550 nm.

30

APPENDIX – 2

Other Types of Fibers: Two additional types of fiber – very large core diameter silica fiber and fiber made

completely of plastic – are normally not employed for data transmission. Silica fiber is typically used in

applications involving high-power lasers and sensors, such as medical laser surgery. All-plastic fiber is useful for

very short data links within equipment because it may be used with relatively inexpensive LEDs. An isolation

system for use as part of a high voltage power supply would be a typical example of an application for plastic

fiber.

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Appendix 3

Do private telecommunications companies own the undersea cables that connect the internet across continents? Yes, the cables are owned by the various PTT or Transit carriers who would be utilizing (tapping) into the cable system. These companies are referred to as the O&M (Operations and Maintenance) companies of the cable

system.

As an example, the SEA ME WE 4 (South East Asia, Middle East and Western Europe) 4 cable system is owned

by the following telecommunication companies:

Algérie Télécom, Algeria

Bharti Infotel Limited, India

Bangladesh Submarine Cable Company Limited (BSCCL), Bangladesh

CAT Telecom Public Company Limited, Thailand

Emirates Telecommunication Corporation (ETISALAT), UAE

France Telecom - Long Distance Networks, France

MCI, UK

Pakistan Telecommunication Company Limited, Pakistan

Singapore Telecommunications Limited (SingTel), Singapore

Sri Lanka Telecom Limited (SLT), Sri Lanka

Saudi Telecom Company (STC), Saudi Arabia

Telecom Egypt (TE), Egypt

Telecom Italia Sparkle S.p.A., Italy

Telekom Malaysia Berhad (TM), Malaysia

Tunisie Telecom, Tunisia

Tata Communications previously Videsh Sanchar Nigam Limited (VSNL), India

Each of these companies pooled in their respective share into the cable system and was allotted specific

bandwidth rights on the cable system, for fixed fees and contributes as per their shareholding for the

maintenance and upkeep of this cable system. The in turn make money, by selling either access to this cable

system or services/accessibility to their own clients.