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                            The Fiber Optic Backbone

                                 Tim J. Morgan

                            University of North Texas
The Fiber Optic Backbone


Introduction

In 1880 Alexander Graham Bell and his assistant Charles Sumner Tainter created a very

early precursor to fiber-optic communications, the Photophone, at Bell's newly

established Volta Laboratory in Washington, D.C. Bell considered it his most important

invention. The device allowed for the transmission of sound on a beam of light. On June

3, 1880, Bell conducted the world's first wireless telephone transmission between two

buildings, some 213 meters apart. Due to its use of an atmospheric transmission medium,

the Photophone would not prove practical until advances in laser and optical fiber

technologies permitted the secure transport of light. The Photophone's first practical use

came in military communication systems many decades later. (Wikipedia 2011)

Fiber Optic communication systems have become the backbone of the Internet and

telecommunications systems around the world. The fiber Optic system provides

bandwidth measured in hundreds of Giga bits per second. The super- broad band of

optical fiber allows Internet service providers to offer multiplicity of multimedia

applications to consumer and business’. Optical Cable can contain up to several hundred

individual fibers with diameters measured in microns. Optical cable has been installed

virtually world-wide. The optical cables stretched across oceans and continents

connecting networks and allowing data to flow at super high rates around the world.

       Fiber Optic communication systems work by transmitting optical signals over

optical fiber. Fiber optic transmission systems all use data links that consists of a

transmitter coupled to one end of a fiber and a receiver coupled on the other end.

Generally Fiber optics systems are configured by transmitting on one fiber- in one

direction and transmitting from the opposite direction on another fiber to provide full
The Fiber Optic Backbone


duplex transmission. Semiconductor LEDs or lasers are used for optical transmitters and

receivers use semiconductor photo detectors.



       Advantages of Fiber Optics

       Less expensive - Several miles of optical cable can be made cheaper than

equivalent lengths of copper wire. Thinner - Optical fibers can be drawn to smaller

diameters than copper wire. Higher carrying capacity - Because optical fibers are thinner

than copper wires, more fibers can be bundled into a given-diameter cable than copper

wires. This allows more phone lines to go over the same cable or more channels to come

through the cable into your cable TV box. Less signal degradation - The loss of signal in

optical fiber is less than in copper wire. 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 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 you money. Digital signals - Optical fibers are ideally suited for carrying

digital information, which is especially useful in computer networks. Non-flammable -

Because no electricity is passed through optical fibers, there is no fire hazard.

Lightweight - An optical cable weighs less than a comparable copper wire cable.

        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
The Fiber Optic Backbone


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

optic cables, you have a direct connection with no echoes.

       Optical fibers are strands of glass consisting of pure silica. As Light travels inside

a fiber, the light is prevented from escaping by fibers Total Internal Reflection (TIR). TIR

is created by materials having different refractive indices. The refractive index

determines the speed of the wave inside the fiber.



       The rate at which light travels in the fiber is determined by refractive index. The

internal reflection of optical fibers is produced by having the optical fiber with an inner

core of high index silica glass surrounded by silica glass of slightly lower index. A

number of techniques are used to obtain the index difference between the core and

cladding.

       Step index optical fiber has a core with a different index than the index of the

cladding. Air has refractive index of 1, water is 1.33, and common glass has a refractive

index of approximately 1.45. Having the core consist of a higher index, surrounded by

the cladding of lower index, increase total internal reflection. The core and cladding are

made-up of silica. Doping the core with a Germanium increases the total reflection

dramatically. Three properties define optical fiber, core radius, core index and the

cladding index.

       A mode of light has specific propagation properties as it travel inside the optical

fiber. The mode propagation is affected by the physical properties within the optical fiber

such as the core index, diameter of the core, cladding index the wavelength. The mode’s

wavelength is unaffected by the length of the optical fiber.
The Fiber Optic Backbone


       Single mode fiber is general used outside the plant and cables have very high fiber

counts that can exceed 288 fibers. Single mode fiber can only accept a single mode

because of its small core diameter.

        Multimode optical fiber has a larger core diameter and can carry many modes

and is generally used in premises applications.

       As the light travels through optical fiber, power is power is lost because of

scattering and absorption. These two factors are directly related to wavelength, resulting

in losses peaking between 1250 nm and 1400 nm. These losses are due to presence of

moisture during the manufacturing process. Power losses are at the minimum around

between 1300 nm and 1500 nm. Optical fiber communication systems general transmit

on these two wavelengths

       Dispersion is a major limiting factor affecting optical fiber transmission. The

pulse broadening caused by dispersion applies to both single mode and multimode optical

fibers. In multimode fiber the light travels through the optical fiber at different speeds,

resulting in a significant difference in arrival time at the receivers As soon as the pulse

enters the optical fiber, echoes of the pulse will appear simultaneously. The echoes of the

pulse will reach the receiver at different times and will be broader. The dispersion of the

pulse is caused by asynchronous nature of the pulse echoes in multimode optical fiber.

       The dispersion in multimode optical fibers limits the data rate of transmission. As

the transmission distance increases, pulse echoes transmitted broaden and are unable to

be filtered by the receiver.

       Dispersion exists in single mode optical fibers and is caused by the dispersion

properties of the refractive index of glass. A material is defined as dispersive if the speed
The Fiber Optic Backbone


is wavelength dependent. Replicas of the pulses now correspond to the individual

wavelengths that constitute the spectrum of the source; the output pulse will certainly be

broadened. The pulse broadening in single mode optical fibers will be much smaller than

the corresponding value in multimode optical fibers.

       There are two types of dispersion in single mode optical fiber. One is the

dispersion properties of glass, determined by the fibers properties. The other is

waveguide dispersion which is determined by the fibers index profile and the core

diameter. Dispersion restricts the rate at data at high rates over long distances. As the

distance product increases, we will be dealing with a weaker signal as the transmission

distance

        The attenuated signal increases the error rate due to the signal-to-noise ratio

outside the tolerance needed for proper performance. The optical signal requires

detection; pulses are reshaped, and retransmitted. This is done by using a repeater at on

long hauls

       As the pulses travel down the optical fiber, they broaden and start overlapping. It

becomes difficult to separate the two adjoining pulses, increasing the error at the receiver.

Reducing the data-rate-distance product will help alleviate the problem. However, this

reduces our ability to transmit data at higher rates over long distances. We will see that

we can either use a repeater or a solution based communication system to overcome the

limitations imposed by dispersion.

       The bandwidth of optical fiber can be dramatically increased by Wavelength

division multiplexing (WDM) technology the wavelength division multiplexing allows

multiple light modes to be combined and launched into the optical fiber at the same time.
The Fiber Optic Backbone


The receiver filters the combined signals and is converted into the individual wavelength

signals. Much like a prism can filter white light and separates it into the primary colors

and the reverse, it can take the primary colors combine them to form white light.

       Summary

       Optical fiber communications system has provided the ultra-broadband long haul

ta system capable of meeting the need of high bandwidth dependent applications and

increasing number of users. As broadband access expands it will require a backbone

capable of meeting the demands of the growing number of 4G enabled devices. The

advent of streaming on-line rentals anytime anywhere will place even more load on

backbone s. The grow enrollment in distance education programs means more student on-

line. The increase in bandwidth on the fiber optic backbone means we can watch movies

at home and use Skype at the same time.
The Fiber Optic Backbone




      References

      Fiber Optic Communications access August 7, 2011

             http://en.wikipedia.org/wiki/Fiber-optic_communication

      Fiber Optic Basics accessed August 8, 2011

             http://www.thefoa.org/tech/ref/basic/nets.html

       Fiber Optic Communication Systems accessed August 8, 2011

             www.gatewaycoalition.org/files/NewEH/htmls/Shankar.doc

				
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