Ch2 Optical Fiber

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Ch2 Optical communication

The telecommunications technology continuous to evolve and the need for

transmission media that satisfy the need of high transmission speed and capacity

leads to use optical ber rather than previous transmission medium such as coaxial

cables.

The optical ber consist core, cladding and coat. The core is the medium where the

light is propagatein, which is made from pure glass (silica) with specic refractive

index, the cladding is made from silica too but with less refractive index.

A ray of light travels more slowly in an optically dense medium than in one lessdense. Refraction occurs when a ray is incident on the interface between twodielectrics of diering refractive indices (eg. !lass and air) . As shown in gure".#(a) ,the ray approaching the interface is propagating in a dielectric of refractiveindex n#and angle $#to the normal of the surface of the interface. %uppose the

dielectric on the other side has a refractive index n"which is less than n#, then therefraction is such that the ray path in this medium at angle $"to the normal , where$"is greater than $#.

&igure ".#'ight rays incident on a high to low refractive index interface (a)refraction (b) the limiting case of refraction showing the critical ray at an angle *c(c) total internal reflection where *+*c.


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The angles of incidence and reection are related to each other by %nell-s law whichstates that

n#sin *#+ n"sin *"

or

sin *# sin *"+ n" n#

As n# is greater than n" ,the refraction angle is always greater than the incidenceangle. /hen the angle of refraction is 01 ,the refracted ray propagates hori2ontallyin parallel to the interface between dielectrics , the angle of incidence must be lessthan 01 , this limit is 3nown as critical angle and has the symbol *c . Referring to%nell-s law the value of the critical angle is given by 456

sin*c+ n" n#

/hen a ray is incident on a dielectric of lower index from a dielectric of higher indexand the angle of incidence of the ray exceeds the critical value then a totalinternal refection will occur476.

8n order to achieve the total internal reection in the core of the ber so the ray willpropagate in a correct way through the core without refraction and at minimum loss,the ray must enter the core with an angle less than the acceptance angle, this anglewhich is shown in gure "." ta3es a shape of cone so it9s called Acceptance :one.

&igure "." The acceptance angle ;a when launching light into an optical ber .4n")#"


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?r it might be given in terms of the relative refractive index difference @ betweenthe core and the cladding

=A+n#(" @)#"

/here

@+n#">n"""n#"

2.1optical ber system

As shown in the gure ".#, the transmission system of optical ber consist ofencoding, transmitting and receiving. The message is sent through electrical cablesto the encoder which converts the message to ono digiti2ed pulses of light. Thedecoder receives the digiti2ed pulses and converts the light wave to electricalwaves, then reconstructed to the original message.

&igure ".# asic :omponents of ?ptical &iber %ystem.


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2.2 light source

The main purpose of the light source is to convert the electrical signals into optical

signals to be transmitted through the ber. There are two maBor types of light

sources that widely used in optical communications , light emitting diode and laser

diode .

Light emitting diode

The light emitting diode ('CD) is cheap , popular and used in short range

communications that wor3 at low bit rates ( up to #"


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a current in the external circuit. 8n the I8= photodiode, the depleted region is made

as large as possible. A lightly doped intrinsic layer separates the more heavily

doped p>types and n>types. The diode-s name comes from the layering of these

materials positive, intrinsic, negative I8=.

!alanche photo diode

The free electrons and holes created by absorbed photons, accelerate, gainingseveral electron Jolts of 3inetic energy. A collision of these fast carriers with neutralatoms causes the accelerated carriers to use some of their own energy to help thebound electrons brea3 out of the valence shell. &ree electron>hole pairs, calledsecondary carriers, appear. :ollision ioni2ation is the name for the process thatcreates these secondary carriers. As primary carriers create secondary carriers, thesecondary carriers themselves accelerate and create new carriers. :ollectively, thisprocess is 3nown as photomultiplication.

Also, AIDs are very temperature sensitive, further complicating circuitreGuirements. 8n general, AIDs are only useful for digital systems because they

possess very poor linearity. ecause of the added circuit complexity and the high

voltages that the parts are subBected to, AIDs are always less reliable than I8=

detectors. This, added to the fact that at lower data rates, I8= detector>based

receivers can almost match the performance of AID>based receivers, ma3es I8=

detectors the rst choice for most deployed low>speed systems. At multi !bit data

rates, however, AIDs rule supreme.

2." modes

There are two types of propagation mode in ber optics cable which are multi>mode

and single>mode. These provide dierent performance with respect to both

attenuation and time dispersion. The single>mode ber optic cable provides the

better performance at a higher cost.

The number of modes in a ber optic cable

depends upon the dimensions of the cable and the variation of the indices of

refraction of both core and cladding across the cross section. There are three

principal possibilities which are multi>mode step index, single>mode step index and

multi>mode graded index.


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#ingle mode step inde$

%ingle>mode propagation is illustrated in gure "." . This diagram corresponds tosingle>mode propagation with a refractive index prole that is called step index. Asthe gure shows, the diameter of the core is fairly small relative to the cladding.ecause of this, when light enters the ber>optic cable on the left, it propagatesdown toward the right in Bust a single ray, a single mode, which is the lowest>ordermode. 8n extremely simple terms, this lowest>order mode is conned to a thincylinder around the axis of the core. The higher>order modes are absent.

&igure "." %ingle>mode step index.

%ulti mode step inde$

:onsider the illustration in &igure ".5 . This diagram corresponds to multimodepropagation with a refractive index prole that is called step index. As you can see,

the diameter of the core is fairly large relative to the cladding. There is also a sharpdiscontinuity in the index of refraction as you go from core to cladding. As a result,when light enters the ber>optic cable on the left, it propagates down toward theright in multiple rays or multiple modes. This yields the designation multimode. Asindicated, the lowest>order mode travels straight down the center. 8t travels alongthe cylindrical axis of the core. The higher modes, represented by rays, bounce bac3and forth, going down the cable to the left. The higher the mode, the more bouncesper unit distance down to the right.

&igure ".5Eultimode step index.


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%ulti mode graded inde$

Eultimode graded index ber has a higher refractive index in the core thatgradually reduces as it extends from the cylindrical axis outward. The core andcladding are essentially a single graded unit. :onsider the illustration in &igure ".7 .

This corresponds to multimode propagation with a refractive index prole that iscalled graded index. Kere the variation of the index of refraction is gradual as itextends out from the axis of the core through the core to the cladding. There is nosharp discontinuity in the indices of refraction between core and cladding. The corehere is much larger than in the single>mode step index case previously discussed.Eultimode propagation exists with a graded index. As illustrated, however, thepaths of the higher>order modes are somewhat conned. They appear to follow aseries of ellipses. ecause the higher>mode paths are conned, the attenuationthrough them due to lea3age is more limited than with a step index. The timedispersion is more limited than with a step index therefore, attenuation and time

dispersion are present, but limited.

&igure ".L Eultimode graded index

2.& attenuation and dispersion

Attenuation is a maBor problem in the optical transmission system which isdened as how much power loss in the optical ber and is normally

expressed in dMm. %everal factors can cause attenuation, but it is

generally categori2ed as either intrinsic loss due to the impurities in the

glass or extrinsic due to external forces such as bending.

Dispersion describes how the optical ber deforms the light pulses travelling

through the ber and is one of the most important factors limiting the bandwidth.

To achieve a reliable transmission of light an optical ber with low attenuation anddispersion is used.


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Ch 3 D'D%

The optical communications continuous to evolve and the need to increase the

transmission speed and capacity conseGuently , this leads to use a new techniGue as

/DE.

'oo3ing bac3 to the previous methods of increasing transmission capacity andspeed , installing a new cables or increasing system bit rate to multiplex moresignals were used. As technology evolve these techniGues were not suitable for theincreasing need of speed and capacity so wdm is developed .

/avelength division multiplexing (/DE) is a technology which multiplexes a number

of optical carrier signals onto a single optical ber by using dierent wavelengths. 8tenables bidirectional communications over one strand of ber, and a multiplicationof the capacity of transmission . 8n other words , 8t is a techniGue for using a ber (oroptical device) to carry many separate and independent optical channels, eachchannel consists of light of a dierent color. Thus a /DE system transmits aNrainbowO. Actually at the wavelengths involved the light is invisible but it9s a goodway of describing the principle.47


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2.1wdm system

The transmission system consists of optical transmitters, optical multiplexer, optical

Pemultiplexer, ampliers and receivers. The optical multiplexer multiplexes the light

from the optical sources , the amplier amplies the optical signal and the

Demultiplexer separate the optical signal bac3 to the original wavelengths. As shownin gure.

&igure wdm system

2.2D'D%

D/DE refers originally to optical signals multiplexed within the #


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These are ampliers placed every F1 to #11 3ilometers to compensate for loss ofoptical power amplication is done by an CD&A, usually consisting of severalamplier stages. D/DE Terminal Demultiplexer This device brea3s the multi>wavesignal bac3 into individual signals. ?ptical %upervisory :hannel (?%:) This channelcarries information about the multi>wave optical signal and may provide data aboutconditions at the site of the intermediate line repeater (component " above). D/DEis sometimes called wave division multiplexing (/DE) and /DE is growing denser asthe technology evolves.

etween multiplexing and demultiplexing points in a D/DE system, there is an areain which multiple wavelengths exist. 8t is often necessary to remove or insert one ormore wavelengths at some point along this span. An optical adddrop multiplexer(?ADE) performs this removal or insertion function, gure. Rather than combining orseparating all wavelengths, the ?ADE can remove one while passing others on.

%ince D/DE system is very eective for dispersion and attenuation, erbium>dopedber ampliers (CD&As) are used to overcome this problems. CD&As are silica based

optical bers that are doped with erbium. This rare earth element has theappropriate energy levels in its atomic structure for amplifying light at #to>

noise ratio and hence on the error rate of the system.:rosstal3 is usually

Guoted as the Nworst caseO condition, this is where the signal in one channel

is right at the edge of its allowed band.

*re+uency Dri,t

D/DE systems have to maintain stable wavelength or freGuency because ofthe closer spacing of the wavelengths. Irecision temperature control of laser


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transmitter is reGuired in D/DE systems to prevent UdriftU o a very narrow

freGuency window of the order of a few !K2.There are another impairments

that eect the D/DE system li3e cross phase modulation, fourwave

mixing ,noise, dispersion, scattering ..etc.

3." (-/- 0rid

An 8TS>T standard is a threshold set by the international telecommunication union

for the telecommunication industry to ensure eHcient service delivery among its

members. The 8TS>T is the primary body that is tas3ed with creating cooperative

standards for rms in the telecommunications eGuipment and systems industry. &or

/DE system interoperability, the operating center freGuency (wavelength) of

channels must be the same at the transmitting end and the receiving end. The 8TS>T

currently recommends F# channels in the : band starting from #


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191.900 1562.23


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Ch2 Optical Fiber

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