signal degradation in optical communication

7/28/2019 SIGNAL DEGRADATION IN OPTICAL COMMUNICATION

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TN402 OPTICAL COMMUNICATIONS

LECTURE 5:

- Signal Degradation In Optical Fibres

- Attenuation

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- Signal Distortion in Fibres

- Dispersion

- Material Dispersion

- Dispersion-shifted fibers

- Plastic Optical Fibers


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Signal Degradation In Optical Fibres

What are the loss or signal attenuation mechanisms in

a fibre?

Why and to what degree do optical signals get

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distorted as they propagate along a fiber?

Signal attenuation is one of the most important

properities of optical fibre because it largely

determines the maximum unmaplified or repeaterless

separation between a Tx and Rx. Attenuation has

large influence on system cost


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Signal Degradation In Optical Fibres

Of equal importance is signal distortion. Distorion

causes optical signal pulses to broaden as they travel

along a fibre.

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For long distances travel, spread pulses will

eventually overlap with neighbouring pulses thereby

creating errorsin the receiver output

The signal distortion mechanism thus limit the

information carrying capacity of a fibre


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Attenuation

The basic attenuation mechanisms of optical energyare:

Absorption, Scattering and Radiative losses

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Scattering is associated both with the fibre materiaand with structural imperfections in the optical

waveguide

Radiative losses orginate from pertubations (bothmicrpscopic and macrosopic) of the fibregeometry


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As light travels along a fiber, its power decreasesexponentially with distance.

If P(0) is the optical power in a fiber at the orgin (z

= 0), then the power P(z) at a distance z further

Attenuation Unit

( ) (0) pz

P z P e

1 (0)ln

( )p

P

z P z

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Where p is attenuation coefficient and isgiven by:


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For simplicity in calculating optical signalattenuation in fibre, the common procedure is toexpress the attenuation coefficient in units of

dB/km

Attenuation Unit

10

10 (0)( / ) log

( )

PdB km

z P z

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z is length in kilometers. Therefore, the unit ofattenuation is dB/km


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Attenuation Unit

This parameter is generally referred to as thefibre loss or the fibre attenuation and it dependson several parameters

Fibre Attenuation Coefficient is:

p scattering absorption bending

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In a typical system, the total loss could be 20 30dB before it needs amplification

So, at 0.2dB/km, this corresponds to a distanceof 100 150km


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Fibre losses depend on the wavelength oftransmitted light

The loss spectrum exhibits a strong peak near

Attenuation

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. .

A secondary minimum is found to occur near1.3m.

Since fibre dispersion is also minimum near1.3m, this low-loss window was used forsecond-generation lightwave systems


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Fibre losses are higher for shorter wavelengths and

exceed 5dBkm in the vision region, making it

unsuitable for long-haul transmission

Attenuation

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Several factors contribute to overall losses.

The two most important among them are material

absorption and Rayleigh scattering


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Optical signal pulses broadens due to distortion

effects as it travels along a fibre. Eventually

neighbouring pulses will overlap

Signal Distortion in Fibres

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After a certain amount of overlap, receiver can no

longer distinguish the individual adjacent pulses

and errors arise when interpreting the received

signal

Pulse spreading in an optical fiber is cased by

Dispersion


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Dispersion, expressed in terms of the symbol t, is

defined as pulse spreading in an optical fiber.

Dispersion

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As a pulse of light propagates through a fiber,

elements such as numerical aperture, core

diameter, refractive index profile, wavelength, andlaser line-width cause the pulse to broaden


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Dispersion

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Dispersion

( )out in

t t t

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Is measured in time (nanoseconds or picoseconds).Total dispersion is a function of fiber length. Thelonger the fiber, the more the dispersion.

( / )total

t L Dispersion km


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Dispersion

The overall effect of dispersion on the performanceof a fiber optic system is known as intersymbolinterference

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rate of change of the input exceeds the dispersionlimit of the fiber, the output data will becomeimperceptible.


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Dispersion

Dispersion is generally divided into:

- Modal dispersion (Intermodal Dispersion)

- Chromatic dispersion (Intramodal Dispersion)

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- -

Modal dispersion is defined as pulse spreading

caused by the time delay between lower-ordermodes (modes or rays propagating straight through

the fiber close to the optical axis) and higher-order

modes (modes propagating at steeper angles)


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Dispersion

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Modal dispersion (Also called Multipath Dispersion)

is problematic in multimode fiber, causingbandwidth limitation, but it is not a problem insingle-mode fiber where only one mode is allowedto propagate


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A pulse of light sent into a fiber broadens in time as

it propagates through the fiber. This phenomenon is

known as pulse dispersion

Pulse Dispersion in Step-Index Fibers (SIF)

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Pulse dispersion is caused by:

- intermodal dispersion

- material dispersion

- waveguide dispersion


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Dispersion

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The rays making larger angles with the axis(those shown as dotted rays) have to traverse alonger optical path length and therefore take alonger time to reach the output end


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Dispersion

Consequently, the pulse broadens as it propagates

through the fiber. Where the output pulses are not

resolvable, no information can be retrieved.

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Thus, the smaller the pulse dispersion, the greater

will be the information-carrying capacity of the

system.


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Dispersion

The intermodal dispersion for a step-index fibercan be derived as follows.

For a ray making an angle with the axis, the

1

1 1

cos

cosAB

ABn ABAC CB

t c c cn n

21

.


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Dispersion

where c/n1 represents the speed of light in amedium of refractive index n1, c being thespeed of light in free space.

1

cosL

n Ltc

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,taken by a ray to traverse a length L of the fiberwould be:


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Dispersion

If we assume that all rays lying between = 0and = c = cos

-1(n2/n1) are present, the timetaken by the extreme rays for a fiber of length L

would be given by:

1min

n Lt

c

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2

1max

2

n Lt

cn

corresponding to rays at = 0

corresponding to rays at

c = cos-1(n2/n1)


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Dispersion

Hence, if all the input rays were excited

simultaneously, the rays would occupy a time

interval at the output end of duration

2

1 1 1max min

2 2

1i

n L n Lnt t

c n cn

24


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21n L L

Dispersion

Finally, the intermodal dispersion in a multimodeSIF is:

12i

c n c

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The quantity represents the pulse dispersion

due to different rays taking different times inpropagating through the fiber, which, in waveoptics, is nothing but the intermodal dispersionand hence the subscript i

i


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Dispersion

We can see that the pulse dispersion isproportional to the square ofNA.

Thus, to have a smaller dispersion, one must

10 sin NA

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ave a sma er NA, w ic o course re uces t eacceptance angle and hence the light-gatheringpower

Acceptance Angle is given by:


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1i

B

Fibre Capacity

Typically fibre capacity is specified in terms ofthe Bit rate-Distance Product (BL)

Where BL is the bit rate (B) times the possible

transmission distance (L)

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In order for neighboring signal pulses to remaindistinguishable at the receiver, the pulse spreadshould be less than 1/B which is a width of a bitperiod.

In general model delay


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2

1 1 1

max min 1in L n Ln

t t

Fibre Capacity

Therefore using:

2

2

1

n cBLn

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The bit rate-distance product is:


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Pulse Dispersion in Parabolic-Index Fibers(PIF)

In a parabolic-index (Graded Index) fiber, the

refractive index in the core decreases

continuously (in a quadratic fashion) from a

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max mum va ue a e cen er o e core o a

constant value at the core-cladding interface.

Since the refractive index decreases as one movesaway from the center of the core, a ray entering

the fiber is continuously bent toward the axis of

the fiber


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Dispersion

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Dispersion

This follows from Snell's law because the ray

continuously encounters a medium of lower

refractive index and hence bends continuously

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away rom e norma .

Even though rays making larger angles with the

fiber axis traverse a longer path, they do so in aregion of lower refractive index (and hence

greater speed).


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Dispersion

The longer path length is almost compensatedfor by a greater average speed such that all raystake approximately the same amount of time intraversing the fiber.

2

422 1 2 2

3

2 12 2 8im

n L n n n L LNA

c n c cn

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This leads to a much smaller pulse dispersion.The final result for the intermodal dispersion ina parabolic-index fiber (PIF) is given by:


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Dispersion

Note that, as compared to a step-index fiber,the pulse dispersion is proportional to thefourth power of NA. For a typical (multimodeparabolic-index) fiber with n2 =1.45 and =0.01, L = 1km, we would get

0.25 / im ns km

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For a typical (multimode) step-index fiber, if we

assume n1 = 1.5, = 0.01, L = 1 km, we wouldget

50 /i ns km


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Dispersion

Comparing the two we find that for a parabolic-index fiber the pulse dispersion is reduced by afactor of about 200 in comparison to a step-

index fiber.

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This is why first- and second-generation opticalcommunication systems used near-parabolic-index fibers.

To further decrease the pulse dispersion, it isnecessary to use single-mode fibers


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Chromatic Dispersion

Chromatic dispersion is pulse spreading due to the

fact that different wavelengths of light propagate at

slightly different velocities through the fiber.

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All light sources, whether laser or LED, have finite

linewidths, which means they emit more than one

wavelength.

Because the index of refraction of glass fiber is a

wavelength-dependent quantity, different

wavelengths propagate at different velocities.


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Chromatic Dispersion

Chromatic dispersion is typically expressed inunits of nanoseconds or picoseconds per (km-nm).

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material dispersion and waveguide dispersion.

tchromatic = tmaterial + twaveguide


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Material Dispersion

Material dispersion is due to the wavelength

dependency on the index of refraction of glass

The dependence of the refractive index on

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wavelength leads to what is known as dispersion


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Material Dispersion

Consider a narrow pencil of a white light beamincident on a prism.

The incident white light

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will disperse into itsconstituent colors.

The dispersion willbecome more evidentat the second surfaceof the prism


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Material Dispersion

Since the refractive index of glass depends on thewavelength, the angle of refraction will be differentfor different colors.

For exam le for crown lass the refractive indices

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at 656.3 nm (orange), 589.0 nm (yellow), and486.1 nm (green) are respectively given by1.5244, 1.5270, and 1.5330.

Thus, if the angle of incidence i= 45 the angle ofrefraction, r, will be r= 27.640, 27.580, and 27.470

for the orange, yellow, and green colorsrespectively.


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cv

Material Dispersion

From the velocity of light in a medium expressiongiven by:

n

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Here n is the refractive index of the medium,which, in general, depends on the wavelength

V is usually referred to as the phase velocity.However, a pulse travels with what is known asthe group velocity, which is given by


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g

cv

n

Material Dispersion

where ng is known as thegroup index and, in mostcases its value is slightly

larger than n

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In the following Table n, ng and Dm aretabulated for pure silica for different valuesof wavelength lying between 700 nm and1600 nm


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Material Dispersion

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1270nm


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Material Dispersion

Variations ofn andng with wavelength for puresilica are given in the following graph.

Notice that ng has a minimum value of around

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.

1270 nm is usually referred to as the zero

material dispersion wavelength, and it is because

of such low material dispersion that the optical

communication systems shifted their operation to

around 1300 nm.


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Material Dispersion

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Material Dispersion

Every source of light has a certain wavelengthspread, which is often referred to as the spectralwidth of the source.

Thus a white li ht source like the sun has a

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spectral width of about 300 nm.

On the other hand, an LED has a spectral width ofabout 25 nm and a typical laser diode (LD)operating at 1300 nm has a spectral width of about2 nm or less.


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Each wavelength component will travel with aslightly different group velocity through the fiber.

This results in broadening of a pulse. This

Material Dispersion

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length of the fiber and to the spectral width of thesource.

Material dispersion coefficient Dm, which ismeasured in ps/km-nm.


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Material Dispersion

Dm represents the material dispersion inpicoseconds per kilometer length of the fiber pernanometer spectral width of the source.

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,characteristic of the material and is (almost) thesame for allsilica fibers.

A negative Dm implies that the shorterwavelengths travel faster; similarly, a positivevalue of Dm implies that longer wavelengthstravel faster.


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L dnn

Material Dispersion

The group delay resulting from material dispersionis given by:

c d

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Therefore material dispersion is an intramodal

dispersion effect and is of particular importancefor single-mode waveguides and for LEDsystems because of broader output spectrumthan a laser diode


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Material Dispersion

The group delay resulting from materialdispersion is also given by:

m m

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Where Dm is absolute value of Materialdispersion and is light source spectral width


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Material Dispersion

Example 1:

In the optical communication systems one uses

= 850 with LED having a spectral width of about

m m

D L

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20 nm. Thus, for a 1-km length of the fiber, thematerial dispersion becomes:

= 84.2 (ps/km-nm) x 1 (km) x 20 (nm) = 1.7 ns


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Material Dispersion

Example 2:

If example 1 is repeated for =1310nm, thedelay will be:

m mD L

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= 2.4 (ps/km-nm) x 1 (km) x 20 (nm) = 0.05ns


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Material Dispersion

Example 3:

In the optical communication systems that are

in operation today, one uses laser diodes (LD)

m mD L

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about 2 nm. Thus, for a 1-km length of thefiber, the material dispersion becomes:

= 21.5 (ps/km-nm) x 1 (km) x 2 (nm) = 43 ps


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Waveguide Dispersion

Waveguide dispersion is due to the physicalstructure of the waveguide

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because only part of the optical powerpropagation along a fibre is confined to the core

Fraction of light power propagating in thecladding travels faster than the light confined tothe core since the index is lower in the cladding


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The amount of waveguide dispersion dependson the fibre design

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Waveguide dispersion usually can be ignored inmultimode fibres but its effect is significant insingle-mode fibres

Material dispersion and waveguide dispersioncan have opposite signs depending on thetransmission wavelength.


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In the case of a step-index single-mode fiber,these two effectively cancel each other at1310 nm, yielding zero-dispersion.

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This makes very high-bandwidth communicationpossible at this wavelength. However, thedrawback is that, even though dispersion isminimized at 1310 nm, attenuation is not.

Glass fiber exhibits minimum attenuation at1550 nm.


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Coupling that with the fact that erbium-dopedfiber amplifiers (EDFA) operate in the 1550-nmrange makes it obvious that, if the zero-

dispersion property of 1310 nm could be shifted-

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window, high-bandwidth long-distancecommunication would be possible.

With this in mind, zero-dispersion-shifted fiberwas developed.


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Polarization-Mode Dispersion (PMD)

Light signal energy at a given wavelength ina single-mode fibre occupies two orthogonalstates or modes

At the start of the fibre the two polarization

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states are aligned


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Where the two different polarizations of light in

a waveguide, which normally travel at the same

speed, travel at different speeds due to random

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mper ec ons an asymme r es, caus ng

random spreading ofoptical pulses.

Although single-mode fiber can sustain only onetransverse mode, it can carry this mode with

two different polarizations


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Slight imperfections or distortions in a fiber canalter the propagation velocities for the twopolarizations.

This phenomenon is called fiber birefringence

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and can be counteracted by polarization-maintaining optical fiber


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Birefringence creates differing optical axes thatgenerally correspond to the fast and slow axes.

Birefringence causes one polarization mode to

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,in the propagation time called Differential GroupDelay (DGD)

DGD is the unit used to escribe PMD. DGD ismeasured in picoseconds


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1

2 2 2 2 2

Total Dispersion

When considering the total dispersion fromdifferent causes, we can approximate the totaldispersion as:

1 2 3......

total n

n

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Where represents the dispersion due to

the various components that make up thesystem


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Total Dispersion

In a fiber, the pulse dispersion is caused, ingeneral, by intermodal dispersion, materialdispersion, and waveguide dispersion.

However, waveguide dispersion is important only

2 2total i m 63

in single-mode fibers and may be neglected incarrying out analysis for multimode fibers.

Considering multimode fibers the total dispersion

is given by


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Total Dispersion

The graph shows the material chromatic andwavelength dispersions for single-mode fiber

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Total Dispersion

The approximate bandwidth of a fiber can be

related to the total dispersion by the following

relationship

.total

BW

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Dispersion-shifted fibers

We have learned that the attenuation of a silicafiber attains its minimum value of about 0.2 dB/kmat around = 1550 nm.

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- -communication systems operated around = 1300 nm, where the dispersion was extremelysmall but the loss was about 1 dB/km

Therefore the repeater spacing was limited by theloss in the fiber.


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Since the lowest loss lies at around =1550 nm,if the zero-dispersion wavelength could be shiftedto the = 1550-nm region, one could have bothminimum loss and very low dispersion.

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This would lead to very-high-bandwidth systemswith very long repeater spacings (~ 100 km)

Extremely efficient optical fiber amplifiers capableof amplifying optical signals in the 1550-nm bandhave also been developed. Thus, shifting theoperating wavelength from 1310 nm to 1550 nmwould be very advantageous


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By reducing the core size and increasing the

value of ,the zero-dispersion wavelength can

be shifted to 1550 nm, which represents the

-

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The current fourth-generation optical

communication systems operate at 1550 nm,

using dispersion-shifted single-mode fibers with

repeater spacing of about 100 km, carrying

about 10 Gbit/s of information


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Variations of dispersions for a typicaldispersion-shifted single-mode fiber (DSF)


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Plastic Optical Fibers (POF)

Plastic optical fibers are made from materials such

as polymethyl methacrylate PMMA (n = 1.49),

polystyrene (n = 1.59), polycarbonates, fluorinated

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, .

These fibers share the advantages of glass optical

fibers in terms of insensitivity to electromagneticinterference, small size and weight, low cost, and

potential capability of carrying information at high

rates.


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The most important attribute of POFs is their largecore diameters of around 1mm as compared toglass fibers with cores of 50 mm or 62.5 mm.

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joints.

They are also more durable and flexible than glass

fibers.

In addition, they usually have a large NA, resultingin larger light-gathering power


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Plastic optical fibers' performance lies somewherebetween conventional copper wires and glassoptical fibers.

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electromagnetic interference.

By comparison, plastic optical fibers are cheaper

and are free from interference.

In addition, signals through copper wires can betapped while it is very difficult to tap signals fromoptical fibers.


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Compared to glass fibers, POFs are much easier toconnect because of their large diameters.

Coupling of light from a source is also very

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.

Thus, although glass optical fibers dominate long-distance data communication, POFs are expected

to provide low-cost solutions to short-distanceapplications such as local area networks (LAN) andhigh-speed Internet access.


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At gigabit rates of transmission, glass fibers areat least 30% more expensive than POFs, whilethe cost of copper increases dramatically.

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Attenuation is one of the important parameters ofan optical fiber.

POF has three low-loss windows, at 570 nm,650 nm, and 780 nm as shown in the graphs


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Typical attenuation spectra of 1-mm-diameter (a) step-index (SI) and (b) graded-index (GI) PMMA plastic fiber


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The loss of SI POF at the 650-nm window is about110 dB/km. This is, of course, very large comparedto silica fibers, which have typical losses of about afew dB/km in this wavelength region.

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The large losses are due to Rayleigh scattering,intrinsic absorption of the material itself, andimpurities and absorption due to vibrational modes

of the molecules.

Because of the high losses, these fibers are used inonly short-distance (a few hundred meters)communication links.

signal degradation in optical communication

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