d. Lecture 3 - Fiber Losses and Dispersion 8 March 06

7/29/2019 d. Lecture 3 - Fiber Losses and Dispersion 8 March 06

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Fiber Losses and Dispersion

By

Dr Neena GuptaAsstt Prof , E&Ec Deptt

Punjab Engineering College

Chandigarh


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Capacity Growth of Optical

Fiber Each Year

Year Capacity (Gb/s)

1980 0.11985 1

1990 3

1995 5

2000 100 (40 practically shown)

2005 1,000 (If limitations due toDispersion &Nonlinearities areovercome)


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The optical world is

approaching towards

1. 50 THz Transmission Window

1000 Channel WDM

100 Gb/s TDM

1000 km Repeaterless transmission

If Nonlinearities can be controlled,

transmission window will be 300THz


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Fiberoptic Communication Systems

What limits the transmission

capability?

Attenuation

Dispersion

Non- Linearities in fibers

Curve &Bending losses

Radiation & Leaky Modes


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Signal Attenuation & Distortion in

Optical Fibers

What are the loss or signal attenuation mechanism in afiber?

Why & to what degree do optical signals get distorted asthey propagate down a fiber?

Signal attenuation (fiber loss) largely determines themaximum repeaterless separation between opticaltransmitter & receiver.

Signal distortion cause that optical pulses to broaden asthey travel along a fiber, the overlap between neighboring

pulses, creating errors in the receiver output, resulting inthe limitation of information-carrying capacity of a fiber.


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Causes of Attenuation

Atomic Absorption of light photons

Scattering of light by flaws and impurities

Reflection of light by splices & Connectors


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Attenuation (fiber loss)

Power loss along a fiber:

The parameter is called fiber attenuation coefficient in a units of for

example [1/km] or [nepers/km]. A more common unit is [dB/km] that isdefined by:

Z=0

P(0) mW

Z= llpePlP

)0()( mw

zpePzP

)0()(

p

]km/1[343.4)(

)0(log

10]dB/km[ p

lP

P

l


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Fiber loss in dB/km

Where [dBm] or dB milliwat is 10log(P[mW]).

z=0 Z=l

]dBm)[0(P

]km[]dB/km[]dBm)[0(]dBm)[( lPlP


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Optical fiber attenuation vs. wavelength


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Wavelength Windows

Attenuation

~ 200 ppb OH

OH

Peaks

First

Window

ThirdSecond

Rayleigh

scattering

1

4( )dbKm[ ]

Infra-red

Absorbtion

Attenuation


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Absorption Loss

z=0 z=L

Attenuation


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Absorption

Absorption is caused by three different mechanisms:

1- Impurities in fiber material: from transition metal ions (must

be in order of ppb) & particularly from OH ions with

absorption peaks at wavelengths 2700 nm, 400 nm, 950 nm &725nm.

2- Intrinsic absorption (fundamental lower limit): electronic

absorption band (UV region) & atomic bond vibration band

(IR region) in basic SiO2.3- Radiation defects


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Light Ray Scattering


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Scattering Loss

Small (compared to wavelength) variation in material density, chemical

composition, and structural inhomogeneity scatter light in other directionsand absorb energy from guided optical wave.

The essential mechanism is the Rayleigh scattering. Since the black body

radiation classically is proportional to (this is true for wavelength

typically greater than 5 micrometer), the attenuation coefficient due toRayleigh scattering is approximately proportional to . This seems to

me not precise, where the attenuation of fibers at 1.3 & 1.55 micrometer

can be exactly predicted with Plancks formula & can not be described

with Rayleigh-Jeans law. Therefore I believe that the more accurate

formula for scattering loss is

4

4

1

5)exp(

Tk

hc

B

scat

eTemperatur:,JK103806.1Js,10626.6-12334 Tkh B


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Absorption & scattering losses in fibers


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Optical Connectors


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Reflection of light by splices


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Bending Loss (Macrobending &

Microbending) Macrobending Loss: The

curvature of the bend is muchlarger than fiber diameter.Lightwave suffers sever loss

due to radiation of theevanescent field in thecladding region. As the radiusof the curvature decreases,the loss increasesexponentially until it reaches

at a certain critical radius. Forany radius a bit smaller thanthis point, the losses suddenlybecomes extremely large.Higher order modes radiateaway faster than lower ordermodes.


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Macro- & Micro-bending Loss


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Microbending Loss

Microbending Loss:

microscopic bends of the

fiber axis that can arisewhen the fibers are

incorporated into cables.

The power is dissipated

through the microbended

fiber, because of the

repetitive coupling ofenergy between guided

modes & the leaky or

radiation modes in the

fiber.


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Available Bandwidth with

Optical Fiber


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Stimulated Inelastic Scattering

Stimulated Raman Scattering

Stimulated Brillouin Scattering


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Stimulated Inelastic Scattering

Elastic scattering means no energy is

transferred from electromagnetic field to the

dielectric mediumInelastic scattering: Optical field transfers

part of its energy to the non linear medium.


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1) Effect and consequences SRS causes a signal wavelength to behave as a pump for longer

wavelengths, either other signal channels or spontaneously scatteredRaman-shifted light. The shorter wavelengths is attenuated by this process,which amplifies longer wavelengths

SRS takes place in the transmission fiber

2) SRS could be exploited as an advantage By using suitable Raman Pumps it is possible to implement a Distributed

Raman Amplifier into the transmission fiber. This helps the amplificationof the signal (in co-operation with the localized EDFA). The pumps aredepleted and the power is transferred to the signal

f fTransmission Fiber


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SRS

Interaction of photons with optical phonons

Scattering of a photon into a photon of

lower frequency (by ~15 THz) that canpropagate in the forward direction plus a

phonon.

The co-propagating downshifted photons

are amplified by the signal and can cause

crosstalks with other WDM channels


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It transfers a part of pump energy to a

counter propagating stokes wavesdownshifted by a frequency of (10 GHz) the

BW is 10 MHz.


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It severely hits the performance of WDM systemsby transferring energy from one channel toneighboring channels.

It converts fraction of (10-6) of input power todownshifted waves (stokes waves).

The energy from a pump wave is transferred to a

Stokes wave (downshifted by 13THz) as the pumpwave propagates through the optical fiber B.W. is40 THz. This happens when the pump power>threshold level


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(SBS)1) Effect and consequences In an intensity modulated system using a source with

narrow linewidth, significant optical power is

transferred from the forward propagating signal to abackward propagating signal when the SBS powerthreshold (around 5 to 10 mW) is exceeded

This limits the amount of light transmitted down the

fiber SBS takes places in the transmission fiber


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Difference between SRS & SBS

Optical phonons participate in SRS

Acoustic Phonons participate in SBS SRS dominates in Forward direction

SBS dominates in Reverse direction


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SBS

Interaction of photons with acoustic phonons

Scattering of a photon into a photon of lower frequency (by ~11GHz) that propagates in the opposite direction plus a phonon.

Brillouin bandwidth is narrow, 20 MHz

The backward propagating downshifted photons are amplified

with distance exponentially

This process reduces SNR because

Signal strength is reduced

Random SBS process introduces noise

Typical thresholds:

Proportinal to Aeff/Leff, laser linewidth, modulation pattern,etc.

4.2 mW for a narrow CW source for SMF 28

2.6 mW for dispersion shift fiber


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Dispersion

Dispersion cause pulse broadening and distortion.


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Fiber Dispersion

Fiber Loss

- 0.35 dB/Km at 1.3mm

- 0.2 dB/Km at 1.5mm

- Minimum Reduction Expected in future

is 0.01dB/Km

Fiber Dispersion

-Material dispersion

- Waveguide Dispersion

- Multimode group Delay Dispersion


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atic Dispersion

(GVD)


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Dispersion.. As pulses travel down the fiber, they spread

and overlap.

This produces inter symbol interference and

makes it difficult to separate the pulses, i.e., itbecomes difficult separate the data bitsincreasing the bit error rate.

This will limit either the data rate (moreseparation between the pulses) or the

maximum distance. Thus, dispersion limits the data transmission

capability in terms of Gbit/s.Km. (data ratedistance product)


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Pulse Spreading due to

Dispersionz=0 z=L

Dispersion


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Dispersion

The optical pulse tend to spread as it propagates down thefiber generating Inter-Symbol-Interference (ISI) andtherefore limiting either the bit rate or the maximum

achievable distance at a specific bit rate Physics behind the effect

The refractive index has a wavelength dependent factor,so the different frequency-components of the optical

pulses are traveling at different speeds

Bit 1 Bit 2Bit 1 Bit 2Bit 1 Bit 2

Bit 1 Bit 2Bit 1 Bit 2


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Cause of Fiber Dispersion

Material

Dispersion

Types of Dispersion

Multimode

Dispersion

Waveguide

Dispersion

- Multimode group delay/dispersion is the variation in

group velocity among the propagation modes at a single

frequency

- Material Dispersion is due to variation in the refractive

index of the core material as a function of wavelength.

- Waveguide dispersion depends upon the fiber design.

The propagation constant which is the function of the ratio offiber dimension i.e. core radius to the wavelen th.


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Dispersion in Optical Fibers

There are two main types of dispersion that cause

pulse spreading in a fiber:

- Chromatic dispersion

- Inter-modal dispersion Dispersion is typically measured as a time spread per

distance traveled (s/km)

Single-mode fiber has only one mode, so inter-modal

dispersion is not an issue In multimode fiber, inter-modal dispersion is the

dominant cause of dispersion, but chromatic

dispersion can be important at 850 nm


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Types of Dispersion in Optical

Fiber


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time

amplitude

amplitude

time

am

plitude

time

am

plitude

time

(a) (b)

(c) (d)


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Fiberoptic Communication Systems

Dispersion.

Dispersion from multimodes.each

mode travels with its own velocity Dispersion in single mode fibersthe

refractive index is a function of thewavelength and we do not have singlewave length sourcesall sources have afinite spectral width


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Slide 10 of 53
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Refractive index

can be increased by adding P2O5 or Germanium Oxide

can be decreased by adding flourine


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Definitions:Refractive index & Propagation constant

Relationship between frequency and wavelength

in electromagnetic radiations: c =x

The refractive index (n) of a material is the ratio ofthe speed of light in the vacuum to the speed of

light in that material: n=c/v.

The propagation speed of an electromagneticradiation depends on the refractive index and the

wavelength and it is determined by the propagation

constant: =2n/


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Chromatic Dispersion

The speed of light is dependent on therefractive index

c = c0/ n

where c0 is the speed of light in a vacuum

The index of refraction, n, varies with thelight transmission wavelength

All light sources (LEDs and LDs) have somecoloration, or variation, in wavelengthoutput

The low wavelength portion of the pulsetravels slower than the high wavelength one

creating pulse spreading


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(continued) Chromatic dispersion is measured in units of

time divided by distance and Tx sourcespectral width(ps/nm-km)

It is zero near 1310 nm in silica optical fibers

It is zero near 1550 nm in Dispersion Shiftedoptical fibers

Even at the dispersion zero, there is somepulse spreading due to the spectral width ofthe light source

Ch ti Di i i


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Chromatic Dispersion in

Optical FiberA high-speed pulse contains a spectrum of l components


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Chromatic Dispersion Problem


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t

Spread,

t0

Spectrum,

12o

Intensity Intensity Intensity

Cladding

CoreEmitter

Very short

light pulse

vg

(2)

vg

(1

)Input

Output

All excitation sources are inherently non-monochromatic and emit within a

spectrum,

, of wavelengths. Waves in the guide with different f ree spacewavelengths t ravel at different group velocities due to t he wavelengt h depenofn1. T he waves arrive at the end of the fiber at different times and hence r

a broadened output pulse.

1999 S.O. Kasap,Optoelectronics(Prentice Hall)

Material Dispersion



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Definitions

Fib Att ti d Ch ti


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Fiber Attenuation and Chromatic

Dispersion

0.1

0.2

0.3

0.4

0.5

0.6

Attenuation(dB/km)

1600 1700140013001200 15001100

Wavelength (nm)

Dispersion-

unshifted

FiberDispersion-

shifted

Fiber

20

10

0

-10

-20 Dispersion(ps/nmkm)

EDFA

band

TrueWave

Fiber

Attenuation

(all Fiber types)

+ Dispersion

Input Pulse Output Pulse

P l i ti M d


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Polarization Mode

Dispersion (PMD)


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PMD

PMD affects fiber optics communication by

spreading the pulse over the distance.

PMD increases the bit error rate hence

limits the transmission distance and

bandwidth of the system.

It causes distortion and limits the no. of

channels transmitted.


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Importance of Higher order

DispersionIf pulse width coincides the zero dispersion

wavelength and 20, the 3 playsimportant role in GVD effects , Forultra short pulses with width less than

0.1 ps it is necessary to include higher

order terms.Typical value of3 =0.1ps3/Km.


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Effect of higher Order

DispersionThe effect of3 is to distort the pulse shape

in such a way that the pulse becomes

asymmetric with oscillations at the edges

For 3 >0 oscillations are at the. trailingedge

For 3


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Nonlinear Effects in WDM


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Stimulated Brillouin scattering

Stimulated Raman scattering

Self-phase modulation

Cross-phase modulation

Four-wave mixing

Nonlinear Effect in the Fiber

N Li Eff t


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Non Linear Effects:

SPM, XPM, FWM

Fiber nonlinearity is caused by the intensity-dependent nonlinearchange in refraction index of the fiber: n = (n0 + n2 I)

In single channel systems, the signal intensity of a given channel

modulates its own refractive index, and therefore its phase. This effectis called Self Phase Modulation (SPM)

In Multi-channel WDM systems, all the other interfering channels alsomodulate the refractive index of the channel under consideration, andtherefore its phase. This effect is called Cross Phase Modulation(XPM)

In Multi-channel WDM systems, intermodulation (or mixing) productsare generated between the WDM channels, as the nonlinearity isquadratic with electric field. This effect is called Four Wave Mixing

(FWM)


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Self phase modulation

Effect and consequences

The refractive index of the fiber is not constant, but has

an Intensity ( I ) dependent factor

The temporal variation of the intensity

of the signal induces a modulation of its

phase. SPM will gradually broaden the

signal spectrum. Once spectrumbroadening is introduced, the signal

experiences a greater temporal

broadening due to chromatic dispersion


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Non Linear Effects:

Self Phase Modulation (SPM)

SPM acts as a negative chirp, which increases with

increasing channel power level and system length

SPM causes a spectral broadening of the optical pulses andthus reduces the dispersion tolerance of the system

At 10 Gbps, its penalty is minimized by distributing

dispersion compensation at each line amplifier site

If dispersion is compensated only at the terminal ends,there will be a residual penalty due to SPM


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SPM

Refractive index is intensity dependent

High pulse intensity

Short pulses

Impairment comes from dispersion

Need to use dispersion-shifted fiber or

dispersion compesation at the receiver Probable spectral crosstalks for WDM

adjacent channels


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Self Phase Modulation

Due to intensity dependent of Refractive

Index in Nonlinear media and leads to

spectral broadening of optical pulses.


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XPM Same physical origin as SPM

Particular for WDM systems

Intensity variations of one pulse alter the phase ofanother channel via nonlinear refractive index of

glass spectral broadening As two pulses (different channels) traverse each

other, one pulses time varying intensity profile

will cause a frequency shift in the other


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Cross Phase Modulation

XPM is always accompanied by self phase

modulation and occurs because of the

refractive index of a wave depends not onlyon the intensity of that wave but also on the

intensity of the co-propagating waves.

XPM is responsible for asymmetricspectrum broadening of co propagating

optical pulses


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Non Linear Effects:

Cross Phase Modulation

XPM acts as a crosstalk penalty, which increases with

increasing channel power level and system length and with

decreasing channel spacing XPM causes a spectral broadening of the optical pulses and

thus reduces the dispersion tolerance of the system

At 10 Gbps, its penalty is minimized by distributing

dispersion compensation at each line amplifier site If dispersion is compensated only at the terminal ends,

there will be a residual penalty due to XPM

FWM


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FWM

Nonlinear interaction between severaldifferent channels in a WDM system

When two waves are interacted, two otherEM waves are generated that isproportional to the cube of the vector sumof the E fields.

Dispersion-shifted fiber for 1.55 micronsignals 25km

N channels N2(N-1)/2 side bands arecreated, causing

Reduction of signals

Interference Cross talk

Typical transmission today uses non-zerodispersion shifted fiber at 1-2ps/nm/km.

2f1-f2 f1 f2 2f1+f2

Frequency


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800 900 1000 1100 1200 1300 1400 1500 1600

UV Absorption

OH- Absorption Peaks in

Actual Fiber Attenuation Curve

Rayleigh

Scattering IR Absorption

Wavelength in Nanometers (nm)

0.2 dB/Km

0.5 dB/Km

2.0 dB/Km

Loss (dB)/km vs. WavelengthS-Band:14601530nm

L-Band:15651625nm

C-Band:15301565nm

Loss Management: Problem

Fiber Attenuation


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Dispersion Management: Problem

Increasing the Bit Rate Higher Bit Rates experience higher signaldegradation due to Chromatic Dispersion:

OA10Gb/s Dispersion

16 Times Greater

Dispersion Scales as (Bit Rate)2

Time Slot

OA2.5Gb/s Dispersion

1)

Dispersion Management: Solution


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Dispersion Compensation

In the Normal(1)

Dispersion RegimeShorter Wavelengths Travel Slower

(BLUE Is SlowerThan RED)

(1) In the Normal Dispersion Regime the

Dispersion Coefficient Is D > 0

While in the Anomalous Regime It Is D < 0

= c/

Dispersion Management: Solution


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Dispersion Compensation (Cont.)

DispersionCompensating Fiber:

By joining fibers with CD ofopposite signs and suitablelengths anaverage dispersion

close to zerocan be obtained;the compensating fiber can beseveral kilometers and the reelcan be inserted at any point inthe link, at the receiver or at thetransmitter

Note: Although the Total Dispersion Is Close to Zero, This Technique

Can Also Be Employed to Manage FWM and CPM Since at Every

Point We Have Dispersion Which Translates in Decoupling the

Different Channels Limiting the Mutual Interaction

Why Require Dispersion


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Why Require Dispersion

Compensation ?

Dispersion Compensating


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Dispersion Compensating

Fiber (DCF)

Di i C ti


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Dispersion Compensation

The Problem

Standard single-mode fiber has zero dispersion at the 1310nm

transmission band. It is not corrected at the 1550nm band.

Dispersion broadens optical pulses as they travel in single-mode

fiber, limiting the ultimate data rate supported by fiber.

A Solution

Recompress the optical pulses using chirped gratings


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THANK YOU

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d. Lecture 3 - Fiber Losses and Dispersion 8 March 06

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