dispersion in optical fibers due to signal degradation

7/29/2019 Dispersion in Optical Fibers due to signal degradation

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Kishori Sharan MathurResearch Scholar, Shri JJT University,Jhunjhunu 333001, Rajasthan, India

[email protected]


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Attenuation

- Absorption, Scattering

Dispersion

- Modal, Chromatic, PMD

Nonlinear effects- SPM, XPM, FWM, SBS, SRS

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Attenuation:

Reduces power level with distance

Dispersion and nonlinear effects:

Erodes clarity with distance and speed

Noise and Jitter:

Leading to a blurred image3


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Attenuation

Dispersion

Nonlinearity

Waveform After 1000 KmTransmitted Data Waveform

Distortion

It May Be a Digital Signal, but Its Analog Transmission

THE DIGITAL SIGNAL IS CARRIED USING ANALOG CARRIER SIGNAL (LASER OR LED)

AND THE TRANSMISSION MEDIA IS NOT IDEAL

THE DATA CARRIED OVER OPTICAL SIGNAL IS

MOSTLY DIGITAL AND HIGH SPEED

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z=0 z=LDispersion

z=0 z=LAttenuation

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DISPERSION

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PRACTICAL EXAMPLE OF DISPERSION

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Dispersion is the spreading of light pulses as they travel downoptical fiber; dispersion results in distortion of the signal,which limits the bandwidth of the fiber

Two general types of dispersion affect DWDM systems;Chromatic dispersion (CD) and polarization mode dispersion(PMD)

Chromatic Dispersion

Polarization Mode Dispersion

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DISPERSION HIERARCHY

TOTAL DISPERSION

(ps/ nm -km)

MULTIMODE

DISPERSION(INTER MODAL)

WAVE GUIDE

DISPERSION

CHROMATIC

DISPERSION(INTRA MODAL)

MATERIAL

DISPERSION

POLARIZATION

MODE DISPERSION(PMD)

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DISPERSION(PULSE DISTORTION)

Limits the information carrying capacity of a fiber

Measure of bandwidth,limits transmitted data rate and distance of optical pulse

Pulse broadening of an optical pulse resulting in intersymbol interference atreceiver end

Types- Inter modal and Intra modal

Inter modal

Different modes travel at different rates

Intra modalPulse spread within a single mode

Types-Material and wave guide dispersion

Material dispersion

Pulse spread caused by variation of refractive index of the fiber core material as a

function of wave length

Pulse spread due to finite spectral emission width of an optical source

Wavelength dependent and increases with increase in spectral width of the source

Wave guide dispersion

SMF confines only 80% of optical energy.20% of optical power propagating in the

cladding travels faster than light confined to the core resulting in pulse spread

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Polarization Mode Dispersion (PMD)Single-mode fiber supports two polarization states

Fast and slow axes have different group velocities

Causes spreading of the light pulse

Chromatic DispersionDifferent wavelengths travel at different speeds

Causes spreading of the light pulse

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WHY IS DESPERSION IS A PROBLEM

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WHEN DISPERSION IS TOO LARGE PULSES

INTEREFERE WITH EACH OTHER

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DISPERSION - CONSEQUENCES

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DISPERSION AND BIT RATE

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BER is a key objective of the optical systemdesign

Goal is to get from Tx to Rx with a BER < BERthreshold of the Rx

BER thresholds are on data sheets

Typical minimum acceptable rate is 10 -12

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BIT ERROR RATIO

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MODAL DISPERSION

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MODAL DISPERSION

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INTERMODAL DISPERSION IN MULTI MODE FIBERS

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CHROMATIC DISPERSION

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60 Km SMF-28

4 Km SMF-28

10 Gbps

40 Gbps

LIMITATIONS FROM CHROMATIC

DISPERSION

t

t

DISPERSION CAUSES PULSE DISTORTION,PULSE "SMEARING" EFFECTS

HIGHER BIT-RATES AND SHORTER PULSES ARE

LESS ROBUST TO CHROMATIC DISPERSION

LIMITS "HOW FAST AND HOW FAR

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different wavelengths propagate at different Speeds

Chromatic dispesion is measured in ps/nm/Km

(picoseconds of dispersion per nanometer of signal

bandwidth per kilometer of distance travelled.

CHROMATIC DESPERSION

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CHROMATIC DISPERSION

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CD CAN BE

POSITIVE (SHORTER WAVELENGTHS THAT TRAVEL

FASTER) OR

NEGATIVE (LONGER WAVELENGTHS THAT TRAVEL

FASTER).

NEGATIVE DISPERSION IS FREQUENTLY USED TO

COMPENSATE FOR EXCESSIVE POSITIVE

DISPERSION IN A FIBER TRANSMISSION NETWORK.


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NORMAL DISPERSION IN GLASS DELAYS THE BLUE LIGHT MORE

THAN THE RED LIGHT, WHILE IN ANOMALOUS DISPERSION THE

RED LIGHT IS DELAYED MORE THAN THE BLUE.

ANOMALOUS DISPERSION SOMETIMES OCCURS AT LONGER

WAVELENGTHS, E.G. IN SILICA (THE BASIS OF MOST

OPTICAL FIBERS) FOR WAVELENGTHS LONGER THAN THE ZERO-

DISPERSION WAVELENGTHOF 1.3 um.
http://www.rp-photonics.com/fibers.htmlhttp://www.rp-photonics.com/zero_dispersion_wavelength.htmlhttp://www.rp-photonics.com/zero_dispersion_wavelength.htmlhttp://www.rp-photonics.com/zero_dispersion_wavelength.htmlhttp://www.rp-photonics.com/zero_dispersion_wavelength.htmlhttp://www.rp-photonics.com/zero_dispersion_wavelength.htmlhttp://www.rp-photonics.com/zero_dispersion_wavelength.htmlhttp://www.rp-photonics.com/zero_dispersion_wavelength.htmlhttp://www.rp-photonics.com/fibers.html


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Affects single channel and DWDM systems

A pulse spreads as it travels down the fiber

Inter-symbol Interference (ISI) leads toperformance impairments

Degradation depends on:laser used (spectral width)

bit-rate (temporal pulse separation)

Different SM types

Interference

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1- Material dispersion

due to the dependence of refractive index towavelength, n()2- Waveguide dispersion

due to the different refractive indices of the

core and claddinglong wavelengths: neff~ncladdingshort wavelengths: neff~ncoreDifferent neffcause different velocity

Different neffcause different velocity

CHROMATIC DISPERSION (INTRA MODAL)

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Output Pulses of Different Lengths of SMF

200 150 100 50 0 50 100 150 200 250 3000

0.25

0.5

0.75

1

Original pulse

SMF 80 km

SMF 100 km

SMF 160 km

T ime (ps)

Amplitud

e


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SUMMED SIGNAL

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The Bit Rate-length Product

Doubling the bit rate (B) would reduce the repeater-less length (L) of optical

communication systems by a factor of 4.

CD is the main limiting factor for repeater-less length.

2

2

0

c

4 B L

D

L = 312 kmL = 18 kmL = 437 km10 Gb/s

L = 4,995 kmL = 294 kmL = 6,993 km2.5 Gb/s

DSF 1.55 m

D ~ 1 ps/nm-km

SMF 1.55 m

D ~ 17 ps/nm-km

SMF 1.3 m

D ~ 1 ps/nm-km

Bit rateRepeater-less length of optical communication systems

L = 312 kmL = 18 kmL = 437 km10 Gb/s

L = 4,995 kmL = 294 kmL = 6,993 km2.5 Gb/s

DSF 1.55 m

D ~ 1 ps/nm-km

SMF 1.55 m

D ~ 17 ps/nm-km

SMF 1.3 m

D ~ 1 ps/nm-km

Bit rateRepeater-less length of optical communication systems


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MATERIAL DISPERSION

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REDUCING MATERIAL DISPERSION

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WAVEGUIDE DISPERSION

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TUNABLE DISPERSION

COMPENSATION(TDC)

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FIBER BRAGG GRATING


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EXPOSING A PHOTOSENSITIVE FIBER TO AN INTENSITY PATTERN OF

ULTRAVIOLET RADIATION WILL PRODUCE OR WRITE A FIBER

BRAGG GRATING. IN ITS BASIC FORM, THE GRATING SELECTIVELY

REFLECTS LIGHT AT THE BRAGG WAVELENGTH (B) SUCH THAT B =

2N, WHERE N IS THE EFFECTIVE INDEX OF REFRACTION OF THE FIBER

AND IS THE PITCH OF THE GRATING IN THE FIBER. DEPENDING ON

THE GRATINGS APODIZATION PROFILE, INTENSITY AND PITCH,

NUMEROUS TYPES OF FUNCTIONS CAN BE DEVISED. A FIBER BRAGG GRATING CONSISTS OF A PERIODIC MODULATION OF

THE INDEX OF REFRACTION ALONG THE CORE OF AN OPTICAL FIBER.

THE COMPLETE MANUFACTURING PROCESS OF COMPONENTS BASED

ON FIBER BRAGG GRATINGS HAS FOUR STEPS: PREPARATION OF THE

PHOTOSENSITIVE OPTICAL FIBER; RECORDING THE GRATING; THERMAL

ANNEALING; AND PACKAGING. FOR HIGH-PERFORMANCE COMPONENTS,NUMERICAL SIMULATION TOOLS HELP OPTIMIZE THE DESIGN PRIOR TO

THESE STEPS. ALL STEPS ARE PERFORMED IN A CLEANROOM

ENVIRONMENT TO ENSURE LONG-TERM RELIABILITY AND IMPROVED

MANUFACTURING YIELD.

FIBER BRAGG GRATING

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A FIBER BRAGG GRATING CONSISTS OF A

PERIODIC MODULATION OF THE INDEX OF

REFRACTION ALONG THE CORE OF AN

OPTICAL FIBER.

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WAVEGUIDE DISPERSION

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Distance (Km) =Specification of Transponder (ps/nm)

Coefficient of Dispersion of Fiber (ps/nm*km)

A laser signal with dispersion tolerance of3400 ps/nm

is sent across a standard SMF fiber which has a Coefficient of Dispersion of17ps/nm*km.

It will reach 200 Km at maximum bandwidth.

Note that lower speeds will travel farther.

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DispersionCompensating Fiber:

By joining fibers with CD of

opposite signs (polarity) andsuitable lengths an averagedispersion close to zero canbe obtained; thecompensating fiber can beseveral kilometers and the

reel can be inserted at anypoint in the link, at thereceiver or at the transmitter

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SOLUTION FOR CHROMATIC

DISPERSION COMPENSATION

Length

Dispersion

+D -D

DISPERSION SAW TOOTHCOMPENSATION

Total dispersion averages to ~ zero

Fiber spool Fiber spoolDCU DCU

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Transmitter

Dispersion

Compensators

Dispersion Shifted Fiber Cable

+100

0

-100

-200-300

-400

-500

C

umulativeDisper

sion(ps/nm)

Total Dispersion Controlled

Distance from

Transmitter (km)

No Compensation

With Compensation


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ELECTRONIC DISPERSION COMPENSATION(EDC)

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Dispersion minimum of standard fiber at 1310

nm

1300 1400 1500 1600 1700

20

0

-20

wavelength / nm -->

Dispersion/ps/nm-km-

->

1200

10

-10

DF

DSF

DFF

SMF

1310 nm

SF = standard fiber

DFF = dispersion flattenedfiber

DSF = dispersion shifted

fiber

NZ-DSF=non zero dispersion

shifted fiber

+NZ-DSF

-NZ-DSF

S band C band L band

FIBER PARAMETERS GRAPH : DISPERSION

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1300 14001500

1600

(wavelength-nm)

Standard single-modeNonzero dispersion-shifted

Nonzero dispersion-shifted

Zero dispersion shifted

Reduced dispersion slope+10

-10

Dispersion

(ps/nm-km)

CHROMATIC DISPERSION IN SM FIBERS

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WAVE PROPAGATION EXAMPLE

electric

field

magnetic

field

PROPAGATION DIRECTION

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POINCARE SPHERE REPRESENTATION OF POLARIZATION

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PMD

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ITU-T RECOMMENDATION

PMD SHOULD BE LESS THAN 0.1 TIMES THE BIT PERIOD


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The optical pulse tends to broaden as it travelsdown the fiber; this is a much weakerphenomenon than chromatic dispersion and it isof little relevance at bit rates of 10Gb/s or less

nx

nyEx

Ey

Pulse As It Enters the Fiber Spreaded Pulse As It Leaves the Fiber

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- caused by asymmetry and stress in the fiber core

that results in birefringence

- An arbitrarily polarized pulse of light entering thefiber can be resolved into two components. Thesepolarization modes will travel at different speedsthrough the fiber. It leads to pulse broadening

- PMD is measured in ps/(Km1/2 )

PMD is important over 40 Gbps

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Caused by ovality ofcore due to:

Manufacturing process

Internal stress (cabling)

External stress (trucks) Only discovered in

the 90s

Most older fiber notcharacterized forPMD

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CHANGES IN POLARIZATION


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CHANGES IN POLARIZATION

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PRINCIPLE OF PMD


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PRINCIPLE OF PMD

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Factors contributing to PMD Bit Rate

Fiber core symmetry

Environmental factors

Bends/stress in fiber

Imperfections in fiber Solutions for PMD

Improved fibers Regeneration

Follow manufacturers recommended installation techniquesfor the fiber cable

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CONTROLLING PMD


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A patented OFS technology creates a "spin" within the fiber

during the draw process. This built-in spin reduces

birefringence by mixing the light between the two

polarizations, which enables the fiber to exhibit ultra low

PMD. In this process, an oscillating sheave imparts spin to the

fiber at the base of the draw tower. The spin then propagates

upward to the neck-down region where the molten glass is

spun first one way and then the other. As the glass cools, the

spin is "locked in" to the fiber.

The angle of the spin and the rate of oscillation can be varied

to impart different end-use characteristics.

CONTROLLING PMD

BY

MANUFACTURING

PROCESS

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EXAMPLE OF PMD


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dispersion in optical fibers due to signal degradation

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