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Kishori Sharan MathurResearch Scholar, Shri JJT University,Jhunjhunu 333001, Rajasthan, India
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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.html7/29/2019 Dispersion in Optical Fibers due to signal degradation
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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
CHROMATIC DISPERSION (INTRA MODAL)
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CHROMATIC DISPERSION (INTRA MODAL)
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
CHROMATIC DISPERSION (INTRA MODAL)
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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
DISPERSION COMPENSATION
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DISPERSION 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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