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7/29/2019 OFC Basics 2
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24.01.2006 Lecture 31
Optical Fiber Basics-Part 2
Prof. Manoj Kumar
Dept. of Electronics and Communication
Engineering
DAVIET Jalandhar
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24.01.2006 Lecture 32
Single-Mode Step Index
Fiber
The Core diameter is 8 to 9mm
All the multiple-mode or multimode
effects are eliminatedHowever, pulse spreading remains
Bandwidth range 100GHz-Km
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24.01.2006 Lecture 33
Typical Core and Cladding
Diameters (mm)
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24.01.2006 Lecture 34
Multiple OFC
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24.01.2006 Lecture 35
Standard Optical Core
Size
The standard telecommunications core sizes in
use today are:
8.3 m (single-mode),
50-62.5 m (multimode)
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24.01.2006 Lecture 36
How a light ray enters an
optical fiber
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Numerical Aperture (NA)
The numerical aperture (NA) is a
measurement of the ability of an optical
fiber to capture light. The NA is also
used to define the acceptance cone of
an optical fiber. OR Numerical aperture
(NA) determines the light accepting
ability of a fiber
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Light Guidance in Optical
Fiber
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Low-order and high-order
modes
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PROPERTIES OF OPTICAL
FIBER TRANSMISSION
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Fiber Loss & 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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What is Group Velocity ?
Group Velocity (Vg) is Considered as thevelocity of energy propagating in thedirection of the axis of the guide fiber.
In order to convey intelligence;Modulation is done. When is done, thereare group velocities those must be
propagating along the fiber.The waves of different frequencies in thegroup will be transmitted with slightlydifferent velocities. Vg = dw/db.
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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 of fiber dimension (i.e. core radius) to the wavelength.
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Dispersion Curves
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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 dispersionDispersion 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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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 some
coloration, or variation, in wavelengthoutput
The low wavelength portion of the pulsetravels slower than the high wavelength one
creating pulse spreading
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Chromatic Dispersion
(continued)
Chromatic dispersion is measured in units oftime divided by distance and Tx sourcespectral width (ps/nm-km)
It is zero near 1310 nm in silica optical fibersIt is zero near 1550 nm in Dispersion Shiftedoptical fibers
Even at the dispersion zero, there is some
pulse spreading due to the spectral width ofthe light source
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Pulse Spreading due to
Dispersion
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Pulse Spreading
time
Pulse from zero-order mode
Pulse from highest-order mode
Pulses from other modes
Resulting pulse
T
T
T
T
T
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Calculation of Pulse Spread
C
C
x
y/2 y/2
Cyx cos
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24.01.2006 Lecture 323
Dispersion Management: Problem
Chromatic Dispersion (CD)
The optical pulse tend to spread as it propagates down thefiber generating Inter-Symbol-Interference (ISI) andtherefore limiting either thebit rate or the maximum
achievable distance at a specific bit ratePhysics behind the effect
The refractive index has a wavelength dependent factor, so thedifferent frequency-components of the optical pulses are traveling atdifferent speeds
Bit 1 Bit 2Bit 1 Bit 2Bit 1 Bit 2
Bit 1 Bit 2Bit 1 Bit 2
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24.01.2006 Lecture 324
Pulse Spreading due to
Dispersion
z=0 z=L
Dispersion
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24.01.2006 Lecture 325
Dispersion Curves
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24.01.2006 Lecture 326
Dispersion Management: Problem
Fiber Dispersion Characteristic
l
Dispersio
nCoefficientps/nm-km
17
0
1310 nm 1550nm
Normal Single Mode Fiber
(SMF) >95% of Deployed Plant
Dispersion Shifted Fiber (DSF)
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24.01.2006 Lecture 327
Dispersion Management: Problem
Increasing the Bit Rate
Higher Bit Rates experience higher signal
degradation due to Chromatic Dispersion:
OA10Gb/s Dispersion
16 Times Greater
Dispersion Scales as (Bit Rate)2
Time Slot
OA2.5Gb/s Dispersion
1)
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24.01.2006 Lecture 328
Dispersion Management: Solution
Direct vs. External Modulation
Laser diodes bias current
is modulated with signal
input to producemodulated optical output
Approach isstraightforward and lowcost, but is susceptible tochirp (spectral broadening)
thus exposing the signal tohigher dispersion
The laser diodes bias current
is stable
Approach yields low chirp andbetter dispersion performance,but it is a more expensiveapproach
Electrical
Signal in
Direct Modulation External ModulationIin
Optical
Signal out
Electrical
Signal inDC Iin
Mod.
Optical
Signal
Unmodulated
Optical Signal
External
Modulator
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24.01.2006 Lecture 329
Dispersion Management: Limitation
Chromatic Dispersion
CD places a limit on the maximum distance a signalcan be transmitted without electrical regeneration:
Fordirectly modulated (high chirp laser)
LD = 1/ B Dl (1)
D dispersion coefficient (ps/km-nm): 17ps/nm*km @1.55ml source line width or optical bandwidth (nm): 0.5nm
B bit rate (1/T where T is the bit period): 2.5Gb/s
LD ~ 47 km (*)
-Forexternally modulated (very low chirp laserf~ 1.2B)
LD ~ 1000 km @ 2.5Gb/s (*)
-LD ~ 61 km @ 10Gb/s (*)@1.55m and 17ps/nm*km
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24.01.2006 Lecture 330
Dispersive propertiesAnomalous dispersion: b2 < 0 orD > 0
short wavelength components (blue) travel faster than longwavelength components (red)
Normal dispersion: b2 > 0 orD < 0 long wavelength components (red) travel faster than short
wavelength components (blue)
Dispersion Management: Solution
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24.01.2006 Lecture 331
Dispersion Management: Solution
Dispersion Compensation
Note: f = c/l
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24.01.2006 Lecture 332
Chromatic Dispersion in
Optical FiberA high-speed pulse contains a spectrum of l components
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24.01.2006 Lecture 333
Explaining Material Dispersion
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24.01.2006 Lecture 334
Chromatic Dispersion
Definitions
Di i M t S l ti
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24.01.2006 Lecture 335
Dispersion Management: Solution
Dispersion Compensation (Cont.)
DispersionCompensating Fiber:
By joining fibers with CD ofopposite signs and suitablelengths an average dispersion
close to zero can 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
Wh R i Di i
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24.01.2006 Lecture 336
Why Require Dispersion
Compensation ?
Di i C ti
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24.01.2006 Lecture 337
Dispersion Compensating
Fiber (DCF) Application
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Thanks