Post on 03-Apr-2018
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Optical Fiber Cable
Communication Systems
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Part-I : Optical Fiber Cable
Part-II : Optical Link Engineering
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Part-I : Optical Fiber Cable
Contents
The need for OFC
OFC Propagation fundamentals
Concept of Critical Angles
Numerical Aperture
Propagation Modes OFC Performance Windows
Commercially available fibers
Optical Fiber Cable Structure
Optical Fiber Cable Splicing Connectors
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The need for OFC
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The need for OFC
More information carrying capacity
Free from EMI, ESI
Low attenuation : 0.25 db/km at 1550 nm
Use of WDM
Switching / routing at Optical signal level
Self healing rings under NMS control
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More information carrying capacity
According to Shannons information capacitytheorem :
C = BW. log2(1+SNR)
whereC = Information carrying capacity (bits/sec)
BW = Bandwidth of the link
SNR = Signal to noise power ratio
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Medium / Link Carrier Information Capacity
Copper Cable
(short distance)
1 MHz 1 Mb
(ADSL Modem)
Coaxial Cable
(Repeater every 4.5 km)
10 MHz 140 Mbps (BSNL)
UHF Link 2 GHz 8 Mbps (BSNL)
2 Mbps (Rly.)
MW Link
(Repeater every 40 km)
7 GHz 140 Mbps (BSNL)
34 Mbps (Rly.)OFC 1550 nm 2.5 Gbps(STM-16Rly.)
10 Gbps (STM-64)
1.28 Tbps (128 Ch. DWDM)
20 Tbps (Possible)
Information Carrying Capacities
of various media : Examples
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Propagation Fundamentals
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Bending of Light Ray
Denser
MediumRI = n1
Rarer
Medium
RI = n2
a
b
n1 > n2 Velocity of light in medium = c/RI
Snells Law : n1
sin a = n2
sin b
a
IncidentRay
Refracted
Ray
ReflectedRay
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Total Internal Reflection
Denser
MediumRI = n1
Rarer
Medium
RI = n2
c
90
n1 > n2 Velocity of light in medium = c/RI
Snells Law : n1
sin a = n2
sin b
IncidentRays
90oRefraction
Total Internal Reflection
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Concepts of Critical Angles
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Critical Angle of Incidence
Denser
Medium
RI = n1
Rarer
Medium
RI = n2
c
90
n1 > n2 Snells Law : n1 sin a = n2 sin b
n1 sin c = n2 sin 90 = n2
Critical Angle of incidence (c) = sin-1 (n2/n1)
Incident Ray
90oRefraction
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How does Optical Fiber propagate light ?
Optical fiber propagates light for angle of incidence > critical angle
RI of Core > RI of cladding
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Concept of Critical propagation angle
Critical Propagation Angle p
Critical angle of
incidence c
p = 90-c
sin p = sin(90-c) = cos c = [1-sin2c]1/2 = [1- (n2/n1)2]1/2
p = sin-1[1- (n2/n1)2]1/2
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Concept of Critical acceptance angle
Critical Propagation Angle p
Snells Law : na sin a = n1 sin p
1. sin a = n1 sin p ; a = sin-1(n1sin p)
2. Acceptance angle = 2a = 2 sin-1(n1sin p)
a
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Numerical Aperture
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Concept of Numerical Aperture
Ability of Optical Fiber to gather light
from source & guide it inside through
total internal reflection
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Mathematical Expression for
Numerical Aperture
Critical Propagation Angle p
NA = sin a = n1 sin p = n1 [1-(n2/n1)2]1/2 = [n1
2-n22]1/2 = (2.n.dn)1/2
a
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Significance of Numerical Aperture
By varying Average RI & differential
RI, NA can be changed over a range
( Ex. 0.1 to 0.3 for Silica Fiber)
NA = [Pin/Ps]1/2
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Propagation Modes
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What is meant by propagation mode
Even within propagating cone, optical fiber can sustain only part of
the rays . The reasons are :
Whenever a ray strikes core-cladding boundary, its phase (wt-bz) has to be equal to 2pk all the time where k is an integer
Rays that meet the above requirements only are sustained as
stable pattern or mode
In other words, rays which have integral number of wavelets between
consecutive reflection points, only are sustainable
The power of launched light is delivered by separate modes within the
fiber. Total output power is the accrual of power carried by different
modes
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Multimode step-index fiber
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Different light waves travel down the fiber.
One mode travels straight down the center of the core.
Other modes travel at different steep angles and bounce
back and forth by total internal reflection.
How to find number of modes
Find V number or normalized cut-off frequency orcharacteristic waveguide parameter V = (pd/l)(n12-n22)1/2
No. of modes in step-index fiber are N= V2/2
Propagation through
Multimode step-index fiber
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Problems with Multi-mode Step-index fiber
Different modes travel different distances ,resulting in different arrival times at the far end
This causes distortion in the transmitted signal
The disparity between arrival times of thedifferent light rays is known as dispersion
High dispersion is an unavoidable characteristicof multimode step-index fiber.
Solutions are : Use Graded Index Fiber Use Single Mode Fiber
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Multimode graded-index fiber
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Propagation through
Multimode graded-index fiber
The cores refractive index is parabolic, being higher at thecenter ( na> nb)
The light rays follow a serpentine path being gradually bentback toward the center by the continuously declining RI.
The modes traveling in a straight line are in a higherrefractive index, so they travel slower than the serpentinemodes
Thus, the arrival time disparity is removed , as all modesarrive at about the same time
No. of modes in graded index fiber are N=V2/4
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Single mode step-index fiber
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Single mode fiber has a much smaller core that allows onlyone mode of light at a time to propagate through the core.
Single-mode fiber exhibits no dispersion caused by multiplemodes
Single-mode fiber also enjoys lower fiber attenuation thanmultimode fiber
Thus, more information can be transmitted per unit of timebecause it can retain the fidelity of each light pulse overlonger distances
Like multimode fiber, early single-mode fiber was generallycharacterized as step-index fiber meaning the refractive indexof the fiber core is a step above that of the cladding ratherthan graduated as it is in graded-index fiber.
Propagation through
Single mode step-index fiber
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Summary of propagation
I t t P t f Si l d fib
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Important Parameters of Single mode fiber
Parameter Description Typical value
Attenuation Loss of signal strength 0.35 db/km at 1310 nm
0.25 db/km at 1550 nm
Core diameter Diameter of core 8 to 10 micro meter
Cladding diameter Diameter of cladding 125 micro meter
Core-cladding RI
ratio
Ratio of RI of core to
cladding
Less than 0.37%
Cut-off wavelength Minimal wavelength at
which fiber supports only
one mode
> 1260 nm
Numerical aperture Ability of Optical Fiber to
gather light from source &
guide it inside through
total internal reflection
0.10 to 0.3
I t t P t
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Important Parameters
Single mode fiber (contd)Parameter Description Typical value
Mode field
diameter
MM fiber carries all light energy
through core as core diameter is large.
But, SM fiber carries 80% light energy
through core and 20% through cladding
as core diameter is small.
Mode field diameter (MFD) is theeffective diameter available for
propagation.
MFD is dependent on wavelengthit
reduces with wavelength. Shorter the
wavelength, more focussed the beam isand more stringent confinement of beam
to core , hence less MFD
When 2 fibers are connected, not only
core-cladding diameters to match but
also MFDs to match
9.3 micro meters for
core diameter of 8.3
micro meters
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More on Cutoff Wavelength Cutoff wavelength is the wavelength above which a single-
mode fiber supports and propagates only one mode oflight.
In other words, an optical fiber that is single-moded at aparticular wavelength may have two or more modes atwavelengths lower than the cutoff wavelength.
The effective cutoff wavelength of a fiber is dependent onthe length of fiber and its deployment
The longer the fiber, the lower is the effective cutoffwavelength.
The smaller the bend radius of a loop of the fiber , thelower is the effective cutoff wavelength.
If a fiber is bent in a loop, the effective cutoffwavelength is lowered.
If a fiber is cabled , the cutoff wavelength of a fiber isreduced
The variations are predictable enough, so that fiber
manufacturers can specify a maximum cable cutoffwavelength for the fiber.
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OFC Performance Windows
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Signal Attenuation in Optical Fiber Attenuation has three components :
- Bending loss (Macro / Micro)
- Absorption loss
- Scattering loss
In bending loss, there are 2 categories
- Macro bending loss (specified by manufacturer)
- Micro bending loss (not specified but included
in total attenuation accountal by manufacturer)
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Macro-bending loss
Macro-bending loss is caused by bendingof the entire fiber axis
The bending radius shall not be sharperthan 30d where d is diameter of cable
One single bend sharper than 30d cancause loss of 0.5 dB
If bending is even sharper, fiber may
break
Mi b di l
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Micro-bending loss Micro-bending loss is caused by micro deformations of fiber
axis which leads to failures in achieving total internal
reflection conditions Micro-bends are small-scale perturbations along the fiber
axis, the amplitude of which are on the order of microns.These distortions can cause light to leak out of a fiber.
Micro-bending may be induced at very cold temperatures
because the glass has a different coefficient of thermalexpansion from the coating and cabling materials. At lowtemperatures, the coating and cable become more rigid andmay contract more than the glass. Consequently, enoughload may be exerted on the glass to cause micro bends.
Coating material is selected by manufacturers to minimizeloss due to micro-bending. The linear thermal expansioncoefficient of coating material shall be compatible with thatof fiber
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Factors causing absorption & attenuation
Scattering of light due to molecular levelirregularities in the glass
Light absorption due to presence of residual
materials, such as metals or water ions, within the
fiber core and inner cladding.
These water ions that cause the water peak
region on the attenuation curve, typically around
1380 nm.
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Absorption loss & Scattering loss
Absorption Loss
Scattering Loss
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Low water peak fiber
Removal of water ions is of particular interest tofiber manufacturers as this water peak regionhas a broadening effect and contributes toattenuation loss for nearby wavelengths.
Some manufacturers now offer low water peaksingle-mode fibers, which offer additional
bandwidth and flexibility compared with standardsingle-mode fibers.
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The three peaks & troughs
Three peaks in attenuation
1050 nm
1250 nm
1380 nm Three troughs in attenuation
850 nm : 3 db/km
1310 nm : 0.35 db/km1550 nm : 0.25 db/km
Performance windows
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Dispersion in Optical Fiber
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Dispersion phenomenon
Dispersion is the time distortion of an optical signal
that results from the differences of time of travel fordifferent components of that signal, typicallyresulting in pulse broadening
As the distance travelled by the signal is more,
broadening of pulse is more In digital transmission, dispersion puts a limit on the
maximum data rate and the maximum distance i.e. theinformation-carrying capacity of a fiber link.
The interference from broadened pulse in the nextinterval shall not lead to erroneous interpretation ofreceived signal
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Types of dispersion
There are 2 types of dispersion :
- Inter-modal dispersion
- Chromatic dispersion
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Inter-modal dispersion in
Multi-mode step-index fiber
The disparity between arrival times of the different modesis known as inter- modal dispersion
Since pulse power is delivered by separate modes whichtravel different distances within fiber, fractions of powerarriving at the end combine to cause spreading of pulse
The amount of pulse spreading over distance L is given bydtmodal(SI) = [L/(2cn2)](NA)2
As already discussed, the solutions to modal dispersionproblem are
Use Graded Index Fiber
Use Single Mode Fiber
I t d l di i i
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Inter-modal dispersion in
Multi-mode Graded-index fiber
The amount of pulse spreading over
distance L is given by
Dtmodal(GI) = (LN1DRI2 )/(8c)where N1 is Core Group RI &DRI is differential RI
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Chromatic dispersion
Individual mode has light of different wavelengths, each traveling along fiber with differentvelocity and resulting in dispersion. This is calledChromatic dispersion
It has 2 components : Material dispersion : The pulse spreading due to
dispersive properties of material
Waveguide dispersion : Dispersion resulting from thelight waves traveling in the core and the inner claddingglasses at slightly different speeds.
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Chromatic dispersion in MM Fiber
In MM(GI) fiber, wave guide dispersion isnegligible
Material dispersion in MM fiber is given byDtmat = D(l) .Dl
where D(l) = (S0l)/[4{1-(l0/l)4}]
l0 is zero dispersion wave-length
S0 is slope of D(l) vs. l curve
at zero dispersion point
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Total dispersion in MM Fiber
In MM(GI) fiber, wave guide dispersion isnegligible
Total dispersion in MM fiber to be
evaluated fromDt2total = Dt
2modal + Dt
2mat
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Dispersion in Single Mode Fiber
No Modal dispersion
Chromatic dispersion exists
Chromatic dispersion vs Wavelength
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Chromatic dispersion vs. Wavelength
in Single Mode Fiber
Fiber dispersion varies with wavelength
The wavelength at which dispersion equalszero is called the zero-dispersion wavelength
(0). This is the wavelength at which fiber has its
maximum information-carrying capacity.
For standard single-mode fibers, this is in theregion of 1310 nm.
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Evolution of Single mode fiber
Single-mode fiber has gone through acontinuing evolution.
There are three basic classes of single-mode
fiber used in modern OFC Systems :
Non dispersion-shifted fiber (NDSF)
Dispersion-shifted fiber (DSF)
Non zero-dispersion-shifted fibers (NZ-DSF).
Explanation about Classes of Single mode fiber
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p g Non dispersion-shifted fiber (NDSF)
1. The initially deployed type used for 1310 nm.
2. This fiber has high dispersion at 1550 nm, hence notsuitable for 1550 nm systems
Dispersion-shifted fiber (DSF)
1. To address the shortcoming of NDSF fiber, fibermanufacturers developed, dispersion-shifted fiber (DSF)
2. This has moved the zero-dispersion point to the 1550 nmregion
Non zero-dispersion-shifted fibers (NZ-DSF)
1. Though DSF worked extremely well with a single 1550nm wavelength, it exhibits serious nonlinearities when
multiple, closely-spaced wavelengths in the 1550 nmwere transmitted in DWDM systems.
2. To address the problem of nonlinearities, non zero-dispersion-shifted fibers (NZ-DSF) were designed bymanufacturers. The fiber is available in both positiveand negative dispersion varieties and is rapidlybecoming the fiber of choice in new fiber deployment.
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Usage of MM and SM Fiber
Multimode fiber is used primarily in systems with
short transmission distances (under 2 km), such as
premises communications, private data networks,
and parallel optic applications. Single-mode fiber is typically used for longer-
distance and higher-bandwidth applications .
Its tremendous information-carrying capacity andlow intrinsic loss have made single-mode fiber the
ideal transmission medium for a multitude of
applications.
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Commercially Available Fibers
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Optical Fiber Sizes
To ensure compatibility among splices/connectors, sizes ofcore & cladding have been standardized
International standards for SM fiber
Cladding diameter : 125 microns (micro meter)
Cladding + coating : 245 microns (micro meter)
Core diameter : 7 to 10 micro meter
International standards for MM fibers
Cladding diameter : 125 microns (micro meter) Cladding + coating : 245 microns (micro meter)
Core diameter : 50 to 62.5 micro meter
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Types of Commercially Available Fibers
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S N Type ITUT
Rec.
Description Indoor/
Outdoor
Application
1 MM50 G.651 Multi Mode Fiber
with 50 micro m.
of Core dia
Outdoor Short-Reach Optical
Transmission for LAN in
Offices and Premises
2 MM62.5 G.651 Multi Mode Fiber
with 62.5 micro m.
of Core dia
Outdoor Short-Reach Optical
Transmission for LAN in
Offices and Premises
3 MM10G G.651 Multi Mode Fiber
with 50 micro m.
of Core dia
Outdoor 10Gigabit Ethernet
Optical Transmission for
LAN in Offices and
Premises
Types of Commercially Available Fibers
Types of Commercially Available Fibers
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Types of Commercially Available Fibers
S N Type ITUT
Rec.
Description Indoor/
Outdoor
Application
4 SM G.652B Single-Mode Fiber Outdoor Large-Capacity & Low-
Loss Transmission in
1550nm Windows
5 LWP G.652D Low-Water-Peak
Single-ModeFiber
Outdoor WDM Optical
Transmission forMetropolitan
Networks
6 SR15 G.652B Bending-
Insensitive
Small Bending
Proof and High
Reliability
Single-Mode
Fiber
Indoor Optical cord and cable
for FTTH / LAN /
Premises
Types of Commercially Available Fibers
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Types of Commercially Available Fibers
S N Type ITUT
Rec.
Description Indoor/
Outdoor
Application
7 SR15E G.652D Bending-Insensitive
Small Bending
Proof and High
Reliability Low-
Water-Peak
Single-ModeFiber
Outdoor Long-Distance Optical
Transmission in
1550nm Windows
8 DS G.653 Dispersion-Shifted
Single-Mode
Fiber
Outdoor Long-Distance Optical
Transmission in
1550nm Windows
9 LA G.655 Large-Effective-Area NZ-DSF
Outdoor Long-DistanceDWDM Optical
Transmission in
the C-&L-Bands
Types of Commercially Available Fibers
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Types of Commercially Available Fibers
S N Type ITUT
Rec.
Description Indoor/
Outdoor
Application
10 SS G.656 Small-Dispersion-
Slope NZ-DSF
Outdoor Long-Distance
DWDM Optical
Transmission in
the C-&L-Bands
11 ULA G.655 Ultra Large-Effective-Area
NZ-DSF
Outdoor Long-DistanceDWDM Optical
Transmission
Utilizing the S-,
C- & L-Bands
12 USS G.656 Ultra Small-
Dispersion-Slope NZ-DSF
Outdoor DWDM Optical
TransmissionUtilizing the S-,
C- & L-Bands
for Metro
Networks
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Optical Fiber Cable Structure
B i t t f O ti l Fib C bl
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Basic structure of Optical Fiber Cable
Optical fiber cable consists of one or more protectiveenclosures, each having one or more bare fibers and
the entirety packaged with a strength member in an
outer jacket.
Basic elements in OF cable are :
Bare fiber
Buffer tube
Strength member
Outer jacket
Bare fiber categories
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Bare fiber categories
Single modeNDSF
DSF
NZ-DSF Multi-mode
Step index
Graded index
Bare fiber description
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Bare fiber description
Two different types of highly pure, solid glass, composedto form the core and cladding.
RI profile : Step index for SM and MM (Step index)
{ RI of core > RI of cladding}
RI profile : Parabolic for MM (Graded index)
{Decreases from centre of core to outer of core}
A protective acrylate coating surrounds the cladding This is applied to the glass fiber as the final step in the
manufacturing process.
This is colour coded for identification of fiber
This coating protects the glass from dust and scratches
This protective coating comprises of two layers: A soft inner layer that cushions the fiber and allows the
coating to be stripped from the glass mechanically
A hard outer layer that protects the fiber duringhandling, particularly the cabling, installation, and
termination processes.
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B ff t b d i ti
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Buffer tube description
Buffer tube is the first shield protectingfiber from damage
It can have one fiber or more
It can be tight buffer or loose buffer
It is colour coded for identification
Feat res of loose b ffer t be
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Features of loose buffer tube
Buffers inner diameter is more than fibers outer diameter
Force applied on buffer does not affect the fiber until theforce is large enough to straighten the fiber inside the
buffer
Loose buffer tube can be filled with gel to prevent entry of
moisture Preparation for and providing connectors/splicing is
laborious
It cannot be installed vertically
Loose buffer tube fiber cables are used out-door
Features of tight buffer tube
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Features of tight buffer tube
Tight buffers inner diameter is same as fibers outer
diameter It can keep the fiber operational despite break, as the fiber
is held in position firmly.
Each buffer can hold one fiber only.
Easy to prepare for and provide connectors / splicing
Can be installed vertically Normal tight buffer tube cables are used in-door
Very strong tight buffer tube cables are used in military /under-sea applications as small separation of fiber endsdue to break does not interrupt services completely.
Features of strength member
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Features of strength member
Purpose is to release fiber from mechanical stressduring installation / operation
Following materials are used as strength members :
Flexible aramid yarn (Ex. Du pont Kevlarwidely used)
Flexible fiber glass roving
Fiber glass rod
Metal wire
Metal rope made from twisted steel wires
Features of outer jacket
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Features of outer jacket
Surrounds the entire assembly of buffer tubeor tubes and strength member
Purpose is to provide environmental
protection to fibers Made of PE
Steel armour is provided for armoured cables
Si i C S
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Single Fiber Cable Structure
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TWO SUITABLE RIP CARDS UNDER THE ARMOUR
HDPE OUTER JACKET (2.0 mm minimum)
CORRUGATED A ISI-304 0R 305 STAINLESSSTELL ARMOUR [0.125 MM (maximum) ]
INNER P.E.SHEATH (1.5 mm minimum)
(OUTER DIA 2.4 mm +/- 0.1 mm)SECONDARY COATING TUBE
NON-HYGROSCOPIC DIELECTRIP TAPE (POLYSTER TAPE)
PRIMARY COATED FIBRE
CENTRAL STRENGTH MEMBER (2.5mm +/- 0.05mm)
WRAPPING ARMIDE YARN(IF REQUIRED)
WATER BLOCKING JELLY
WATER BLOCKING THIXOTROPIC JELLY
ONE SUITABLE RIP CARD UNDER THE INNER SHEATH
CONSTRUCTIONAL DIAGRAM OF
24 FIBRE ARMOURED OPTIC CABLE (TC. 55. 2006 Rev.1)
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CABLE CORE
HDPE OUTER JACKET
STAINLESS STELL ARMOURCORRUGATED A ISI-304 0R 305
HDPE OUTER JACKET
CABLE CORE
INNER P.E.SHEATH
RIP CARD
INNER P.E.SHEATH
CORRUGATED STAINLESS STELL ARMOUR
ARMOURED OPTIC FIBRE CABLE
CROSS SECTIONAL VIEW OF
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Multiple Fiber Cable Structure
Structure details of
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Structure details of
24 fiber cable used in Railways
6 tubes & 4 fibers per tube
Colour coded
St th f O ti l Fib C bl
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Strength of Optical Fiber Cable
One common misconception about optical fiber isthat it must be fragile because it is made of glass.
While traditional bulk glass is brittle, the ultra-
pure glass of optical fibers exhibits both high
tensile strength and extreme durability.
Tensile strength is of the order of 44000 to 60000
kg per sq.cm
(For copper it is only 7500 kg per square cm.)
Bending Parameters
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Bending Parameters
Optical fiber and cable are easy to install because it is
lightweight, small in size, and flexible. Precautions are needed to avoid tight bends, which may
cause loss of light or premature fiber failure.
Bending radius shall be > 30 d (where d is dia. of cable)
Splice trays and other fiber-handling equipment, suchas racks, are designed to prevent fiber-installation errorssuch as this.
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Fiber geometry parameters
Splice yields and system losses have a profound impact onthe quality of system performance and the cost ofinstallation / maintenance
Splice-loss requirement is typically around 0.1 dB.
The three fiber geometry parameters that have thegreatest impact on splicing performance include thefollowing: Cladding diameter
Core/clad concentricity (or core-to- cladding offset)
Fiber curl
These parameters are controlled during the fiber-manufacturing process
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Cladding Diameter
The cladding diameter tolerance controlsthe outer diameter of the fiber, with tightertolerances ensuring that fibers are almost
exactly the same size. During splicing, inconsistent cladding
diameters can cause cores to misalign wherethe fibers join, leading to higher splice
losses.
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Core/clad concentricity
How well the core is centered in the cladding glassregion
Tighter core/clad concentricity tolerances help
ensure that the fiber core is centered in relation tothe cladding.
This reduces the chance of cores that do not matchup precisely when two fibers are spliced together.
Core/clad concentricity is determined during thefirst stages of the manufacturing process.
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Fiber Curl
Fiber curl is the inherent curvature along aspecific length of optical fiber that is
exhibited to some degree by all fibers.
It is a result of thermal stresses that occurduring the manufacturing process. Tighter
fiber-curl tolerances reduce the possibility
that fiber cores will be misaligned duringsplicing, thereby impacting splice loss.
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Splicing of fibers
What is splicing
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Splicing is permanent connection of two pieces of fiber
Two types of splices : Mid-span splicing of two fibers
Fibers from two cables are spliced after laying drum by drum
Cuts in fiber run are attended by splicing certain minimum lengthcable piece at either end
Pig-tail splicing Pig-tail is fiber with factory installed connector at one end The free fiber of pig-tail is spliced connected to cable
Two techniques of splicing Mechanical splicing
Fusion splicing
Mechanical Splicing
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Mechanical Splicing
Mechanical splicing is of slightly higher losses (about0.2 db) and less-reliable performance
System operators use mechanical splicing for emergencyrestoration because it is fast, inexpensive, and easy.
Mechanical splices are reflective and non-homogenous
Fusion Splicing
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Fusion Splicing
Fusion splicing provides a fast, reliable, low-loss,fiber-to-fiber connection by creating a homogenous
joint between the two fiber ends.
The fibers are melted or fused together by heating the
fiber ends, typically using an electric arc.
Fusion splices provide a high-quality joint with the
lowest loss (in the range of 0.01 dB to 0.10 dB for
single-mode fibers) and are practically non-reflective.
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Connectors
B i b t t
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Basics about connectors
Fiber optic connector facilitates re-mateable connection
i.e. disconnection / reconnection of fiber Connectors are used in applications where
Flexibility is required in routing an optical signalfrom lasers to receivers
Reconfiguration is necessary
Termination of cables is required
Connector consists of 4 parts :
Ferrule
Connector body
Cable Coupling device
Typical connector is shown in figure
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Parts of connector and their descriptionThe Ferrule The fiber is mounted in a long, thin cylinder, the ferrule, which acts as a
fib li t h i Th f l i b d th h th t t
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fiber alignment mechanism. The ferrule is bored through the center at a
diameter that is slightly larger than the diameter of the fiber cladding.
The end of the fiber is located at the end of the ferrule. Ferrules are
typically made of metal or ceramic, but they may also be constructed ofplastic.
The
Connector
Body
Also called the connector housing, the connector body holds the ferrule.
It is usually constructed of metal or plastic and includes one or more
assembled pieces which hold the fiber in place. The details of these
connector body assemblies vary among connectors, but bonding and/or
crimping is commonly used to attach strength members and cablejackets to the connector body. The ferrule extends past the connector
body to slip into the coupling device.
The Cable The cable is attached to the connector body. It acts as the point of entry
for the fiber. Typically, a strain-relief boot is added over the junction
between the cable & the connector body, providing extra strength
The
Coupling
Device
Most fiber optic connectors do not use the male-female configuration
common to electronic connectors. Instead, a coupling device such as
an alignment sleeve is used to mate the connectors. Similar devices
may be installed in transmitters & receivers to allow these devices to
be mated via a connector. These devices are also known as feed-
through bulkhead adapters.
Characteristics of connectors
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Parameter Description
Insertion loss 1. Loss due to use of connector
(unavoidable)2. Manufacturers specify typical value
3. Use of strain relief boot over the
junction between the cable &
connector body and attachingstrength member to the connector
minimize the insertion loss
Repeatability
(loss)
Connector is re-useable (up to 500
times). The increase in loss shall beless than the repeatability loss
Suitability Suitable to SM / MM fiber
Return loss Important factor for SM fibers (shall be
less than 60 db)
FC Connector
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Insertionloss
Repeatability Fiber type Application
0.5 to 1.0 db 0.20 db SM / MM Transmission
NW
FDDI Connector
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Insertion
loss
Repeatability Fiber type Application
0.2 to 0.7 db 0.20 db SM / MM FDDI LAN
(Fiber
distributed data
interface)
LC Connector
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Insertion loss Repeatability Fiber type Application
0.15 db (SM)
0.10 db (MM)
0.20 db SM / MM High density
interconnection
MT Array Connector
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y
Insertionloss
Repeatability Fiber type Application
0.3 to 1.0 db 0.25 db SM / MM Ribbon fiber
cables
SC Connector
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Insertion loss Repeatability Fiber type Application
0.2 to 0.45 db 0.10 db SM / MM Transmission
NW
SC Duplex Connector
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Insertion loss Repeatability Fiber type Application
0.2 to 0.45 db 0.10 db SM / MM Transmission
NW
ST Connector
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Insertion loss Repeatability Fiber type Application
0.40 db (SM)
0.50 db (MM)
0.40 db (SM)
0.20 db (MM)
SM / MM Inter/Intra
Building
Steps in attaching connectors to fiber
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1. Cut the cable one inch longer than the required finished
length.2. Carefully strip the outer jacket of the fiber with no
nick fiber strippers. Cut the exposed strength members,and remove the fiber coating. The fiber coating may beremoved two ways: by soaking the fiber for two minutes
in paint thinner and wiping the fiber clean with a soft,lint-free cloth, or by carefully stripping the fiber with afiber stripper. Be sure to use strippers made specificallyfor use with fiber rather than metal wire strippers asdamage can occur, weakening the fiber.
Steps in attaching connectors to fiber
( td )
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(contd..)3. Thoroughly clean the bared fiber with isopropyl alcohol
poured onto a soft, lint-free cloth such . NEVER cleanthe fiber with a dry tissue. Note: Use only industrialgrade 99% pure isopropyl alcohol. Commerciallyavailable isopropyl alcohol is for medicinal use and isdiluted with water and a light mineral oil. Industrial
grade isopropyl alcohol should be used exclusively.
4. The connector may be connected by applying epoxy orby crimping. If using epoxy, fill the connector withenough epoxy to allow a small bead of epoxy to form atthe tip of the connector. Insert the clean, stripped fiberinto the connector. Cure the epoxy according to theinstructions provided by the epoxy manufacturer.
Steps in attaching connectors to fiber
( td )
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(contd)
5. Anchor the cable strength members to the connectorbody. This prevents direct stress on the fiber. Slide theback end of the connector into place (where applicable).
6. Prepare the fiber face to achieve a good optical finish bycleaving and polishing the fiber end. Before theconnection is made, the end of each fiber must have asmooth finish that is free of defects such as hackles, lips,and fractures. These defects, as well as other impuritiesand dirt change the geometrical propagation patterns oflight and cause scattering.
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Part-II: Optical Link Engineering
(Single Mode Fiber Systems)
C
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Contents
Considerations in Optical Link Engineering
(Single Mode Fiber Systems)
Selection of Components
Link Power Budget
Rise time budget
ons erat ons nOptical Link Engineering
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Starting point : Point to point link
Understanding System performance criteria
Choice of components available
Link Power Budget analysis
To determineOFC link meets the attenuationrequirement or amplifiers are to be added
System rise time analysis
To verify that the dispersion is within tolerablelimits
System performance vis--vis cost constraints
Point to Point Link
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The simplest transmission link is a point to point
line having transmitter at one end and receiver onthe other.
This type of link forms the basis for engineeringmore complex system architectures.
The design of an optical link involves manyinterrelated variables among the fiber, source,photo detector,, so that the actual link design andanalysis may require several iterations before theyare finalized.
Data
source
Optical
Tx
Optical
Rx
Data
User
Optical Fibre
System performance criteria
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System performance criteria :
Desired transmission distance.
Data rate - channel band width.
Bit Error Rate (BER)
System performance should be ensured
over the expected system life time.
y p
Choice of components
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To fulfil these requirements, the designer
has a choice of the following components
and their associated characteristics :
Multimode or single-mode optical fiber
Core size
Core refractive index profile
Bandwidth or dispersion
Attenuation
Numerical aperture or mode field diameter
Choice of components (contd)
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LED or laser diode optical source
Emission wavelength.Spectral line width.
Output power
Effective radiating area
Emission patternNumber of emitting modes
Pin or avalanche photodiode
Responsivity
Operating wave lengthSpeed
Sensitivity
Link power & Rise-time Budget Analysis
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Two analysis are usually carried out :
Link power budget Rise time budget
The link power budget analysis, power margin between
transmitter and receiver sensitivity required for specified
BER. The power margin is then distributed to connectors, splice,
fiber loss + any other additional margins including
degradation of components due to aging.
Once the link power budget has been established, the risetime budget (analysis) is to be carried out to ensure that
over all system performance has been met.
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Selection of components
Decide the wavelength
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To carry out link power budget, first decide the
wave length of operation, short distance - 800 -900 nm, longer distance of 1300 - 1550 nm.
Once the wave length is decided the system
performance is interrelated to 3 major components
namely, receiver, transmitter and fiber.
Generally, characteristics of two of these elements
are chosen and then the characteristics of the 3rd
one is computed to meet the system performancerequirement.
If the components ha e been o er / nder
Decide the detector & source
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If the components have been over / under
specified, a design iteration may be needed.
Generally, to start with a suitable photo detector
is chosen which can detect successfully the optical
signals at the highest operating speed, i.e, suitable
for the desired band width. Then suitable optical source is chosen to suit the
transmission speed (band width)
Optical power level is estimated using a particular
fiber.
Introduction of booster amplifier is also examined
at this stage.
Pin Photo Diode Vs APD
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Pin Photo Diode Vs. APD
The minimum optical power level that mustfall on the photo detector to satisfy the BER
requirement at the specified data rate.
Cost of components. Complexity of the receiver design /
maintenance for eg. Pin photo diode
receiver is simpler, more stable with
variation in the temperature and less
expensive than APD.
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Pin photo diode bias voltages are normally
less than 5 V and APDs bias voltage ranges
from 40 V to several hundred V.
APDs are more sensitive to low optical
power levels.
Pin Photo Diode Vs. APD (contd)
LED Vs. LD
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LED Vs. LD
The system parameters involved in decidingbetween LED and LD are :
Signal dispersion.
Data rate.
Transmission distance.
Cost.
LED source in 800 - 900 nm region can
work up to data rate - distance of 150 mbps
- km
LED Vs. LD (contd)
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Single mode fiber with LD source can
provide ultimate bit rate at data rate distanceproduct of over 500 Gbps - km with 1550 nm.
Laser diodes are capable of coupling 10 - 15
dB more optical power into a fiber than anLED which enables greater repeater less
transmitter distance with a laser.
The disadvantages of LD are its cost and itscomplexity of transmitter circuitry.
( )
Selection of Fiber
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Choice between single and multimode fiber. Choice depends on type of light source and the amount
of dispersion that can be tolerated.
Light emitted diodes (edge emitting type) withmultimode fibers data rates of greater than 500 Mb/sover several kilometers are possible.
Loss (attenuation) characteristics of a cabled fiber,excess loss that results from the cabling process mustbe considered in addition to the basic attenuation of the
fiber. Connector, splice, loses also to be considered.
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Link Power Budget
C i f Li k
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Constituents of Link
An optical power loss model for a point-to-
point link is shown below :
Power received on photo detectorPR= Power output of the source PS Total power
loss PT
OpticalTx OpticalRx
Splice ( Joints)Connector Connector
Optical Fibre
Example-I
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Example I
The data speed is 2.5 Gb/s. BER - 10 -9
Source 1550 nm laser diode.
Power output level into a fiber flylead = + 3dBm. Detector, InGaAs APD of sensitivity - 32dBm at
2.5 Gb/s.
Optical cable of loss 0.3 dB/km.
Distance = 60 km.
Illustration of Link Loss Budget
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g
Ex.-II System Requirement :
Data rate 20 Mb/s
BER - 10 -9
Selected detector is silicon pin photo diode operating on 850nm.
For 20 MB/s data rate, the required receiver input level is42 dBm.
(Receiver sensitivity varies with data rate)
GaAlAs LED is chosen which can couple - 13 dBm average
optical power level into a fiber flylead with 50 m corediameter.
Cable length 20 km, attenuation 1dB /km
Receiver Sensitivity
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Pr = 11.5 log B - 71.0 dBm For InGaAs APD
= 11.5 log B60.5 dBm For InGaAs pin
diode
B is the bandwidth in Mbps
Table for calculation of Power Budget
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Output / sensitivity / loss Power margin (dB)Component /
Loss
Parameter Ex. I Ex. II Ex. I Ex. IIOptical output Laser ,+3 dBm LED, -13dBm - -
Detector Type,sensitivity
APD,-32 dBm at 2.5
Gbps
PIN diode-42dBm at 20
Mbps
- -
Allowed loss 35 dB 29 dB 35 29
Source
connector loss
1 dB 1dB 34 28
Jumper +
Connector Loss
3 + 1 dB 3+1dB 30 24
Cableattenuation 18 dB (60 km ,@ 0.3 dB /km) 20 dB (20 km,@ 1dB/km) 12 4
Jumper +
Connector loss
3 +1 dB 3+1dB 8 0
Receiver
connector loss
1 dB 1dB 7 (final) 0 (final )
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Rise time budget
RISE TIME
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RISE TIME
Rise Time budget analysis is a means for ensuring theminimum permissible data rate over an optical fiber link.
Total system rise time is given by equation below :
t = t12
+ t22
+ t32
+t42
+ .
The following are the 3 commonly encountered rise timeelements in link designing :
Transmitter Rise Time. Dispersion Rise Time of the fiber.
Receiver Rise Time.
Li it f Ri ti
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Limits for Rise-time
Conventionally, the total transition time
degradation of a digital link should not exceed
70% of an NRZ bit period or 35 percent of a bit
period for RZ bit rate.
The transmitter rise time are primarily due to light
source and its drive circuitry.
The receiver rise time results from the band widthof the receiver front end.
H t t t
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How to compute trx
Rise time of the receiver is time interval between 10% and
90% of the rise of output. This is related to the bandwidth
with the following empirical formula
trx = 350 / Brx ( Brx is Rx bandwidth) The fiber rise time is the total dispersion time down the
fiber.
In case of single mode fiber multimodal dispersion is not
present and hence the total dispersion is due to chromaticdispersion only.
Example of rise time budget
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Optg. Wavelenth =1310 nm.
Laser Source rise time = 25 psec
Spectral width of laser = 1 nm.
Fibre dispersion = 2 psec/nm-km
Total lenth =60km
Therefore, material dispersion related rise time =2x 1 x 60
= 120 psec.
R i b d idth 2 5 GH
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Receiver band width = 2.5 GHz
Receiver rise time = 350/2500 = 0.14 n sec. Substituting all these values we get the total rise
time = (25 +120 +140 )
=186 p sec
This value is less than the maximum allowable
70 % of bit interval time for 2.5 Gbps NRZ data
string. ( which is 280 nsec.)
Thus, we can finalise this design to be adequate.
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THANK YOU