OFC equipment .ppt

7/28/2019 OFC equipment .ppt

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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


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


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




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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



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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



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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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p g

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


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


0.3 to 1.0 db 0.25 db SM / MM Ribbon fiber

cables

SC Connector


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NW

SC Duplex Connector


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NW

ST Connector


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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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p g

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.


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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.


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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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p g g

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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p g

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

OFC equipment .ppt

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