Fiber Optic1

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Fiber OpticFiber Optic

By Sanghamitra Mojumder

December 8,2007


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Definition

Fiber-optic communications is based on the principle that light in a glass

medium can carry more information over longer distances than electrical

signals can carry in a copper or coaxial medium. The purity of today's

glass fiber, combined with improved system electronics, enables fiber to

transmit digitized light signals well beyond 100 km (60 miles) without

amplification. With few transmission losses, low interference, and high

bandwidth potential, optical fiber is an almost ideal transmission

medium.


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Less expensive - Several miles of optical cable can be made cheaper

than equivalent lengths of copper wire.

Thinnerand lighter- Optical fibers can be drawn to smaller

diameters than copper wire.

Higher carrying capacity/High Bandwidth - Because optical fibers

are thinner than copper wires, more fibers can be bundled into a

given-diameter cable than copper wires. This allows more phone lines

to go over the same cable or more channels to come through the

cable into your cable TV box.

Less signal degradation - The loss of signal in optical fiber is less than

in copper wire.

contd.

Advantages of Fiber Optics


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Light signals - Unlike electrical signals in copper wires, light signals fromone fiber do not interfere with those of other fibers in the same cable.

This means clearer phone conversations or TV reception

Low power - Because signals in optical fibers degrade less, lower-powertransmitters can be used instead of the high-voltage electricaltransmitters needed for copper wires. Again, this saves your provider andyou money.

Digital signals - Optical fibers are ideally suited for carrying digitalinformation, which is especially useful in computer networks.

High Security : Very difficult to Tap without breaking.

Non-flammable - Because no electricity is passed through optical fibers,

there is no fire hazard.

Non-corrosive -Because transmission media is optical fiber no corrosionas compared to Copper


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ref [6]

Disadvantages

Polarization dependence

Wavelength dependence

Limited bend radiusIf physical arc of cable too high, light lost or wont reflect

Will break

Difficult to splice

Mechanical vibration becomes signal noise

Advantages of Fiber Optics


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Profound influence on network architecture

Optical Fiber

Very long distances (>1000 km)

Very high speeds (>40 Gbps /wavelength)

Nearly error -free (BER of 10)

Dominates long distance transmission

Distance less of a cost factor in communications

Plentiful bandwidth for new services

Light sources (lasers, LEDs ) generate pulses of light that are

transmitted on optical fiber

Optical

Electricalsignal

Electricalsignal

Optical fiber

Optical

Opticalsource

Modulator Receiversignal


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Cladding

Core

Cladding JacketLight

qc

qc

Geometry of optical fiber

Total Internal Reflection in optical fiber

Transmission in Optical Fiber

Very fine glass cylindrical core surrounded by concentric layer of glass (cladding) Core has higher index of refraction than cladding Light rays incident at less than critical angle is completely reflected back

into the core


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Multimode : Thicker core, shorter reach

Rays on different paths interfere causing dispersion & limiting bit rate

Single mode: Very thin core supports only one mode (path)

More expensive lasers, but achieves very high speeds

Multimode fiber: multiple rays follow different paths

Singlemode fiber: only direct path propagates in fiber

Direct path

Reflected path

Multi & Single Mode Fiber


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Fiber Optic Connectors

Fiber-to-fiber interconnection can consist of a splice, a permanent

connection, or a connector, which differs from the splice in its

ability to be disconnected and reconnected. Fiber optic connector

types are as various as the applications for which they were

developed. Different connector types have different characteristics,

different advantages and disadvantages, and different performance

parameters. But all connectors have the same four basic

components.


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Parts of a Fiber Optic Connector

The Ferrule:

The fiber is mounted in a long, thincylinder, the ferrule, which acts as a

fiber alignment mechanism. Theferrule is bored through the center ata diameter that is slightly larger thanthe diameter of the fiber cladding.The end of the fiber is located at theend of the ferrule. Ferrules aretypically made of metal or ceramic,

but they may also be constructed ofplastic.

contd


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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 cable

jackets to the connector body. Theferrule extends past the connector

body to slip into the coupling device.

contd.


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The Cable:

The cable is attached to the

connector body. It acts as

the point of entry for thefiber. Typically, a strain-

relief boot is added over the

junction between the cable

and the connector body,

providing extra strength to

the junction.

contd.


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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 theconnectors. Similar devices may be

installed in fiber optic transmitters

and receivers to allow these devices

to be mated via a connector. These

devices are also known as feed-

through bulkhead adapters.


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Splicing

Splicing is only needed if the cable runs are too long for one straight

pull or you need to mix a number of different types of cables (like

bringing a 48 fiber cable in and splicing it to six 8 fiber cables)

Splices are "permanent" connections between two fibers. There are

two types of splices, fusion and mechanical, and the choice is usuallybased on cost or location.

Fusion Splices are made by "welding" the two fibers together usually

by an electric arc.

Mechanical Splices are alignment gadgets that hold the ends of two

fibers together with some index matching gel or glue between them.


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ST ( straight tip )

This was developed by the US company, AT&T, to overcome many of

The problems associated with the SMA and is now the most popular

choice of connector for multimode fibers. It is also available for

Single mode systems.

The problem of repeatability is overcome by fitting a key to the

connector and a corresponding keyway cut into the adapter. There is

now only one position in which the connector can fit into the adapter.

The screw thread of the SMA has been replaced by a bayonet fitting so

that there is no worry about the connector becoming loose whenexposed to vibration.

The ferrule is spring loaded so that the pressure on the end of the

ferrule is not under the control of the person fitting the connector.

There is no SMA worries about how tight to do up the nut.


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FC Connectors

These connectors are used for single-mode and multimode fiber-

optic cables. FC connectors offer extremely precise positioning of

the fiber-optic cable with respect to the transmitter's opticalsource emitter and the receiver's optical detector. FC connectors

feature a position locatable notch and a threaded receptacle. FC

connectors are constructed with a metal housing and are nickel-

plated. They have ceramic ferrules and are rated for 500 mating

cycles. The insertion loss for matched FC connectors is 0.25 dB.

From a design perspective, it is recommended to use a lossmargin of 0.5 dB or the vendor recommendation for FC

connectors.


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LC Connectors-

Lucent Connector / Local Connector

LC connectors are used with single-mode and multimode fiber-optic

cables. The LC connectors are constructed with a plastic housing andprovide for accurate alignment via their ceramic ferrules. LC

connectors have a locking tab. LC connectors are rated for 500

mating cycles. The typical insertion loss for matched LC connectors is

0.25 dB. From a design perspective, it is recommended to use a loss

margin of 0.5 dB or the vendor recommendation for LC connectors.


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Subscriber connector (SC)

Also available in PC and APC versions and suitable for single mode and

Multimode systems.

This connector is designed for high performance telecommunication and

cable television networks.

There is a different feel about this connector when compared with theprevious types. The body is of light plastic construction and has a more

domestic or office feel about it.

It has low losses and the small size and rectangular shape allows a high

packing density in junction boxes. It plugs into the adapter with a very

positive click action, telling us its definitely engaged.


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Flat Type, PC and APC

Many of the connectors are offered in different finishing styles called flat finish,

physical contact (PC), and angled physical contact (APC). So we see the connector

name with a PC or APC added on the end. If nothing is mentioned, we assume a

flat finish.

A flat finish is simply polished to produce a smooth flat end to the fiber so that

the light comes straight out of the connector within the acceptance angle of theother fiber.

In the case of the PC finish, the fiber is polished to a smooth curve. There are

two benefits of a PC connector. As the name implies, the two fibers make physical

contact and therefore eliminates the air gap resulting in lower insertion

losses. The curved end to the fiber also reduces the return loss by reflecting the

light out of the fiber.

The APC finish results in very low return losses, It is simply a flat finish set at an

angle, typically 80. The effect of this is that when the Fresnel reflection occurs

much of the reflected power is at an angle less than the critical angle and is not

propagated back along the fiber.


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Media interface connector ( MIC ),

fixed shroud duplex ( FSD )

or fiber data distributive interface ( FDDI )

Unlike the other connectors, this one has two fibers within thesame cover. This allows signals to be routed in two directions at

the same time. This is called duplex operation.

It uses STPC ceramic ferrules, otherwise it is another all plastic

connector, with a similar feel to the SC. It is intended to beused in local area networks (LANs) to interconnect computer

systems and other pieces of office equipment.


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MT-RJ Connectors Mechanical Transfer

Registered Jack

MT-RJ connectors are used with single-mode and multimode fiber-

optic cables. The MT-RJ connectors are constructed with a plastic

housing and provide for accurate alignment via their metal guide pins

and plastic ferrules. MT-RJ connectors are rated for 1000 mating

cycles. The typical insertion loss for matched MT-RJ connectors is

0.25 dB for SMF and 0.35 dB for MMF. From a design perspective, it isrecommended to use a loss margin of 0.5 dB or the vendor

recommendation for MT-RJ connectors.


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Comparison

Inter-/Intra-

Building,

Security, Navy

SM, MM

Typ. 0.40 dB

(SM)

Typ. 0.20 dB

(MM)

Typ. 0.40 dB

(SM)

Typ. 0.50 dB

(MM)

ST

DatacomSM, MM0.10 dB0.20-0.45 dBSC Duplex

DatacomSM, MM0.10 dB0.20-0.45 dB

SC

High Density

InterconnectionSM, MM0.25 dB0.30-1.00 dB

MT Array

High Density

InterconnectionSM, MM0.2 dB

0.15 db (SM)

0.10 dB (MM)LC

Fiber Optic

NetworkSM, MM0.20 dB0.20-0.70 dB

FDDI

Datacom,

Telecommunica

tions

SM, MM0.20 dB0.50-1.00 dBFC

ApplicationsFiber TypeRepeatabilityInsertion LossConnector RepeatabilityInsertion LossConnector


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Thank you

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