Opt Lecture 01

8/4/2019 Opt Lecture 01

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Optical CommunicationsSemester 2/2005

Lecture 1

Introduction


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What is lightwave technology?

Lightwave technology uses light as the

primary medium to

carry information.

The light often is guided through

optical fibers (fiberoptic technology).

Most applications use invisible

(infrared) light. (HP)


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Why lightwave technology?

Most cost-effective wayto move

huge amounts of information (voice,

data)quickly and reliably.

Light is insensitive to

electrical interference.

Fiberoptic cables have less weight

and consume less space than

equivalent electrical links.

(HP)


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Use Of Lightwave Technology

Majority applications:

Telephone networks

Data communication systems

Cable TV distribution

Niche applications:

Optical sensors

Medical equipment


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Basic Fiber-Optic System

Transmitter(laser diode or LED).

Fiber-optic cable.

Receiver(PIN diode or avalanchephotodiode).

Most fiber systems are digital but analog is

also used.


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Basic Link Design

Transmitter Connector Cable

ReceiverCableSplice


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Typical Long-haul System

Terminal

EquipmentAmplifier

Unit

Regenerator

Unit

Terminal

EquipmentAmplifier

Unit

Amplifier

Unit

Amplifier spans: 30 to 120 km

Regenerator spans: 50 to 600 km

Terminal spans: up to 600 km (without regenerators)

up to 9000 km (with regenerators)

Two pairs of single-mode fiber


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Typical RegeneratorUnit

Pulse re-shaping & re-timing

Power

Supply

Telemetry &

Remote Control

Modulation & bit

rate dependent!


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Typical AmplifierUnit

Optical Amplifiers

Power

Supply

Telemetry &

Remote Control

Modulation & bit

rate independent!


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How fast is fiber optics?

Copper wire (twisted pair) up to ~ 100 Mb/sec (short distances) 1,500 phone calls

2 TV channels

2 Bibles/sec

Coaxial cable (also copper) Up to ~1 Gb/sec (short distances) 15,000 phone calls

20 TV channels (> 200 with data compression)

20 bibles/second

Optical Fiber up to 50 Tb/s (50,000 Gb/s) (long distances) 0.78 billion phone calls

1 million TV channels

1 million Bibles/second

(Light travels in fibers at about 2/3 the speed of light, but so do electrical signals in wire!)


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Company Types

Component Manufacturers

Lasers/LEDs, photodetectors,couplers, multiplexers, isolators,fibers, connectors

Subsystem Manufacturers Transmitters, receivers, amplifiers

(EDFA), repeaters

System Manufacturers

Point-to-point, SONET/SDH, WDM

Installers & Service Providers

Link signature, fault location

Port 1

Port 2

Port 3

Port 4

COMMON

DWDM


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Physical Basics

LW Technology


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The Carrier - Light

RaysWavesParticles

Absorption

Emission

Interference Refraction

Reflection

Bandgap

Conduction band

Valence band

n0

n1

n0


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Light Properties - Wavelength

P

Distance

Field

Strength

1000pm (picometer) =1nm (nanometer) 1000Qm =1 mm

(millimeter)

1000nm (nanometer) =1Qm (micrometer) 1000mm =1 m (meter)

Wavelength P: distance to complete one sine wave


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Electromagnetic Spectrum

Frequency

Wavelength 1 Mm 1 km 1 m 1 mm 1 pm1 nm

1 kHz 1 MHz 1 GHz 1 THz 1 ZHz1 YHz

c = f P n

c: Speed of light ( 2.9979 m/s )f: Frequency

P Wavelength

n: Refractive index

(vacuum: 1.0000; standard air: 1.0003; silica fiber: 1.44 to 1.48)


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LW Transmission Bands

Near Infrared

Frequency

Wavelength 1.6

229

1.0 0.8 m0.6 0.41.8 1.4

UV

(vacuum) 1.2

THz193 461

0.2

353

Longhaul Telecom

Regional Telecom

Local Area Networks

850 nm

1550 nm

1310 nm

CD Players

780 nm

HeNe Lasers

633 nm


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Wavelength and Color Names

Wavelength (and color) can be controlled by type and amount ofdopants (alloy materials) used to make the P and N sides of the lightemitting diode.

Light emitting diodes (LEDs) with visible light output are usedfor indicator lights, etc.

LEDs with infra-red output used as electro-optic (EO)converters for step or graded index fibers

Construction of two parallel semi-reflecting surfaces on the diode

with proper spacing relative to desired wavelength producesenhancement of one wavelength, yielding almost monochromaticLASER radiation (laser diode -- LD), used for single-mode fiber

Proper efficient coupling of light into the fiber core is a majordesign consideration as well (not discussed here)

400nm

Ultra-

violet* blue

500nm 600nm 700nm 850nm 1300nm 1550nm

green red Infra-red**not visible to

human eyes

850, 1300

and 1550

nm arelocal minima

in the fiber

transmission

spectrum,

wavelengths

often used

for fiber

systems.


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Optical Power

Power(P):

Transmitter: typ. -6 to +17dBm (0.25 to 50mW)

Receiver: typ. -3 to -35 dBm (500down to 0.3 W)

OpticalAmplifier: typ. +3 to +20dBm (2 to 100mW)

Laser safety

International standard: IEC 825-1

United States (FDA): 21 CFR 1040.10

Both standards considerclass I safe under reasonable forseeableconditions of operation (e.g., without using optical instruments, suchas lenses or microscopes)


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Snells Law

Demonstration with glass of water

Material with higher

dielectric constant I,slowerwave speed, c2,

larger index n2.

Line perpendicular to interface at

point where ray intersects interface.

Angle of Refraction F

Angle of

Incident Ray

D

Angle of

Reflected

Ray R

R=D and

Sin(R)=Sin(D)

Material with lower

dielectric constant I,fasterwave speed, c1,

smaller index n1.

no=1/co=IoQo :vacuum (orair)n1=1/c1=I1Qo :lower index mediumn2=1/c2=I2Qo :higher index medium

Snells law:

n2Sin(D) = n1Sin(F)

Incident ray power

is partly in reflected

ray, partly in refracted

ray.


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Total Internal Reflection When angle of incidence is beyond B, ~100% of optical

power is reflected internally

some sources measure angle from the perpendicularline rather than from the interface, so inequality isstated differently

When you (or a fish) go under a smooth water surface(e.g., a swimming pool), you can see up to the air only

inside of a circle. Outside that circle, you see onlyreflections from the surface.

B

Location of your (underwater) eye


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What is an optical fiber?

Its basically, a highly transparent light pipe

Input

Light Low index

cladding

High index

Core

Total internal

reflection

up to many kilometers


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The Logarithmic Scale

0 dBm = 1 mW

3 dBm = 2 mW

5 dBm = 3 mW

10 dBm = 10 mW

20 dBm = 100 mW

-3 dBm = 0.5 mW

-10 dBm = 100 QW-30 dBm = 1 QW-60 dBm = 1 nW

0 dB = 1

+ 0.1 dB = 1.023 (+2.3%)

+ 3 dB = 2

+ 5 dB = 3

+ 10 dB = 10

-3 dB = 0.5

-10 dB = 0.1

-20 dB = 0.01

-30 dB = 0.001

dB = 10 log10 (P1/ P0) dBm = 10 log10 (P / 1 mW)


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Interference

Incoherent light adds up optical power

Coherent light adds electromagnetic fields

Zero phase shift:

constructive interference

180 phase shift:

destructive interference+ =

+ =


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Coherence

Coherent lightPhotons have fixed phaserelationship (laser light)

Incoherent lightPhotons with random phase(sun, light bulb)

Coherence length (CL)Average distance over whichphotons lose their phaserelationship

1/e

1

CL


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Reflections

Reflections: root cause for many problemsReturn loss definition:

RL = 10 * log

Pr

Pi

P reflected

P incident


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Polarization

y

x

z

SOP: linear

horizontal

SOP: linear

vertical Most lasers are highly polarized

Degree of polarization (DOP):DOP = Ppolarized / Ptotal

State of polarization (SOP):

describes the orientation

and rotation of thepolarized light


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Brief quantum description of gain process


28/78

Optical Resonator


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Focusing to overcome diffraction


30/78

Why use Guided Waves?


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OpticalWaveguides


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OpticalWaveguide Properties


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Waveguide Principles

Waves propagating in a waveguide are called MODES

Perpendicular Polarised Wave

ElectricField Transverse to the direction of Propagation(TEMODE)

Parallel Polarised Wave

ElectricField Parallel to the direction of Propagation(TMMODE)


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A History of Fiber Optic Technology


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The Nineteenth Century John Tyndall, 1870

water and light experiment

demonstrated light used

internal reflection to follow a

specific path

WilliamWheeling, 1880

piping light patent

never took off

Alexander Graham Bell, 1880

optical voice transmission

system

called a photophone

free light space carried voice

200 meters

Fiber-scope, 1950s

Light


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The Twentieth Century

Glass coated fibers developed to reduce optical

loss Inner fiber - core

Glass coating - cladding

Development of laser technology was important tofiber optics

Large amounts of light in a tiny spot needed

1960, ruby and helium-neon laser developed 1962, semiconductor laser introduced - most

popular type of laser in fiber optics

cladding

core


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The Twentieth Century (continued) 1966, Charles Kao and Charles Hockman proposed optical fiber could be

used to transmit laser light if attenuation could be kept under 20dB/km

(optical fiber loss at the time was over 1,000dB/km) 1970, Researchers at Corning developed a glass fiber with less than a

20dB/km loss

Attenuation depends on the wavelength of light


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The Twentieth Century /Present

Late 1970s, early 1980s:

Second-generationtechnology

Sources/receivers: visible and

near-IR (600 to 920 nm)Fibers: individual multi-mode

fiber

Mid -1980s to present::

Third generation technology

Sources/receivers: near-IR

(1300, 1550 nm)

Fibers: individual single-mode

fibers

Present:

Fourth generation technology

1550 nm operation to usefiberamplifiers

Several wavelengths perfiber(WDM)

Wavelength addressablenetworks


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RealWorld Applications

Military

1970s, Fiber optic telephone link installed aboard the U.S.S. Little Rock

1976, AirForce developed Airborne Light FiberTechnology (ALOF)

Commercial

1977, AT&T and GTE installed the first fiber optic telephone system

Fiber optic telephone networks are common today

Research continues to increase the capabilities of fiber optic

transmission


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The Future Fiber Optics have immense potential bandwidth (over 1

teraHertz, 1012 Hz)

Fiber optics is predicted to bring broadband services to the

home

interactive video

interactive banking and shopping

distance learning

security and surveillance

high-speed data communication

digitized video


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Advantages of Fiber Optics

Immunity from

Electromagnetic (EM)

Radiation and

Lightning LighterWeight

Higher Bandwidth

Better Signal Quality

Lower Cost

Easily Upgraded

Ease of Installation


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Advantages of Fiber Optics

Less expensive Highercarrying capacity

Less signal degradation.

Less interference

Low power losses

Safer

Lightweight

Flexible

HIGHER SPEEDCOMMUNICATIONS

Why are fiber-optic systems revolutionizing telecommunications?

Compared to conventional metal wire (copper wire), optical fibers are:


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Why Not Fibers?

Lack of bandwidth demand

HDTV requires high bandwidth

Lack of standardsTelecomm industry

Computer industry

Radiation darkeningDepends on dose, exposure, glass materials, impurity

types and levels

Clears with time


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Fiber Optic Components - Fiber

Extremely thin strands of ultra-pure glass

Three main regions

center: core (9 to 100 microns)

middle: cladding (125 or 140 microns)

outside: coating or buffer(250, 500 and 900 microns)


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Fiber Structure

Core and cladding are both transparent,

usually glass, sometimes plastic.

Core has higherindex of refraction. Light propagates down the core, reflecting

from cladding.


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Fiber Communication


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Fiber Optic Components - Light Emitters

Two types Light-emitting diodes

(LEDs)

Surface-emitting

(SLED): difficult to

focus, low cost Edge-emitting

(ELED): easier to

focus, faster

Laser Diodes (LDs)

narrow beam fastest

i i i d


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Communications Diode Laser &

Modulator

Grating

InGaAsP

Multiquantum

Well Layers

p-InP/InGaAs

Current

BlockingLayers

n-InP substrate

Laser

Modulator

Frequency Stability~10-5

Lifetime >> 25 years

Maximum modulation speed ~ 40 GHz ( 25 psec ber bit)

(hard to do) - but fibers can carry more information than this

L li ht d LED li ht d


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Laser light and LED light compared

LED are an extended source; light appears as manyindependent light modes each small element of the LED is spatially incoherent

minimum focused size is an image of the LED and this is much largerthan the core of a single-mode fibre and hence coupling efficiency ispoor

Multimode fibre is normally used with LED

Ideal laser light is a single ordered light beam

It is spatially and temporally coherent

Laser light can be focused to a very small spot

Multi-mode fibre

Single-mode fibre


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Fiber Optic Components - Detectors

Two types Avalanche photodiode

internal gain

more expensive

extensive support electronics required

PIN photodiode

very economical

does not require additional support circuitry

used more often


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52/78

Refraction and reflection


53/78

Meaning of refractive index

Refractive index, n defined by:n

cVlightofSpeed !,

n1

n2

U1 U1

U2

2211 sinsin UU nn !

Here n1 < n2


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Typical Fiber Construction

Core - Thin glass center of the fiber where the light travels

Cladding - Outer optical material surrounding the core that reflects the light backinto the core

Buffercoating - Plastic coating that protects the fiberfrom damage and moisture

Hundreds or thousands of these optical fibers are arranged in bundles in optical

cables. The bundles are protected by the cable's outer covering, called ajacket.


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Typical Fiber Structure

Many fibers may be gathered in a protectivecovered cable, with steel or kevlar plastic rope(not shown) incorporated for pulling strength.

High index glass core

Lower index glass cladding

typical

light

ray

Plastic protective jacket, prevents mechanical

damage to outside surface offiber. Can be removed

forsplicing by cutting or dissolving. Typically

color coded for identification of each fiber.


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Principles of Operation - Refraction

Light entering an optical fiber bends in towards the

center of the fiber refraction

Refraction

LED or

LASER

Source


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Principles of Operation - Reflection

Light inside an optical fiber bounces off the cladding -

reflection

Reflection

LED or

LASER

Source


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Principles of Operation - Critical Angle

If light inside an optical fiber strikes the cladding toosteeply, the light refracts into the cladding - determined

by the critical angle

Critical Angle

P i i l f O i A l f


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Principles of Operation - Angle of

Incidence Also incident angle Measured from perpendicular

Incident Angles

Principles of Operation Angle of


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Reflection

Also reflection angle

Measured from perpendicular

Reflection Angle

Principles of Operation Angle of


61/78


Refraction

Also refraction angle

Measured from perpendicular

Refraction Angle


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Principles of Operation - Angle Summary

Refraction Angle

Three important angles The reflection angle always equals the incident angle

Reflection Angle

Incident Angles


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Meridional ray representation

Principles of Operation - Index of


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Principles of Operation - Index of

Refraction

n = c/v

c= velocity of light in a vacuum

v= velocity of light in a specific medium

light bends as it passes from one mediumto another with a different index of

refraction

air, n is about 1

glass, n is about 1.4

Light bends in towards normal -

lower n to higher n

Light bends

away from

normal - higher

n to lower n

P i i l f O i S ll L


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Principles of Operation - Snells Law

The amount light is bent by refraction is given by Snells

Law:

n1sinU

1= n2sinU2

Light is always refracted into a fiber(although there will

be a certain amount ofFresnel reflection)

Light can either bounce off the cladding or refract intothe cladding

Principles of Operation Snells Law


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Principles of Operation - Snell s Law

Example 1

Calculate the angle of refraction at the air/core interface

Solution - use Snells law: n1sinU1 = n2sinU2 1sin(30) = 1.47sin(Urefraction) Urefraction = sin-1(sin(30)/1.47)

Urefraction = 19.89

nair= 1

ncore = 1.47

ncladding = 1.45Uincident = 30

Principles of Operation - Snells Law


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Principles of Operation - Snell s Law

Example 2

Calculate the angle of refraction at the core/claddinginterface

Solution - use Snells law and the refraction angle from

Example 1

1.47sin(90 - 19.89) = 1.45sin(Urefraction) Urefraction = sin-1(1.47sin(70.11)/1.45) Urefraction = 72.42

nair= 1ncore = 1.47

ncladding =

1.45

Uincident = 30



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Calculation

The angle of incidence that produces an angle ofrefraction of 90 is the critical angle

n1sin(Uc) = n2sin() n1sin(Uc) = n2 Uc = sin-1(n2 /n1)

Light at incident angles

greater than the critical

angle will reflect back

into the coreCritical Angle,Uc

n1 = Refractive index of the core

n2 = Refractive index of the cladding

P i i l f O ti A t


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Principles of Operation - Acceptance

Angle and NA

The angle of light entering a fiber which follows the

critical angle is called the acceptance angle, E

E = sin-1[(n12-n2

2)1/2]

Numerical Aperture (NA)

describes the light- gathering

ability of a fiber

NA = sinE

Critical Angle,Uc

n1 = Refractive index of the coren2 = Refractive index of the cladding

Acceptance Angle,E


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Numerical Aperture (NA)

Acceptance / Emission Cone

NA = sin U = n2core - n2cladding

Principles of Operation - Acceptance


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Principles of Operation Acceptance

Cone

There is an imaginary cone of acceptance with an angleE

The light that enters the fiber at angles within the

acceptance cone are guided down the fiber core

Acceptance Cone

Acceptance Angle,E


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For example, a typical silica fibre has n1=1.48 and n2 =1.45

giving an NA of 0.3.

For a large (extended) source, such as an LED, which also

emit light over a wide range of angles, the product of the NA

and the fibre entrance aperture area determines the fraction of

the LED output light that can be coupled into the LED.Normally

this fraction is small.

A laser is effectively a very small source (it is said to bespatially coherent) and can be matched to the fibre to give high

power coupling efficiency

LED

LASER

i i l f O i l


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Principles of Operation - Formula

Summary

Index ofRefraction

Snells Law

Critical Angle

A

cceptanceA

ngle

Numerical Aperture

v

cn !

2211 sinsin UU nn !

!

1

21sinn

ncU

2

2

2

1

1

sin nn !

E

2

2

2

1sin nnN !! E

Basic Step-Index (SI) Fiber Design


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Basic Step-Index (SI) Fiber Design

Refractive

Index (n)

Diameter(r)

Cladding

Primary coating

(e.g., soft plastic)

Core

1.480

1.460

Most common designs: 100/140 or 200/280 Qm

Plastic optical fiber(POF): 0.1 - 3 mm , core 80 to99%

140 Qm

100 Qm

R i Fib P V l


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Representative FiberParameter Values


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Fiber Types

SM step index MM step index MM graded index

Multi-mode (Step-index), Graded


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Multi mode (Step index), GradedIndex, Single Mode

Cross sectional views ( should be circles*)Multi-mode Graded Index Single Mode

125Qm

~10Qm~80Qm

Accurate alignment less needed

for splicing. Higher loss. Major

time dispersion of short optical

pulses due to different geometric

paths. Less used today, but

historically important.

Accurate alignment less needed

for splicing. Higher loss. Reduced

dispersion due to lowerwave speed

in central rays, higher wave speed

(lowerindex) in outer part of core.

Used for last mile and service drops

with single mode for long runs.

Accurate alignment neededfor splicing. Best low loss.

Most widely used fiber type

for long spans.

*non-circularity is an

artifact of computer

artwork software.

M h i l t t f i l d


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Mechanical structure of single-mode

and multimode step/graded index fibers

Documents

Opt Lecture 01