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FACULTY DEVELOPMENT PROGRAMME

TODAYS TOPIC

OPTICAL FIBER COMMUNICATION

Mr.Rajkumar D BhureAssoc.Prof.,ECE Dept. JBIET

DEPT OF ELECTRONICS & COMMUNICATION ENGINEERING,JBIET

By:- Mr. Rajkumar D Bhure

Assoc.Prof. ECE Dept. JBIET

Optical Fiber Communication

Spectrum for communication

Attenuation Vs Wavelength

Fiber vs. CopperOptical fiber transmits light pulses

Can be used for analog or digital transmissionVoice, computer data, video, etc.

Copper wires (or other metals) can carry the same types of signals with electrical pulses

Advantages of OFC over conventional Copper Wire Communication

Small Size and light Weight Abundant Raw Material Availability Higher Band Width Low Noise Low transmission loss and high signal security Highly Reliable and easy of maintainance Fibers Are Electrically Isolation Highly transparent at particularo Wave Length No possiblityof ISI and echoes cross talketc. Immunity to natural hazardous.

Index of Refraction(n)When light enters a dense medium like

glass or water, it slows downThe index of refraction (n) is the ratio of

the speed of light in vacuum to the speed of light in the medium n=c/v

Water has n = 1.3 Light takes 30% longer to travel through it

Fiber optic glass has n = 1.5Light takes 50% longer to travel through it

Material Used Basic materials are plastic and

Sand(Sio2)The Dopents used increase or decrease

RI are:- GeO2 and P2O5--- To increase RI

B2O3------ To Decrease RI

Elements ofOptical communication System

Optical FiberCore

Glass or plastic with a higher index of refraction than the cladding

Carries the signal

CladdingGlass or plastic with a lower index

of refraction than the core

BufferProtects the fiber from damage

and moisture

JacketHolds one or more fibers in a

Plastic Optical FiberLarge core (1 mm) step-index multimode

fiberEasy to cut and work with, but high

attenuation (1 dB / meter) makes it useless for long distances

Singlemode FiberSinglemode fiber has a core diameter of 8 to 9

microns, which only allows one light path or modeImages from arcelect.com (Link Ch 2a)

Index of refraction

Multimode Step-Index FiberMultimode fiber has a core diameter of 50 or

62.5 microns (sometimes even larger)Allows several light paths or modesThis causes modal dispersion – some modes

take longer to pass through the fiber than others because they travel a longer distance

See animation at link Ch 2f

Index of refraction

Step-index MultimodeLarge core size, so source power can be

efficiently coupled to the fiberHigh attenuation (4-6 dB / km)Low bandwidth (50 MHz-km)Used in short, low-speed datalinksAlso useful in high-radiation environments,

because it can be made with pure silica core

Singlemode FIberBest for high speeds and long distancesUsed by telephone companies and CATV

Multimode Graded-Index FiberThe index of refraction gradually changes

across the coreModes that travel further also move fasterThis reduces modal dispersion so the bandwidth

is greatly increased

Index of refraction

Step-index and Graded-indexStep index multimode was developed first,

but rare today because it has a low bandwidth (50 MHz-km)

It has been replaced by graded-index multimode with a bandwidth up to 2 GHz-km

Types of Optical Fibers

Total Internal Reflection

1 1 2 2

1 2 1 2

:sin sin

90 deg

sin sin

Snells Lawn n

flection Condition

When n n and as increases eventually

goes to rees andn

n n orn

is called the Critical angleFor there is no pro pagating refracted ray

Light Ray confinement

nSinθ0=n1Sinθ; SinΦ=n2/n1;

NA= n1(2Δ)1/2 ; ΔT=L Δ n12/cn2

Acceptance AngleThe acceptance angle (qi) is the largest incident angle ray that can be coupled into a guided ray within the fiber

The Numerical Aperture (NA) is the sin(qi) this is defined analogously to that for a lens

D FullAccep ceAngle

1 1 12 2 22 2 2

1 2 1 2

( ) (2 ) (2 )

NA n n n n

n n n nWhere n

= - = D = D

- +D º º

Sources and WavelengthsMultimode fiber is used with

LED sources at wavelengths of 850 and 1300 nm for slower local area networks

Lasers at 850 and 1310 nm for networks running at gigabits per second or more

Sources and WavelengthsSinglemode fiber is used with

Laser sources at 1300 and 1550 nmBandwidth is extremely high, around 100

THz-km

Fiber Optic SpecificationsAttenuation

Loss of signal, measured in dBDispersion

Blurring of a signal, affects bandwidthBandwidth

The number of bits per second that can be sent through a data link

Numerical ApertureMeasures the largest angle of light that can be

accepted into the core

Attenuation and Dispersion

Measuring BandwidthThe bandwidth-distance product in

units of MHz×km shows how fast data can be sent through a cable

A common multimode fiber with bandwidth-distance product of 500 MHz×km could carryA 500 MHz signal for 1 km, orA 1000 MHz signal for 0.5 km

From Wikipedia

Numerical ApertureIf the core and cladding have almost the

same index of refraction, the numerical aperture will be small

This means that light must be shooting right down the center of the fiber to stay in the core

Fiber Manufacture

Three MethodsModified Chemical Vapor Deposition

(MCVD)Outside Vapor Deposition (OVD)Vapor Axial Deposition (VAD)

Modified Chemical Vapor Deposition (MCVD)

A hollow, rotating glass tube is heated with a torch

Chemicals inside the tube precipitate to form soot

Rod is collapsed to crate a preform

Preform is stretched in a drawing tower to form a single fiber up to 10 km long

Modified Chemical Vapor Deposition (MCVD)

Outside Vapor Deposition (OVD)A mandrel is coated with a porous preform in

a furnaceThen the mandrel is removed and the preform

is collapsed in a process called sinteringImage from csrg.ch.pw.edu.pl

Vapor Axial Deposition (VAD)Preform is fabricated

continuouslyWhen the preform is

long enough, it goes directly to the drawing towerImage from

csrg.ch.pw.edu.pl

Drawing Apparatus

The fiber is drawn from the preform and then coated with a protective coating

Fiber Performance

AttenuationModern fiber material is very pure, but there is

still some attenuationThe wavelengths used are chosen to avoid

absorption bands850 nm, 1300 nm, and 1550 nmPlastic fiber uses 660 nm LEDs

Image from iec.org (Link Ch 2n)

Optical Loss in dB (decibels)

If the data link is perfect, and loses no powerThe loss is 0 dB

If the data link loses 50% of the powerThe loss is 3 dB, or a change of – 3 dB

dB = 10 log (Power Out / Power In)

Data LinkPower In Power Out

Absolute Power in dBmThe power of a light is measured in

milliwattsFor convenience, we use the dBm units,

where-20 dBm = 0.01 milliwatt-10 dBm = 0.1 milliwatt0 dBm = 1 milliwatt10 dBm = 10 milliwatts20 dBm = 100 milliwatts

Three Types of DispersionDispersion is the spreading out of a light

pulse as it travels through the fiberThree types:

Modal DispersionChromatic DispersionPolarization Mode Dispersion (PMD)

Modal DispersionModal Dispersion

Spreading of a pulse because different modes (paths) through the fiber take different times

Only happens in multimode fiberReduced, but not eliminated, with graded-

index fiber

Chromatic DispersionDifferent wavelengths travel at different

speeds through the fiberThis spreads a pulse in an effect named

chromatic dispersion(group Delay)

T=1/vg =1/L *dВ/dώChromatic dispersion occurs in both

singlemode and multimode fiberLarger effect with LEDs than with lasersA far smaller effect than modal dispersion

Modal DistributionIn graded-index fiber, the off-axis modes go

a longer distance than the axial mode, but they travel faster, compensating for dispersionBut because the off-axis modes travel

further, they suffer more attenuation

Equilibrium Modal DistributionA long fiber that has lost the high-order

modes is said to have an equilibrium modal distribution

For testing fibers, devices that can be used to condition the modal distribution so that measurements will be accurate

Mode StripperAn index-matching substance is put on

the outside of the fiber to remove light travelling through the cladding

Mode ScramblerMode scramblers mix light to excite every

possible mode of transmission within the fiberUsed for accurate measurements of

attenuation Figure from

fiber-optics.info (Link Ch 2o)

Semiconductor Optical Sources

Source Characteristics Important Parameters

Electrical-optical conversion efficiencyOptical powerWavelengthWavelength distribution (called linewidth)Cost

Semiconductor lasersCompactGood electrical-optical conversion efficiencyLow voltagesLos cost

Semiconductor Optoelectronics Two energy bands Conduction band (CB)Valence band (VB)

Fundamental processesAbsorbed photon creates an electron-hole pairRecombination of an electron and hole can emit a photon

Types of photon emission Spontaneous emission

Random recombination of an electron-hole pair Dominant emission for light emitting diodes (LED)

Stimulated emission A photon excites another electron and hole to recombine Emitted photon has similar wavelength, direction, and phase Dominant emission for laser diodes

Basic Light Emission Processes

Pumping (creating more electron-hole pairs)Electrically create electron-hole pairsOptically create electron-hole pairs

Emission (recombination of electron-hole pairs)Spontaneous emission Simulated emission

Semiconductor Material Semiconductor crystal is required Type IV elements on Periodic Table

SiliconGermanium

Combination of III-V materialsGaAs InPAlAsGaP InAs

Direct and Indirect Materials

Relationship between energy and momentum for electrons and holes Depends on the material

Electrons in the CB combine with holes in the VB Photons have no momentum

Photon emission requires no momentum change CB minimum needs to be directly over the VB maximum Direct band gap transition required

Only specific materials have a direct bandgap

Light Emission The emission wavelength depends on the energy band gap

Semiconductor compounds have differentEnergy band gapsAtomic spacing (called lattice constants)

Combine semiconductor compoundsAdjust the bandgapLattice constants (atomic spacing) must be matchedCompound must be matched to a substrate

Usually GaAs or InP

12 EEEg

hc 24.1

Common Semiconductor CompoundsGaAs and AlAs have the same lattice constants

These compounds are used to grow a ternary compound that is lattice matched to a GaAs substrate (Al1-xGaxAs)

0.87 < l < 0.63 (mm)

Quaternary compound GaxIn1-xAsyP1-y is lattice matched to InP if y=2.2x1.0 < l < 1.65 (mm)

Optical telecommunication laser compoundsIn0.72Ga0.28As0.62P0.38 (l=1300nm)

In0.58Ga0.42As0.9P0.1 (l=1550nm)

Optical SourcesTwo main types of optical sources

Light emitting diode (LED)Large wavelength contentIncoherentLimited directionality

Laser diode (LD)Small wavelength contentHighly coherentDirectional

Band-gap and refractive index engineering.

Heterostructured LED

Avoiding losses in LED

Carrier confinement

Photon Confinement

Double heterostructure

Burrus type LED

Shown bonded to a fiber with index-matching epoxy.

Double Heterojunction LED (important)

n+ GaAs

p Al GaAs

p GaAs (active region)n AlGaAs

Metal contact

Fiber Optics

Double HeterostructureThe double heterostructure is invariably used

for optical sources for communication as seen in the figure in the pervious slide.

Heterostucture can be used to increase:Efficiency by carrier confinement (band gap

engineering)Efficiency by photon confinement (refractive

index)The double heterostructure enables the source

radiation to be much better defined, but further, the optical power generated per unit volume is much greater as well. If the central layer of a double heterostructure, the narrow band-gap region is made no more than 1m wide.

Photon confinement - Reabsorption problem

Source of electrons

Source of holes

Active region (micron in thickness)

Active region (thin layer of GaAs) has smaller band gap, energy of photons emitted is smaller then the band gap of the P and N-GaAlAs hence could not be reabsorbed.

Carrier confinement

p+-AlGaAsn+-AlGaAs p-GaAsholes

electrons

Simplified band diagram of the ‘sandwich’ top show carrier confinement

Light Emitting Diodes (LED) Spontaneous emission

dominates Random photon emission

Spatial implications of random emission Broad far field emission

pattern Dome used to extract more of

the light Critical angle is between

semiconductor and plastic Angle between plastic and air

is near normal

Spectral implications of random emission Broad spectrum

kTp245.1

Laser Diode Stimulated emission dominates

Narrower spectrum More directional

Requires high optical power density in the gain region High photon flux attained by creating an optical cavity Optical Feedback: Part of the optical power is reflected back into

the cavity End mirrors

Lasing requires net positive gain Gain > Loss Cavity gain

Depends on external pumping Applying current to a semiconductor pn junction

Cavity loss Material absorption Scatter End face reflectivity

Laser Diode Stimulated emission dominates

Narrower spectrum More directional

Requires high optical power density in the gain region High photon flux attained by creating an optical cavity Optical Feedback: Part of the optical power is reflected back into

the cavity End mirrors

Lasing requires net positive gain Gain > Loss Cavity gain

Depends on external pumping Applying current to a semiconductor pn junction

Cavity loss Material absorption Scatter End face reflectivity

Optical Feedback

Easiest method: cleaved end faces End faces must be parallel Uses Fresnel reflection

For GaAs (n=3.6) R=0.32 Lasing condition requires the net cavity gain to be one

g: distributed medium gain a: distributed loss R1 and R2 are the end facet reflectivity's

1exp21 LgRR

Phase Condition

The waves must add in phase as given by

Resulting in modes given by

Where m is an integer and n is the refractive index of the cavity

mL z 22

Longitudinal Modes

The optical cavity excites various longitudinal modes

Modes with gain above the cavity loss have the potential to lase

Gain distribution depends on the spontaneous emission band

Wavelength width of the individual longitudinal modes depends on the reflectivity of the end faces

Wavelength separation of the modes Dl depends on the length of the cavity

Mode SeparationWavelength of the various modes

The wavelength separation of the modes is

A longer cavityIncreases the number of modesDecrease the threshold gain

There is a trade-off with the length of the laser cavity

1121 mmnLmm

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