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FACULTY DEVELOPMENT PROGRAMME
TODAYS TOPIC
OPTICAL FIBER COMMUNICATION
BY
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
cable
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 3
1 2 1 2
21 2
1
1
:sin sin
Re
90 deg
sin sin
c c
c
c
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
#
tan
1
2
f ff
D FullAccep ceAngle
NA
º =
=×
1 1 12 2 22 2 2
1 2
1 2 1 2
( ) (2 ) (2 )
and 2
NA n n n n
n n n nWhere n
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
If the data link loses 90% of the powerThe loss is 10 dB, or a change of – 10 dB
If the data link loses 99% of the powerThe loss is 20 dB, or a change of – 20 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
gg EE
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
Metal contact
Epoxy
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
2
1
1
n
nR
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
m
nL2
Longitudinal Modes
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
m
nL2
1
1121 mmnLmm
nLm
nL
2
2 2
2