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Optical Fiber Communications
Outline
History
Types of fiber
Light propagation
Losses in optical fiber
Optical fiber classification
Sources
Detectors
Optical fiber system link budget
Introduction
 EM waves are guided
through media composed
of transparent material
 Without using electrical
current flow
 Uses glass or plastic
cable to contain the light
wave and guided them
 Infinite bandwidth – carry
much more information
History
Photophone
 Alexander Graham Bell
 Mirrors and detectors transmit sound wave via
beam light
 Awkward, unreliable, no practical application
 Smoke signals and mirrors
Uncoated fiber cables
 1930, J.L. Baird and C.W. Hansell
 scanning and transmitting TV image
History
 1951 – light transmission via bundles of fibers –
leads to fiberscope – medical field
 1958 – light amplification – stimulated emission
 1960 – laser invention
 1967 – fiber cable with clad
 1970 – low loss optical cable. < 2 dB/km
 1980 – optical cable refined – affordable optical
communication system
 1990 – 0.16dB/km loss
History
1988 – long haul transmission system
1988 – SONET
1990 – optical voice and data network are
common
Advantages
 Wider bandwidth
 Better than metallic cables
 Up to several thousand GHz
 Speed up to several Gbps
 Immunity to crosstalk
 glass fiber/plastic are non-conductor to electrical
current
 immune to adjacent cables
 Immunity to static interference
 immune to static noise – EMI, lightning etc.
Advantages
 Environmental Immunity
 more resistant to environment, weather variations
 wider temperature range operation
 less affected by corrosive liquids and gases
 Safety and convenience




safer and easier to install and maintain
no current and voltage associated
no worry about explosion and fire caused
lighter and compact, flexible, lesser space required
Advantages
 Lower transmission loss
 lesser loss compared to metallic cables
 0.19 dB/km loss @ 1550 nm
 amplifiers can be spaced more farther apart
 Security
 virtually impossible to tap into a fiber cable
 Durability and reliability
 last longer, higher tolerance to changes in environment and
immune to corrosion
 Economics
 Approximately the same cost as metallic cables
 less loss between repeaters. Lower installation and overall
system’s cost
Disadvantages
 Interfacing cost
 Optical cable – transmission medium
 Needs to be connected to standards electronics
facilities – often to be expensive
 Strength
 lower tensile strength
 can be improved with kevlar and protective jacket
 glass – fragile – less required for portability
 Remote electrical power
 need to be include electrical line within fiber cable for
interfacing and signal regeneration
Disadvantages
Loss due to bending
 bending causes irregularities in cable
dimension – the light escapes from fiber core –
loss of signal power
 prone to manufacturing defect
Specialized tools, equipment and training
 tools to splice, repair cable
 test equipment for measurements
 skilled technicians
Optical Spectrum
Optical Communication systems
Types of fiber
 Optical fiber construction
Types of fiber
 Optical fiber construction
 special lacquer, silicone, or acrylate coating –
outside of cladding – to seal and preserve the
fiber’s strength, protects from moisture
 Buffer jacket – additional cable strength
against shocks
 Strength members – increase a tensile
strength
 Outer polyurethane jacket
Types of fiber
 fiber cables – either glass, plastic or both
 Plastic core and cladding (PCP)
 Glass core – plastic cladding (PCS)
 Glass core – glass cladding (SCS)
 Plastic core – more flexible - easier to install
 but higher attenuation than glass fiber – not as good
as glass
 Glass core – lesser attenuation – best propagation
characteristics
 but least rugged
 Selection of fiber depends on its application – trade off
between economics and logistics of particular application
Physics of light
 Physics of light
 Einstein and Planck – light behaves like EM wave and
particles – photon – posses energy proportional to its
frequency
E p  hf
E p  energy of the photons
h  Planck constant
f  light frequency
E p  hf
Ep 
hc

light propagation
 the lowest energy state – grounds state
 energy level above ground state – excited state
 if energy level decays to a lower level – loss of
energy is emitted as a photons of light
 The process of decaying from one level to another
– spontaneous decay or spontaneous emission
 Atoms can absorbs light energy and change its
level to higher level – absorption
E p  E2  E1
light propagation
 Optical power
 flow of light energy past a given point in a specified time
d
(energy)
P
t (time)
dQ

dt
P = optical power
dQ = instanteneous charge
dt = instanteneous change in time
light propagation
 Optical power
 generally stated in decibel to define power level (dBm)
 P 
dBm  10 log 

 1mW 
 Question
 10 mW in dBm?
light propagation
Velocity of Propagation
 in vacuum – 3 x 108 m/s
 but slower in a more dense material than free space
 when it passes through different medium or from one
medium to another denser material – the ray changes
its direction due to the change of speed
light propagation
 from less dense to more denser material
– the ray refracted closer to the normal
 from more denser material to less denser
material – the ray refracted away from the
normal
light propagation
 Refraction
 Occurs when the light travels between two
different material density and changes it speed
based on the light frequency
 Refractive Index
 the ratio of the velocity of propagation of a light
ray in a given material
light propagation
n  cv
n = refractive index
c = speed of light
v = speed of light in a given material
light propagation
 Snell’s Law
 how a light ray reacts when it meets the
interface of two transmissive materials that
have different indexes of refraction
light propagation
 Snell’s Law
 angle of incidence
 angle at which the
propagating ray strike
the interface with
respect to the normal
 angle of refraction
 the angle formed
between the
propagating ray and the
normal after the ray
entered the 2nd medium
light propagation
 Snell’s Law
n1 sin1  n2 sin2
n1 = refractive index material 1
n 2 = refractive index material 2
1 = angle of incidence
 2 = angle of refraction
light propagation
 Question
 medium 1 – glass = 1.5
 medium 2 – ethyl alcohol = 1.36
 angle of incident – 30o
 determine the angle of refraction
light propagation
 Critical Angle
 the angle of incident ray in which the refracted
ray is 90o and refracted along the interface
light propagation
 Critical Angle
 the minimum angle
of incident at which
the refracted angle is
90o or greater
 the light must travel
from higher refractive
index to a lesser
refractive index
material
light propagation
 Critical Angle
n2
sin 1  sin  2
n1
 2  90
n2
sin  c  (1)
n1
n2
 c  sin
n1
1
light propagation
Acceptance Angle
 the maximum
angle in which
external light rays
may strike the
air/glass interface
and still propagate
down the fiber
light propagation
Acceptance Angle
n2  n2
2

 sin 1 1
in(max)
n
0
in (max) = acceptance angle
n0 = refractive index of air
n1 = refractive index of fiber core
n2 = refractive index of fiber cladding
 sin 1 n2  n2
1
2
in(max)

light propagation
Numerical Aperture - NA
 to measure the magnitude of the acceptance
angle
 describe the light gathering or light-collecting
ability of an optical fiber
 the larger the magnitude of NA, the greater the
amount of external light the fiber will accept
light propagation
Numerical Aperture - NA
NA  sin in
NA  n12  n22
in  sin 1 NA
θin = acceptance angle
NA = numerical aperture
n1 = refractive index fiber core
n1 = refractive index fiber cladding
Optical Fiber Configurations
 Mode of propagation
 single mode
 only one path for light
rays down the fiber
 multimode
 many higher order path
rays down the fiber
Optical Fiber Configurations
 Index Profile
 graphical presentation of the magnitude of the
refractive index across the fiber
 refractive index – horizontal axis
 radial distance from core – vertical axis
Optical Fiber Configurations
 Index Profile
 step index –
single mode
 step index –
multimode
 graded
index multimode
optical fiber classification
 Single Mode Step Index
 dominant – widely used in telecommunications
and data networking industries
 the core is significantly smaller in diameter
than multimode cables
optical fiber classification
 Multimode Step Index
 similar to single mode – step index fiber
 but the core diameter is much larger
 light enters the fiber follows many paths as it
propagate down the fiber
 results in different time arrival for each of the
path
optical fiber classification
 Multimode Mode Graded Index
 non uniform refractive index – decreases
toward the outer edge
 the light is guided back gradually to the center
of the fiber
optical fiber classification
 Comparison
 Single mode step index
 (+) minimum dispersion – same path propagation –
same time of arrival
 (+) wider bandwidth and higher information txn. rate
 (-) small core – hard to couple light into the fiber
 (-) small line width of laser required
 (-) expensive – difficult to manufacture
optical fiber classification
 Comparison
 Multimode step index




(+) relatively inexpensive, simple to manufacture
(+) easier to couple light into the fiber
(-) different path of rays – different time arrival
(-) less bandwidth and transfer rate
 Multimode graded index
 intermediate characteristic between step index
single and multimode
losses in optical fiber
Attenuation
 power loss – reduction in the power of light
wave as it travels down the cable
 effect on system’s performance by reducing:




system’s bandwidth
information tx rate
efficiency
overall system’s capacity
losses in optical fiber
 Attenuation
P
 10log  out
A
(dB)
 P
 in




A (dB) = total reduction in power level
P out = cable output power
Pin
= cable input power
losses in optical fiber
Attenuation
 depends on signal’s wavelength
 generally expressed as decibel loss per km
 dB/km
losses in optical fiber
 Attenuation
P  Pt 10 Al /10
P = measured power level
Pt = transmitted power level
A = cable power loss
l = cable length
optical power in decibel units is
P(dBm)= Pin(dBm)-A(dB)
P= measured power level (dBm)
Pin =transmit power (dBm)
A= cable power loss, attenuation (dB)
losses in optical fiber
 Question




Single-mode optical cable
input power 0.1 mW light source
0.25 dB/km cable loss
determine
 optical power 100 km from the transmitter side
losses in optical fiber
 Absorption Loss
 absorption due to impurities – absorb lights and
convert it into heat
 contributors:
 Ultraviolet – ionized valence electron in the silica material.
 infrared – photons of light absorbed by glass’s atom –
converted into random mechanical vibrations - heating
 ion resonance – caused by OH- in in the material. OHtrapped in the glass during manufacturing process
losses in optical fiber
 Absorption Loss
losses in optical fiber
 Material – Rayleigh, Scattering Losses
 permanent submicroscopic irregularities
during fiber drawing process
 when the light propagates and strike one of the
impurities, they are diffracted – causes the light
to disperse and spread out
 some continues down the fiber, some escapes
via cladding – power loss
losses in optical fiber

losses in optical fiber
 Chromatic – Wavelength, Dispersion Loss
 many wavelengths being txn. from LED
 each wavelength travels at different velocity
 arrives at end of fiber at different time
 resulting in chromatic distortion
 solution: using monochromatic light source
losses in optical fiber
 Radiation Losses
 loss due to small bends and kinks in the fiber
 two types of bend:
 microbend – difference in the thermal contraction
rates between core and cladding. Geometric
imperfection along the axis.
 constant radius bend – excessive pressure and
tension during handling and installation
losses in optical fiber
 Modal Dispersion Losses
 pulse spreading
 difference in the propagation times of light rays
that take different path
 occur only in multimode fiber
 solution: use graded index fiber or single mode
step index fiber
losses in optical fiber
 Coupling Losses
 imperfect physical connection
 three types of optical junctions:
• Light source to fiber connection
• Fiber to fiber connection
• Fiber to photodetector connection
 Caused by:
•
•
•
•
Lateral displacement
Gap dispalcement
Angular displacement
Imperfect surface
losses in optical fiber
 Coupling Losses
 Lateral Displacement
 axis displacement between 2 pieces of adjoining
fiber cable
 amount of loss – couple tenth to several decibels
 Gap displacement – miss alignment




end separation
the farther apart, the greater the light loss
if the two fiber is spliced, no gap between fiber
if the two fiber is joined with a connector, the ends
should not touch each other
losses in optical fiber
 Coupling Losses
 angular displacement (misalignment)
 less than 2o, the loss will typically less than 0.5 dB
 imperfect surface finish
 end fiber should be polished and fit together
squarely
losses in optical fiber
 coupling loss
Light Sources
Light source for optical communication
system
 efficiently propagated by optical fiber
 sufficient power to allow light to propagate
 constructed so that their output can be
efficiently coupled into and out of optical fiber
Light Sources

Light Sources
 LED
 p-n junction diode
 made from a semiconductor (AlGaAs)
 emits light by spontaneous emission
Sources
 Homojunction LED
 p-n junction
 two different mixture of the same type of atoms
 Heterojunction LED
 made from p type semiconductor material from one set
of atom and n type semiconductor material from another
set
 Burrus Etched well surface emitting LED
 for higher data rate
 the well helps concentrate the emitted light ray
 allow more power to be coupled into the fiber
 ILD
Injection Laser Diode
Sources
Sources
Sources
Light Detectors
PIN diodes
 light doped material between two heavily
doped n and p type semiconductor
 most common as light detector
APD
 avalanche photo diode
 more sensitive than PIN diode
 require less additional amplification
Detectors
 Characteristic of Light detectors
 responsivity
 a measure of conversion efficiency of photodetector
 ratio of output current to the input optical power
 dark current
 the leakage current that flows through photodiode when
there is no light input
 transit time
 time of light induced carrier to travel across the depletion
region of semiconductor
 spectral response
 the range of wavelength values that a given photodiode
will respond
 light sensitivity
 the minimum optical power a light detector can receive
and still produce a usable electrical output signal
Lasers
 LASER-Light amplification stimulated by the emission of radiation
 --laser technology deals with the concentration of light into a very small,
powerful beam
 --there are 4 types of lasers
 1)Gas lasers: Helium and Neon enclosed in a glass tube laser, CO2 lasers
 --Output is continuous mono chromatic (one colour)
 2)Liquid lasers: organic dye enclosed in a glass tube for an active medium
 --A powerful pulse of light excites the organic dye
 3)Solid lasers: solid, cylindrical crystal such as ruby, for the active medium.
Ruby is excited by a tungsten lamp tied to an alternating-current power
supply.
 --Output is continuous
 4)Semiconductor lasers: Made from semiconductor p-n junctions and are
commonly called Injection laser diodes (ILD’s).
 -- a direct-current power supply controls the amount of current to the active
medium
Laser characteristics
All lasers use
 an active material to convert energy into laser light
 a pumping source to provide power or energy
 optics to direct the beam through the active material to
be amplified
 optics to direct the beam into a narrow powerful cone of
divergence
 a feedback mechanism to provide continuous operation
 an output coupler to transmit power out of the laser