Jim Downing CoPI Subject Matter Expertise SME National Center for Telecommunications Technologies Ma

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Jim DowningCo-PI / Subject Matter Expertise
(SME)National Center for Telecommunications
TechnologiesMay 23-25, 2005
3rd Annual National Conference for High
Performance Computing Technology
Communications TechnologyIntroduction to
Fiber-Optic Communications
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Historical Highlights

• B.C. Greeks use fire for signals and Heron of
  Alexandria shows sunlight traveling in a water
  leak from a bucket.
• 1790s Paul Revere sees light in the Old North
  Church and French engineer Claude Chappe
  invents the optical telegraph.
• 1800s The Heron experiment was repeated by Daniel
  Collodon in Switzerland and Jaques Babinet in
  France, and was popularized by John Tyndall
  in 1850s England. Alexander Graham Bell
  patents the photophone in 1880.

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Bells Photophone
Alexander Graham Bell patented this photophone in
1880 (Courtesy of AMP Inc.).
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Historical Highlights

• 1920s John Logie Baird (England) and Clarence W.
  Hansel (U.S.) patented hollow glass pipes or
  transparent rods to transmit televison
  signals.
• 1954 Abraham Van Heel at the Technical
  University at Delft in Holland first added a
  transparent cladding to fiber after a
  suggestion the by American optical physicist
  Brian O'Brien.
• 1960s Optical fibers had attenuation of about
  1dB/m. Work by Elias Snitzer of American
  Optical and Will Hicks at Mosaic Fabrications
  led to the development of small-core single
  mode fiber. Charles K. Kao and George Hockham
  suggest that long-distance communications
  with lt 20dB/km fiber would eventually become a
  reality.
• 1970 First low-loss fiber made at Corning.

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First Low-Loss Fiber(20 dB/km)
Donald Keck, Robert Maurer, and Peter Schultz
(left to right), who made the first low-loss
fibers in 1970 at Corning. Courtesy Corning,
Incorporated.
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Why Fiber Optics?
Greater information carrying capacity. The
bandwidth increase for fiber optic systems is
about ten times that of conventional electrical
systems. This is equivalent to THz of information
carrying capability, using a 10Gb/s per channel
DWDM system. Lower loss. Fiber attenuation is
less than 1dB/km, and instead of amplifiers every
kilometer or so as with conventional systems,
amplification is only needed for about every
200km of optical fiber. Lower cost per bit. The
result of the first two advantages, fiber optics
systems carry much more information for the same
cost. Electrical isolation. Fibers are immune to
disturbances caused by lightning and
electromagnetic interference (EMI) and are much
more secure because the signal cannot be accessed
unless the fiber itself is breached. Small size
and weight. Fibers take up much less space and
are lighter than the equivalent copper
carrier. Environmental ruggedness. Fiber cables
can be designed to last in harsh environments.
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Fiber and Copper Cable Comparison Capacity
Equivalent information carrying capacity fiber
(right) and copper cables.
(Courtesy of Lucent Technologies)
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Fiber and Copper Cable Comparison Speed,
Capacity and Repeater Spacing
(From Sterling)
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Fiber and Copper Cable Comparison Bandwidth

(Courtesy of Siecor Corp.)
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Basic Telecommunications

• Analog and Digital Signals
• Modulation
• Network Topologies
• Transmission Media
• System Concepts

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Digital Transmission
(Courtesy of AMP Inc.)
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Modulation
(Courtesy of AMP Inc.)
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Time Division Multiplexing
(from Gokhale)
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Network Topologies
(From Sterling)
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Seven Layer OSI Model
gtgtgtgtgt
(from Gokhale)
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Transmission Media

• Copper
• Original PSTN, CATV, earlier LAN backbones, T-1
  and cable internet access
• Wireless
• Cellular phones and radio, wireless modems and
  LANS, personal communications systems,
  multipoint distribution systems
• Lightwave
• Fiber backbone and long haul, optical networks,
  integrated optics,
• free space optics

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The Transfer Function and Decibels
 
 
The decibel (dB) is often used to describe a
voltage or power ratio as in a communication
transfer function. The transfer function
expressed in decibels is
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System Loss
Problem A fiber optic communications system has
an output power of 2mW for an input of 3mW. Find
the loss in dB. Solution
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Basic Communications System
(Courtesy of AMP Inc.)
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Fundamentals of Optics
Geometrical Optics. Light as a particle
(refraction, Snells Law, reflection, scattering)
Wave Optics. Light as a wave (the
electromagnetic wave, polarization, coherence,
interference, diffraction) Quantum Optics. The
quantum nature of light (Plancs Law, absorption,
emission) Nonlinear Optics. High power anomalies
(four-wave mixing, phase modulation, Brillouin
scattering, Raman scattering) Optical Power. The
measure of light (power and energy, Watts, dBm)
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Refraction
Refraction is the bending of light rays as they
pass through a medium, accompanied by a change in
velocity. The ratio of the speed of light in a
vacuum to the speed of light in that medium (v)
is called the refractive index and it is given
by   where c is the vacuum speed of light or
3.0 x 108 m/s. Note that the optical path length
(S), or the apparent length of the optical
element can be determined from     where L is
the actual length of the element. One
complication with respect to the refractive index
is that it is wavelength dependent.
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Refractive Index of Various Materials
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Snells Law
n1 sin ?1 n2 sin ?2

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Refraction and Total Internal Reflection
n2 gt n1 Light refracted at all angles. n1 gt
n2 Light refracted until refracted angle
is 90 and incident angle reaches the
critical angle. All light is reflected for
incident angles greater than the critical
angle.
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Law of Reflection
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Fresnel Reflection
In the early 1700s Fresnel developed his laws of
reflection, which determined the fraction of
light reflected as a function of incident angle.
For both parallel and perpendicular polarizations
at normal incidence, Fresnel reflection reduces
to     Example Find the fraction of incident
light reflected in air at normal incidence
from (a) fused silica and (b) BK-7
glasses. Solution (a) (b)  
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Fresnel Reflection n2 gt n1
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Fresnel Reflection n1 gt n2
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Electromagnetic Wave
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Polarization and Coherence
Polarization Polarization describes the direction
of the electric field oscillation. In most
examples, we have assumed the electric field is
oriented such that it only oscillates along the
y-axis or is linearly polarized in the
y-direction. Most light is actually randomly
polarized and oscillates in any of the planes
perpendicular to the z-axis. We will see later
how polarization can be controlled somewhat in
optical fibers. Unless otherwise stated, we will
assume linear polarization in our discussions and
problems. Linearly polarized light can be
generated by passing a randomly polarized beam
through a polarizing material. The material
reduces the amplitude of all other orientations.
Circular and elliptical polarizations are also
possible
Coherence If the phase difference is zero, then
we have coherence. Temporal coherence is
equality between the time dependent parts of the
equations or implies that the wavelengths are
equal. This would indicate monochromatic
(single color) light. Spatial coherence
implies that the waves are in phase at a point in
space. Incoherent light means that the phase is
always changing.
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Interference
Constructive Interference
Destructive Interference
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Diffraction

Light spreads out as it passes through an
aperture. Interference of waves on a screen
causes light and dark rings.
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Diffraction Gratings
Transmission Grating
Reflection Grating
d sin ? m ?
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Scattering

Light scattering is the spreading apart of light
caused by an interaction with matter. A common
phenomenon responsible for our blue sky,
scattering is divided into two basic categories.
Rayleigh scattering, often referred to as
molecular scattering, is caused by particles with
diameters less than or equal to one-tenth of the
wavelength of light. Light is scattered equally
in all directions perpendicular to the plane of
polarization, with shorter wavelengths scattered
more than longer ones. Mie scattering is
scattering from particles larger than one-tenth
wavelength. Often cause by impurities, the
scattering is out of phase and propagates mostly
in the direction of the original light wave.
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Electromagnetic Spectrum
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Absorption and Emission

Plancs Law states that E h ? Where E is
the energy in Joules, h is Plancs constant
(6.626x10-34 J s) and ? is the frequency of the
light.
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Nonlinear Optics
Four-wave mixing is similar to harmonic
generation in which two frequencies are added to
form a third. If three high power evenly spaced
frequency signals are combined, occasionally
a fourth signal is generated which has the same
spacing but at the next lowest frequency. This
can be a problem if that frequency is already in
use, or four-wave mixing can be used to
generate a fourth frequency when
needed.   Phase modulation is the result of a
change in the refractive index of a material with
a change in light intensity at higher powers. In
self-phase modulation, the changes in phase with
intensity broaden the linewidth of a particular
signal. Cross-phase modulation arises when
self-phase modulation also causes phase changes
in another signal, resulting in linewidth
broadening at another wavelength.
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Nonlinear Optics
Brillouin ScatteringBrillouin scattering occurs
at optical powers high enough to generate small
acoustic waves in the material. The resulting
change in density alters the refractive index,
shifting the frequency slightly. For long pulses
and narrow linewidths, stimulate Brillouin
scattering can occur at much lower power
levels. Raman ScatteringIn stimulated Raman
scattering, light is absorbed andsome energy is
lost or gained from molecular vibrations.The
reemitted light is shifted from the original
energyby plus or minus the molecular vibrational
energy.Using Raman scattering energy can be
transferredfrom one wavelength to another.
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Optical Power
  We cannot measure the energy of photons, but
the optical power of the signal can be detected
with a photodetector. The optical power is given
by     It is often more convenient when
dealing with fiber optic systems to use the
decibel-milliwatt form of power or dBm given
by     The conversion from dBm to Watts is
then     The dBm form allows for
simplification of system analysis in that the
output can be determined directly by    
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Properties of Optical Fibers
Acceptance Angle and Numerical Aperture
Modes Dispersion Attenuation Reflection
and Bending Losses Special Fibers
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Acceptance Angle andNumerical Aperture
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Fiber Modes
While the numerical aperture tells us much about
optical fiber performance, we must again use the
wave model of light to understand fiber modes.
The numerical aperture implies that any
acceptance angle between normal incidence and
the acceptance half-angle will propagate down the
fiber. This is not actually the case. The
geometry of the fiber shape and the existence of
forward traveling and backward travelling
(reflected) waves yield constructive and
destructive wave interference, allowing only
certain ray angles or modes to propagate. The
numerical results are obtained by applying
Maxwell's electromagnetic field equations and
examining the boundary conditions, which yield a
set of Bessel functions describing allowable
modes. For our purposes, we can describe a
characteristic waveguide parameter or "V number"
which allows us to simplify the analysis of
propagating modes. The V number is given
by where a is the radius of the fiber core and
? is the wavelength of light. For single mode
propagation, we must have The number of modes
propagating in a step index fiber is approximated
by
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Types of Fiber
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Fiber Dispersion

Output Pulse
Input Pulse
Modal dispersion
Material dispersion
Waveguide dispersion
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Fiber Attenuation
Typical fiber attenuation (Courtesy of Corning
Glass Works).
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Fiber Transmission Bands
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Fresnel Reflection at a Fiber Interface

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Bending Losses
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Special Fibers
Plastic Optical Fiber (POF) Plastic fiber is
much cheaper to make but has very high
attenuation. For short distance networks (such
as in automobiles) POF may be the solution. Low
OH Fiber OH or water bands have been a problem
in the past, but manufacturers have developed
fiber with very low OH cont which is usuable
throughout the fiber transmission
regions. Polarization Maintaining Fibers (PMF)
PMF keeps the polarization of the incoming light
and keeps cross-coupling between polarization
modes at a minimum. PMF are used in lithium
niobate modulators and Raman amplifiers. Erbium
Doped Fibers (EDF) Primarily used for Erbium
doped fiber amplifiers (EDFA). Photosensitive
Fibers Grmanium and germanium-boron doped fibers
generate photosensitivity for fabrication of
Bragg gratings. Other Fibers Reduced cladding
fibers, lensed fibers and holey or crystal fibers
are among those currently under development.
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Dispersion Shifted Fiber
Waveguide dispersion offsets chromatic
dispersion at 1.31µm in step index single mode
fiber. Zero dispersion-shifted fibers have
increased waveguide dispersion to achieve zero
total dispersion near 1.55 µm where fiber
absorption is minimum. Non-zero
dispersion-shifted fibers move the total
dispersion slope outside of the erbium-fiber
band to avoid four-wave mixing. Low (but not
zero) dispersion is achieved with layered core
structures.
(From Sterling)
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Fiber Performance Parameters
(From Sterling)
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Fiber and Cable Fabrication
Fiber Fabrication Fiber Cables
Connectors Splices
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Fiber Fabrication
Fused Silica Glass Fused silica is the medium of
choice for optical fiber communications. While
other fiber materials are adequate and even
advantageous to some applications, only a
high-quality glassy melt of silicon dioxide SiO2
has the purity needed to make ultra-low-loss
fiber. To obtain the pure material, usually a
fused silica soot is deposited on a surface by
reacting SiCl4 with oxygen. The reaction of
GeCl4 and oxygen produces a soot of germanium
dioxide (GeO2) for use as a dopant. Both
germanium dioxide and phosphorus pentaoxide
(P2O5) are used to increase the refractive index
of the pure silica and boron trioxide (B2O3) and
fluorine (F) are used to decrease the index.
Various procedures have been defined to deposit
the silica soot and prepare a preform, which is a
single glass rod with the refractive index
profile of the finished fiber material.
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Fiber Preform Fabrication
Rod-in-Tube Method
Double Crucible Method
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Fiber Preform FabricationModified Chemical
Vapor Deposition (MCVD)
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Fiber Preform FabricationOutside Vapor
Deposition (OVD)
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Fiber Preform FabricationAxial Vapor Deposition
(AVD)
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Fiber Draw Process
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Typical Fiber Sizes
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Fiber Cable Structure
(Courtesy of Hewlett-Packard)
(Courtesy of Belden Electronic



Wire and Cable)
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Connector Losses NA and Diameter Mismatch
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Connector LossesLateral, Rotational and
Separation Displacement
(Courtesy of AMP Inc.)
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Types ofConnectors
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Splices
Mechanical Splice (Courtesy of 3M)
Fusion Splicer (Courtesy of Siecor Corp.)
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Sources and Transmitters
Conduction P-N Junction Diode The Light
Emitting Diode (LED) The Laser Comparison
of LED and Laser Performance Transmitters
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Conduction

Electrons move toward positive source
terminal. Holes appear to move in opposite
direction.
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P-N Junction Diode
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Periodic Table
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LED Emission Process

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LED Structure

Homojunction LED
Heterojunction LED
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LED Output Patterns

Surface Emitting LED
Edge Emitting LED
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Burrus Diode

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Stimulated Emission

An External photon stimulates a conduction band
electron to fall to the valence band. A second
photon is then emitted at the same wavelength.
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Positive Feedback

After stimulated emission occurs, photons reflect
off mirrors and stimulate additional emission on
each pass.
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Population Inversion

High current densities cause electrons to move to
the conduction band. Population inversion occurs
when electrons move to the conduction band faster
than than they drop to the conduction band by
stimulated emission.
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Single Quantum Well Laser (SQWL)

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Fabry-Perot Laser Diode

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Distributed Feedback Laser (DFBL)

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Vertical CavitySurface Emitting Laser (VCSEL)
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Laser Diode Output Power

(Courtesy of AMP Inc.)
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LED and Laser General Characteristics
(From Sterling)
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LED and Laser Linewidth
(Courtesy of AMP Inc.)
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LED and Laser Fiber Coupling

(From Sterling)
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LED and Laser Transmitters
(From Sterling)
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Detectors and Receivers
The PN Photodiode The3 PIN Photodiode
The Avalanche Photodiode Detector Performance
Characteristics Receivers
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Photodiode Operation
Photons with energy equal to or grater than the
gap energy cause electrons to move to the valence
band. A current is then generated proportional to
the light striking the photodiode.
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PIN and Avalanche Photodiode Operation
Avalanche Photodiode
PIN Photodiode
(Courtesy of AMP Inc.)
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Detector Responsivity
(From Sterling)
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Detector/Amplifier Characteristics
(From Sterling)
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Eye Pattern
(From Sterling)
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Fiber Optic Devices
Couplers Modulators Optical Amplifiers
WDM Components Optical Switches
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Fiber Couplers
Three Port Coupler
Four Port Directional Coupler
Star Coupler
(From Sterling)
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Optical Add/DropMultiplexer
(From Sterling)
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Direct and Indirect Modulation
(From Sterling)
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Repeaters and Regenerators
A repeater consists of an optical receiver, an
electronic amplifier and an optical transmitter.
An optical signal is converted to an electrical
signal, amplified, and then converted back to an
optical signal to be sent down the fiber again.
The use of the term "repeater" has faded
somewhat as most amplifiers contain circuitry to
"clean up" the signal before retransmission.
The purpose of a regenerator is to remove the
noise from a digital signal and generate a clean
signal for further transmission. Discrimination
and retiming circuits separate the signal from
the noise and make sure the timing of pulses is
correct. Since most amplification is now done
optically, the regenerator may not amplify at
all. While the terminology is not entirely
consistent, regenerators are generally classified
as one of two types. If the regenerator
amplifies and reshapes the signal, it is a 2R
amplifier, while a 3R amplifier amplifies,
reshapes and retimes. An amplify only device, or
optical amplifier is a 1R device.
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Erbium Doped Fiber Amplifier (EDFA) System
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Semiconductor Optical Amplifier
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Raman Optical Amplifiers
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Fabrication of Fiber Grating

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Array Waveguide Grating
(Courtesy of Photonics Integrated Research, Inc.)
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Optical Cross Connect
(From Sterling)
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Micro Electro-Mechanical Systems (MEMS)
MEMS switches allow rapid switching of 2-D and
3-D arrays of miniature switches. Scalable
elements consist of a mirror and an electrostatic
actuator. Switches, interleavers, add drop
multiplexers and optical cross connects can all
be made with MEMS technology.
MEMS Switch Array Mirrors move up when on.
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MEMS
16 X 16 Switch Array Dual Axis
Mirrors for M X N Arrays
(Coutesy of JDS Uniphase)
(Courtesy of Coventor)
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Modulation, Network Topographies and Multiplexing
Modulation SONET Wavelength Division
Multiplexing
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Modulation Formats
(From Sterling)
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SONET
Transmission Rates
Network Topologies
(From Sterling)
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Wavelength Division Multiplexing (WDM)
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WDM Application
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Dense Wavelength Division Multiplexing (DWDM)
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Coarse Wavelength Division Multiplexing (CWDM)
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WDMStandards and Capacity
ITU-T Recommended Frequencies for 100 MHz Channel
Spacing DWDM
DWDM Single Fiber Transmission Capacity
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Optical Communications Systems
Power Budget LANs Telephone Fiber
Networks All Fiber Network
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Ethernet Link Power Budget
(From Sterling)
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Fiber LAN Standards
Enterprise System Connection (ESCON)
Fiber Distributed Data Interface (FDDI)
(From Sterling)
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Telephone Fiber Network
(From Sterling)
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SONET TelephoneTransmission Rates
(From Sterling)
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FDDI Dual Ring Structure
(From Sterling)
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Premises Networks
Distributed Network
Centralized Network
(Courtesy of AMP Inc.)
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Fiber to the Curb (FTTC)
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Fiber Optics Test and Measurement
Power Meter Optical Time Domain
Reflectometer (OTDR) Oscilloscope Optical
Spectrum Analyzer
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Other Lightwave Applications
Integrated Optics Free Space Optics
Millimeter Waves
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Integrated Optics
Integrated optics involves the fabrication of
multiple planar optical functions on a single
monolithic device. Many laser devices such as a
semiconductor laser coupled with an
electro-absorption modulator, multiple wavelength
lasers and other amplifiers can be fabricated on
the same substrate. Other functions include
array waveguides, couplers and lithium niobate
modulators.
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Free Space Optics (FSO)
Atmospheric optical links or free space optics
uses fiber optic wavelengths and
technologies. Systems can be set up in a few
hours to a few days. FSO transmitters and
receivers have a range of several miles. FSO can
be used at network edges and other short links
without good current solutions.