Chapter 9 Introduction to Fiber Optics

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Chapter 9Introduction to Fiber Optics

• Information Technology in Theory
• By Pelin Aksoy and Laura DeNardis

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Objectives

• Understand the importance of fiber-optic
  technologies in the information society
• Identify the fundamental components of a
  fiber-optic cable
• Understand the principles by which light travels
  within a fiber-optic cable, including refraction
  and total internal reflection

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Objectives (continued)

• Understand how a single fiber can carry multiple
  signals through wavelength division multiplexing
• Learn about the advantages and disadvantages of
  fiber optics as a transmission medium for various
  applications
• Gain exposure to cutting-edge fiber-optic
  approaches

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Background on Optical Communications

• Fiber-optic communication
• Trapping light inside an optical fiber
• Can carry any form of information
• Fiber is an optical medium, which means it is
  capable of transmitting light
• Based on total internal reflection (TIR)

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Tyndalls Experiment
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Structure of Fiber-Optic Cables

• Core
• Cladding
• Coating

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General Structure of Fiber-Optic Cables
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General Structure of Fiber-Optic Cables
(continued)
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Structure of Fiber-Optic Cables Cladding

• Cylindrical material made of glass or specialized
  plastic
• Central portion of the fiber
• Light signal carrying the information travels
  through the core
• The diameter of the core can range from a couple
  of micrometers (µm-one millionth of a meter) to a
  couple of millimeters (mm-one thousandth of a
  meter)

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Structure of Fiber-Optic Cables Jacket

• Surrounds the cladding
• Insulates and protects the fiber from physical
  damage and environmental effects, such as
  moisture, that might interfere with the inner
  workings of the cable
• Usually made of opaque plastic or another type of
  material

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How Light Travels Through Fiber

• TIR is the basis of fiber-optic communication
• TIR may be considered to be an extreme case of
  refraction
• When a light ray strikes a boundary of two
  materials with different RIs, it bends, or in
  other terms, refracts to an extent that depends
  on the ratio of the RIs of the two materials

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Refraction

• The phenomenon that causes a spoon inside a clear
  glass of water to appear shorter and bent to an
  observer looking from the outside
• Light rays that strike the water/air boundary
  bend to create an image that is shorter than the
  actual height of the spoon in the water, because
  water has a higher RI than air
• TIR is the phenomenon that makes the side of an
  aquarium act as a mirror when viewed at an
  appropriate angle

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Refractive Index

• Refractive index of an optical medium Speed of
  light in a vacuum (300,000,000 meters per
  second)/speed of light in the optical medium

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Refractive Index (continued)
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Total Internal Reflection
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Total Internal Reflection (continued)
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Total Internal Reflection (continued)
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Types of Fiber-Optic Cables

• Single-mode fiber
• Multi-mode fiber
• Step index fiber
• Graded index fiber

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Single-Mode Fiber

• Very small core diameter, on the order of a few
  micrometers (µm) less than that of a human hair
• The cladding diameter is tens of micrometers,
  making it much larger than the core
• Single-mode fiber can transmit information at
  very high data rates across large distances
• The small diameter restricts only a single mode
  of light to carry all of the information

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Multi-Mode Fiber

• Has a much larger core diameter, ranging from
  tens to hundreds of micrometers
• Used for lower-bandwidth, shorter-distance
  applications than single-mode fibers
• Information-carrying capacity is less than that
  of single-mode fibers due to the physical effects
  they impose on the signals traveling through them

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Step Index Fiber

• A fiber-optic cable with a uniform refractive
  index throughout its core is classified as a step
  index fiber
• Both single-mode and multi-mode

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Graded Index Fiber

• A cable whose core refractive index is
  non-uniform and varies gradually is classified as
  a graded index fiber
• The value of the RI of a graded index fiber is
  highest in the center of the core and gradually
  diminishes towards the cladding

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Refractive Index Profiles
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Fiber-Optic Communication Systems

• Pulses of electricity corresponding to the bits
  arrive at the input transducer
• A device that converts one form of energy to
  another
• Devices similar to light bulbs are used as
  optical transducers at the input of fiber-optic
  cables to convert electricity into light
• Light emitting diodes (LED)
• Laser diodes (LD)

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Fiber-Optic Communication Systems (continued)

• Electrical signals arriving at the input of
  optoelectronic devices are used to modulate the
  light source
• The modulated optical signal is emitted by the
  source and coupled into the cable
• Once the light is trapped inside the cable, it
  travels to the other end, where it is demodulated
  and an output transducer
• Photodiode (PD) or phototransistor

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Fiber-Optic Communication Systems (continued)
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Wavelength Division Multiplexing

• Fiber-optic cables can carry multiple signals due
  to their large bandwidths
• Dense WDM (DWDM)
• A large number of channels are multiplexed onto a
  single fiber-optic cable
• Course WDM
• A smaller number of channels multiplexed

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Wavelength Division Multiplexing (continued)
A fiber-optic communication system employing WDM
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Fiber-Optic Technology Benefits/Drawbacks

• High transmission rate
• Immunity to electromagnetic interference
• Low attenuation
• High security
• Small weight and size
• Low power consumption
• High installation cost
• Difficulty in splicing

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High Transmission Rate

• Fiber supports extremely high transmission rates
• Many applications require these rates
• Video
• Music sharing
• Transmission of medical images
• Satellite imagery
• Cable TV broadcasting
• Internets transmission backbone
• A fiber optic communication system with 160 WDM
  channels could transmit close to 26 terabits per
  second (i.e. 26,000,000,000,000)

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Immunity to Electromagnetic Interference

• Light rather than electricity
• Immune to electromagnetic interference (EMI)
• The presence of EMI directly affects the
  transmission rate
• Thus, the immunity to EMI enables the large
  capacity of fiber optics
• Fiber used to replace copper in noisy
  environments
• A safer option in an industrial environment,
  where the presence of an electrical signal could
  present a fire hazard

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Low Attenuation

• Low attenuationon the order of fractions of a
  dB/km in some transmission systems
• Glass used for fiber-optic cables is much purer
  than the glass we find in a car windshield
• The purer the glass, the lower the signal
  degradation
• Low attenuation translates directly to efficient
  long-distance communications, such as
  transatlantic or other suboceanic cable routes

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High Security

• Unlike wireless or copper-based transmission
  systems, fiber-optic systems more difficult to
  crack
• You cannot tap into a fiber cable by merely
  slicing the cable jacket and clipping a crocodile
  clip on the cable, as you can with a copper cable
• Banks, embassies, and many governmental
  departments that require private communications
  use heavy-duty, robust fiber-optic cabling
  systems due to its high level of security

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Small Weight and Size

• Because fiber cables are either made of glass or
  plastic, they are much more lightweight than a
  metal like copper
• A fiber as thin as a strand of hair has much more
  capacity than a thick, heavy coaxial copper cable
• In applications used on aircraft and satellite
  systems, where weight is a major issue, and in
  applications where space is a concern, fiber
  optics has obvious advantages

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Low Power Consumption

• Because fiber-optic cables have low attenuation
  characteristics, the amount of power that must be
  supplied in the cable is much lower than in
  copper-based systems across the same distance
• Efficient LEDs and LDs as well as efficient light
  detectors are readily available to reduce the
  overall power consumption of an optical link

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High Installation Cost

• High planning, installation, and maintenance cost
  
• To properly install, lay, and align these cables,
  personnel usually require extensive training,
  because these cables can be sensitive to
  stretching or other physical effects
• Elaborate installations are often required for
  important applications to minimize these effects
• The cost of installing fiber-optic systems is
  decreasing because more efficient components and
  techniques are being developed to make
  installation easier

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Difficulty in Splicing

• Because fibers are made of either plastic or
  glass, they are more difficult to splice (i.e.
  bring together) than copper cables
• Sophisticated splicing techniques, such as fusion
  or using a connector, are needed to bring two
  pieces of fiber together
• Splicing fiber-optic cables is generally more
  difficult than soldering or crimping two pieces
  of copper wire together

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Commercial Cables

• Fiber optics used in a wide array of
  applications
• Medicine
• Telecommunications
• Computing
• Computer networking
• Transoceanic cables

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Commercial Cables (continued)

• Fiber optics used in a wide array of medical
  applications
• Endoscopy
• The process of inspecting various organs of the
  body using a device called an endoscope
• A fiber-optic cable as part of an endoscope can
  be used to provide the light and transport the
  imagery of various organs of the body such as the
  stomach

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Structure of Commercial Cables
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Summary

• Fiber-optic cables are made up of a few basic
  components, including the core, cladding, and
  coating or jacket
• Light within a fiber-optic cable is carried from
  one end to the other based on a phenomenon called
  total internal reflection
• The core and the cladding have different
  refractive indices, so that light that traverses
  the core and hits the core-cladding boundary
  undergoes refraction
• Light rays that strike the core-cladding boundary
  at an angle greater than the critical angle
  undergo total internal reflection

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Summary (continued)

• Lasers and LEDs are the primary sources that are
  used to insert light into a fiber-optic cable
• Photodiodes and phototransistors are transducers
  that convert light back into electricity
• Fiber-optic cables may be classified as either
  single-mode or multi-mode single-mode fibers
  have a smaller core diameter and can support
  higher transmission rates across longer distances
  than multi-mode fibers
• Fiber-optic cables may also be classified
  according to the nature of their cores
  refractive index
• Step index fibers have a uniform refractive index
  throughout their core, whereas graded index
  fibers have a refractive index that varies
  gradually through their core

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Summary (continued)

• WDM is a technique used to multiplex multiple
  channels of information and transmit them across
  a single fiber-optic cable the 1.3-?m and
  1.55-?m wavelengths are the most popular, due to
  the low attenuation that optical signals undergo
  at these wavelengths
• Advantages of fiber-optic cables include high
  transmission rates, immunity to EMI, low
  attenuation, high security, small weight and
  size, and low power consumption
• Disadvantages include higher installation and
  maintenance costs and difficulty in splicing

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Summary (continued)

• Commercial fiber-optic cables are sold with
  multiple strands of fibers packaged as a single
  cable with extra components that increase its
  strength
• Established but unused fiber-optic systems are
  called dark fiber
• Various commercial applications of fiber-optic
  cables include transoceanic communications,
  computer networking, computing, and medicine