OPTICAL FIBERS

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OPTICAL FIBERS

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What are Fiber Optics

• Long thin strands of very pure glass about the
  size of human hair
• Arranged in bundles called optical cables
• Used to transmit light signals over long
  distances
• Hundreds of thousands arranged in bundles to form
  optical cables

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What is an Optical Fiber?
An optical fiber is a waveguide for light
consists of core inner part where wave
propagates cladding outer part used to keep
wave in core buffer protective
coating jacket outer protective shield
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• Passage of light from a material with a high
  index of refraction(n1) to a material with a
  lower index of refraction(n2)
• At the critical angle light will not go into n2
  but instead travel along the surface between the
  two media

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What are Optical Fibres ? 

• Optical Fibres are fibres of glass, usually about
  120 micrometres in diameter, which are used to
  carry signals in the form of pulses of light over
  distances up to 50 km without the need for
  repeaters. These signals may be coded voice
  communications or computer data

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• The optical fiber can be used as a medium for
  telecommunication and networking because it is
  flexible and can be bundled as cables.
• The light transmitted through the fiber is
  confined due to total internal reflection within
  the material.
• In telecommunications applications, the light
  used is typically infrared light
• Fibers are generally used in pairs, with one
  fiber of the pair carrying a signal in each
  direction
• Fibers, like waveguides, can have various
  transmission modes. The fibers used for
  long-distance communication are known as single
  mode fibers, as they have only one strong
  propagation mode.

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• Multi-mode fibers, where light transmitted in the
  different modes arrives at different times,
  resulting in dispersion of the transmitted
  signal.
• single mode equipment is generally more expensive
  than multi-mode equipment.
• single-mode optical fiber, data rates of up to 40
  Gbit/s are possible in real-world use on a single
  wavelength. Wavelength division multiplexing can
  then be used to allow many wavelengths to be used
  at once on a single fiber

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Types of optical fibers

• Single mode
• only one signal can be transmitted
• use of single frequency
• Multi mode
• Several signals can be transmitted
• Several frequencies used to modulate the signal

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Types of Fibres
Multi-mode step index
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Single-mode step index
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multi-mode graded index
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GRIN
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Typical core and cladding diameters

• Type Core (mm) Cladding (mm)
• Single mode 8 125
• Multimode 50 125
• 62.5 125
• 100 140

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Launching the Light

• Factors that effect the Launching of Light

• Intensity
• Area
• Acceptance Angle
• Fresnell Loss

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Signal Production

• Convert electrical input to modulated light

• 2 Basic Schemes

On/Off Linear Variation

• 2 Common Devices used

Light Emitting Diode (LED) Laser Diode (LD)
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Through the Wire

• Light Propagates through the wire due to total
  internal reflection

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Fibre can be bent!!
Illustration of total internal reflection
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Total internal reflection

• Trapping light in the fiber

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Total Internal Reflection
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Types of fiber ends
beam patterns can be spherical cylindrical
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Fibers carry modes of light

• a mode is
• a solution to the wave equation
• a given path/distribution of light

higher modes gives more light, which is not
always desirable
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Controlling the of Modes

• From the V parameter, we see that we can reduce
  the number of modes in a fiber by reducing
• (1) NA (2) diameter (wrt ??)
• This is exactly the case in single mode
  fibers.

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The V Parameter
a fiber radius?o incident wavelength

• known as the V-parameter or the fiber parameter
• an important parameter that governs the number of
  modes
• parameters that relates yucky EM wave solutions
  for both core and cladding

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How Fibers Work

• The classical understanding of fiber optics comes
  once again from out longtime friend, Snells Law!

• Step index fibers Total Internal Reflection

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Optical Fiber Bandwidth

• Bandwidth Limitation
• Light entering at different angles reach the end
  of the cable at different times
• Smearing is produced uncertainty of beginning
  and end of signal

• less smearing higher the bandwidth
• smearing can be reduced by reducing the size of
  the fiber core

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Areas of Application 

• Telecommunication's
• Optical fibres are now the standard point to
  point cable link between telephone substations.
• Local Area Networks (LAN's)
• Multimode fibre is commonly used as the
  "backbone" to carry signals between the hubs of
  LAN's from where copper coaxial cable takes the
  data to the desktop. Fibre links to the desktop,
  however, are also common.

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• Cable TV
• As mentioned above domestic cable TV networks use
  optical fibre because of its very low power
  consumption.
• CCTV
• Closed circuit television security systems use
  optical fibre because of its inherent security,
  as well as the other advantages mentioned above.
• Optical Fibre Sensors

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• Long-haul trunks
• common in telephone networks
• Metropolitan trunks
• to join phone exchanges in metro areas
• Rural exchange trunks
• connect exchanges of different phone
  companies

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• Subscriber loops
• central exchange to subscriber
• LANs
• Can support hundreds of stations on a campus

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Other Applications

• Endoscope
• X-ray Imaging
• Night Vision

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Advantages of optical Fibres

• Can carry much more information
• Much higher data rates
• Much longer distances than co-axial cables
• Immune to electromagnetic noise
• Light in weight
• Unaffected by atmospheric agents

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Disadvantages of optical Fibres

• expensive
• need to convert electrical signal into optical
  signal when transmitting and convert it back to
  electrical signal when receiving

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The Optical Transmitter

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• The source of the optical signal can be either a
  light emitting diode, or a solid state laser
  diode.
• The transmitter converts an electrical analog or
  digital signal into a corresponding optical
  signal.
• The most popular wavelengths of operation for
  optical transmitters are 850, 1300, or 1550
  nanometers.

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Optical Receivers

• Converts modulated light from the cable into the
  original signal
• Photodiode Pin or Avalanche type
• High gain internal amplifiers
• Large sensitive detecting area several microns
  thick

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The Optical Receiver

• The receiver converts the optical signal back
  into a replica of the original electrical signal.
  The detector of the optical signal is either a
  PIN-type photodiode or avalanche-type photodiode.

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Degradation of the Signal

• Glass must be extremely pure
• Most general purpose optical fiber
• Signal losses per km traveled
• 850nm 60-75
• 1300nm 50-60
• 1550nm 40
• Excessive bending

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Signal Regeneration

• Optical regenerators spliced along the cable to
  boost weakened signals
• Optical Regenerator
• Optical fibers with specially doped coating
• Doped portion is pumped with a laser
• When signals enters energy from the laser allows
  doped material to imitate lasers
• Doped molecules now emit a stronger signal with
  the same initial characteristics

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Optical Fiber - Transmission Characteristics

• Act as wave guide for 1014 to 1015 Hz
• Portions of infrared and visible spectrum
• Light Emitting Diode (LED)
• Cheaper
• Wider operating temp range
• Last longer
• Injection Laser Diode (ILD)
• More efficient
• Greater data rate
• Wavelength Division Multiplexing
• - Multiple beams of light at different
  frequencies can be transmitted simultaneously

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Global crossing fibre networks
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Atlantic crossing networks
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