LIGHT COMMUNICATION

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LIGHT COMMUNICATION
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Fiber vs. Metallic Cables

• Advantages
• Larger bandwidth
• Immune to cross-talk
• Immune to static interference
• Do not radiate RF
• spark free
• No corrosion, more environment resistive

• Disadvantages
• Initial cost of installation high
• Brittle
• Maintenance and repair more difficult and more
  expensive

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Typical Fiber Optical Communication System
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Elements of a Fiber Data Link

• Transmitter emits light pulses (LED or Laser)
• Connectors and Cables passively carry the pulses
• Receiver detects the light pulses

Cable
Transmitter
Receiver
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Repeaters

• For long links, repeaters are needed to
  compensate for signal loss

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Optical Fiber

• Core
• Glass or plastic with a higher index of
  refraction than the cladding
• Carries the signal
• Cladding
• Glass or plastic with a lower index of refraction
  than the core
• Buffer
• Protects the fiber from damage and moisture
• Jacket
• Holds one or more fibers in a cable

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Singlemode Fiber

• Singlemode fiber has a core diameter of 8 to 9
  microns, which only allows one light path or mode
• Images from arcelect.com (Link Ch 2a)

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Multimode Step-Index Fiber

• Multimode fiber has a core diameter of 50 or 62.5
  microns (sometimes even larger)
• Allows several light paths or modes
• This 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
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Multimode Graded-Index Fiber

• The index of refraction gradually changes across
  the core
• Modes that travel further also move faster
• This reduces modal dispersion so the bandwidth is
  greatly increased

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Attenuation

• Absorption
• interaction of light with electrons molecule
  vibration
• Rayleigh Scattering
• caused by compositional fluctuations in glass
  material. Energy escapes not converted
• Material Fabrication
• caused impurities (transition metal ions)
• Fiber Fabrication
• caused by fiber imperfections (defects/stresses)
  Leads to Mie scattering which is ?
  independent
• Deployment/Environmental
• caused by bends and microbends
  Leads to mode conversions

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Three Types of Dispersion

• Dispersion is the spreading out of a light pulse
  as it travels through the fiber
• Three types
• Modal Dispersion
• Chromatic Dispersion
• Polarization Mode Dispersion (PMD)

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Modal Dispersion

• Modal Dispersion
• Spreading of a pulse because different modes
  (paths) through the fiber take different times
• Only happens in multimode fiber
• Reduced, but not eliminated, with graded-index
  fiber

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Chromatic Dispersion

• Different wavelengths travel at different speeds
  through the fiber
• This spreads a pulse in an effect named chromatic
  dispersion
• Chromatic dispersion occurs in both singlemode
  and multimode fiber
• Larger effect with LEDs than with lasers
• A far smaller effect than modal dispersion

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Polarization Mode Dispersion

• Light with different polarization can travel at
  different speeds, if the fiber is not perfectly
  symmetric at the atomic level
• This could come from imperfect circular geometry
  or stress on the cable, and there is no easy way
  to correct it
• It can affect both singlemode and multimode fiber.

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• Light Sources
• ?? Light Emitting Diode (LED)
• ?? simple construction and drive circuitry
• ?? best for short distances, modest bit rates,
  and low channel capacity
• ?? Semiconductor Laser Diode
• ?? high drive currents and complex circuitry
• ?? produce high power for higher bit rates and
  long distances

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• Light Sources LED
• ?? Usually a P-N junction aluminium-gallium
  arsenide (AlGaAs) or
• Gallium-arsenide-phosphide (GaAsP)
• Spontaneous emission through recombination of
  electrons and holes
• Works in forward bias, energy released as a
  photon
• A photon a quantum of E/M wave energy

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• Light Sources Laser Diode
• Light Amplification by Stimulated Emission of
  Radiation
• ?? A laser diode Is an LED with two important
  differences
• ?? (1) The operating current is much higher in
  order to produce OPTICAL GAIN
• ?? (2) Two of the ends of the LD are cleaved
  parallel to each other. These ends act as
  perfectly aligned mirrors which reflect the light
  back and forth through the "gain medium" in order
  to get as much amplification as possible
• ?? The typical response time of a laser diode Is
  0.5 ns. The line width is around 2 nm with a
  typical laser power of 10's of milliwatts. The
  wavelength of a laser diode can be 850 nm, 1300
  nm, or 1500 nm.

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Photo-Detectors ?? Must detect down to the order
of 10-14 W ?? Need high conversion efficiency
between light and electrical energy ?? Must
respond fast for high bandwidth ?? Must have
low-noise power and good light-collecting
properties ?? Ideally, they must operate at low
voltage, be easy to use, be robust and immune to
changes in ambient conditions, have a long life,
be reliable and inexpensive ?? Two devices stand
out ?? Positive-intrinsic-negative (PIN)
diodes ?? Avalanche photodiodes (APD)
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Detection Procedure

• ?? Photons collide with the electrons in the
• valence band
• ?? The electrons absorb photon energy, hv, and
  cross the band gap into the conduction band with
  charge q.
• ?? Incident optical power, P, transfers to the
• device with efficiency ?.
• ?? The generated photocurrent is
• ?? We resort to mean values because the whole
• photo-detection process is stochastic.

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Detectors PIN Diode
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Detectors The APD Device
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Detectors Characteristics ?? Responsivity ??
Measure of conversion efficiency, a ratio of the
output current to the input optical
power (A/W) ?? Dark current ?? Leakage current
flowing with no light input ?? Transit time ??
Time it takes a photo-induced carrier to cross
the depletion Region ?? Spectral response ?? A
relative spectral response vs. wavelength or
frequency curve displays the range or system
length possible for a given wavelength.