Optical Communication

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Optical Communication

• From Sound to Light and Back

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Apparatus
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Sound Waves
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Longitudinal Waves

• ? Longitudinal (Compression)
• ? Waves parallel to direction of travel

Credit http//www.physicsclassroom.com
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Wave Properties

• ? Amplitude
• ? Frequency
• ? Wavelength
• ? Velocity

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Changing Wave Properties

• What happens when you change the pitch of
• the sound?

Frequency is changed. How?
What happens when you change the volume of the
sound?
Amplitude is changed. How?
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How does sound come from sound waves?

• ? Air and other media have matter
• ? Matter oscillates when waves pass through
• ? Sound Pressure
• ? The deviation in equilibrium pressure caused by
  a sound wave

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How We Vocalize

• Vocal Chords are infolded membranes stretched
  across larynx
• Chords come close together, air pressure builds,
  pushes them apart
• Chopped flow of air sustained
• Steady oscillations create sound

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How the Ear Works
Credit http//magnatone.com/HEARIN-HEALTH.html
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The Light Transmitter

• ? Sound waves enter microphone
• ? Microphone is an electret
• ? Contains permanently charged plate
• ? Sound waves cause plate to vibrate
• ? Electrical potential differences caused by
  vibrations
• ? Voltage differences cause light in Light
  Emitting Diode (LED) to be modulated

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Modulation
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Fiber Optics

• ? Cables made of thin glass or plastic strands
• ? Not affected by electromagnetic interference
• ? Propagate light over long distances with no
  energy loss (Total Internal Reflection)

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Snells Law

• ? Describes relationship between angles of
  incidence and refraction between two different
  media
• ? Media possess a Refractive Index (n)
• ? Measures how much speed of light is slowed down
  by the medium
• ?The more light is slowed, the higher its
  Refractive Index
• ? Speed of light in a vacuum 3 x 108 m/s
• ? Refractive Index n 1
• ? Same as in air

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Refraction and Reflection
Angle of incidence
Angle of refraction
Both angles are taken from normal
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Snells Law
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Total Internal Reflection

• ? There is a special case of Snells Law
• ? When going from high density to low density,
  there is a point after which all of the light is
  reflected
• ? This point is the Critical Angle

To Longitudinal Axis
To Normal
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Critical Angle

• All of the light will be reflected when angle
  of incidence is greater than the critical angle

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Total Internal Reflection

• ? Allows optical fibers to carry light very long
  distances without any loss of energy

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Whats Wrong Here??
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Critical angle was not exceeded every time
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Calculation

• ? Find the critical angle to the longitudinal
  axis in a standard optical fiber.
• n of cladding 1.343
• n of core 1.557

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Solution
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Receiver and Speaker

• Photodarlington on receiver converts light energy
  back into electrical signals
• Signals are amplified through circuitry

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Speaker

• Speakers have both permanent magnets and
  electromagnets
• Electromagnet

Composed of magnetic metal wrapped in coil of
wire Current runs through wire
Creates magnetic field around metal
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Speakers

• Both types of magnets have polar orientation
• Electromagnets can change orientation
• How?

By changing direction of current Alternating
Current (AC)
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Speakers

• Amplifying circuitry switches electrical signals
• Current constantly reversing
• Polar orientation changes many times per second

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Speakers

• Changing polar orientation changes interaction
  with permanent magnet
• Electromagnet will move up and down as current
  alternates

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Speakers
Movement of coil causes speaker cone to move up
and down, creating longitudinal sound waves