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Optical Communication
From Sound to Light and Back
Apparatus
Optical Fiber
Transmitter
Receiver
Speaker
Sound Waves
Longitudinal Waves
‫ ٭‬Longitudinal (Compression):
‫ ٭‬Waves parallel to direction of travel
Credit: http://www.physicsclassroom.com
Wave Properties
‫٭‬
‫٭‬
‫٭‬
‫٭‬
Amplitude
Frequency
Wavelength
Velocity
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?
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
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
How the Ear Works
Credit: http://magnatone.com/HEARIN-HEALTH.html
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
Modulation
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)
Snell’s 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
Refraction and Reflection
1 1
 

2 
1 = Angle of incidence
2 = Angle of refraction
Both angles are
taken from normal
Snell’s Law
sin 1 n2

sin 2 n1
Total Internal Reflection
‫ ٭‬There is a special case of Snell’s 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 Normal:
n 2 
C  sin  
n1 
1
To Longitudinal Axis:
C ,Longitudinal  90  C
Critical Angle
refraction  90
sin refraction 1

All of the light will
be reflected when
angle of incidence
is greater than the
critical angle
Total Internal Reflection
‫ ٭‬Allows optical fibers to carry light very
long distances without any loss of
energy
What’s Wrong Here??
Critical angle was not exceeded
every time
Calculation
‫ ٭‬Find the critical angle to the longitudinal
axis in a standard optical fiber.
n of cladding = 1.343
n of core = 1.557


n
1
C  sin  2 
n1 
C ,Longitudinal  90  C
Solution
1.343 
C  sin 

1.557 
C  59.605
C ,Longitudinal  90  50.605
1
C ,Longitudinal  30.395
Receiver and Speaker
• Photodarlington on receiver converts
light energy back into electrical signals
• Signals are amplified through circuitry
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
Speakers
• Both types of magnets have polar
orientation
• Electromagnets can change orientation
• How?
By changing direction of current
Alternating Current (AC)
Speakers
• Amplifying circuitry switches electrical
signals
• Current constantly reversing
• Polar orientation changes many times
per second
Speakers
• Changing polar
orientation changes
interaction with
permanent magnet
• Electromagnet will
move up and down
as current alternates
Speakers
Movement of coil causes speaker cone to move
up and down, creating longitudinal sound
waves