Principles of Fiber Optic Transmission (I)
In This Lecture
Elements of an Optical Fiber
communication
 Optical fibers
 Amplitude Modulation
Elements of an Optical Fiber communication
• Fiber optic links contain three basic elements
– transmitter
– optical fiber
– receiver
User
Input(s)
Transmitter
Electrical-to-Optical
Conversion
Optical Fiber
Receiver
User
Output(s)
Optical-to-Electrical
Conversion
• Transmitter (TX)
– Electrical interface encodes user’s information
through AM, FM or Digital Modulation
– Encoded information transformed into light by
means of a light-emitting diode (LED) or laser
diode (LD)
User
Input(s)
Electrical
Interface
Data Encoder/
Modulator
Light
Emitter
Optical
Output
• Receiver (RX)
– decodes the light signal back into an electrical
signal
– the data decoder/demodulator converts the
signals into the correct format
Optical
Input
Light Detector/
Amplifier
Data Decoder/
Demodulator
Electrical
Interface
User
Output(s)
Optical fibers
 Optical fibers carry light energy from the
transmitter to the receiver.
 An optical fiber may be made of glass or plastic.
 The fiber can carry light around corners and
over great distances.
Comparison of fiber
and copper cables
Connectors
 The connector is attached to the optical
fiber and allows it to be mated to the
transmitter or receiver to provide solid
contact.
 The connector must align the fiber end
precisely with the light source or receiver
to prevent signal loss.
Advantages of optical fibers
 Can carry much more information
 Less expensive .
 Thinner
 Much higher data rates
 Much longer distances than co-axial cables
 Immune to electromagnetic noise
 Light in weight
 Unaffected by atmospheric agents
Amplitude Modulation
• One method used for converting electrical signals into light
signals for transmission is amplitude modulation (AM).
.
Electrical energy
with continuously
varying voltage
is converted into
converted into
Light with
continuously
varying
brightness
Amplitude modulation
requires two components: a
carrier and a signal that is
imposed on
the carrier—also known as
the intelligence
—to change it in some way
Amplitude modulation requires two
components: a carrier and a signal that
is imposed on
the carrier—also known as the
intelligence
—to change it in some way
 To modulate the amplitude of the light in a fiber optic
transmitter, the intelligence is sent through a circuit
that changes it to a continuously varying voltage.
 As the intelligence changes, the voltage controlling
the light rises and falls, varying the light’s intensity to
match the intelligence.
 Amplitude modulation suffers from two
problems that can affect the quality of the
signal: attenuation and noise.
 Noise : can be considered data without
meaning; that is, data that is not being
used to transmit a signal, but is simply
produced as an unwanted by-product of
other activities.
Attenuation :Attenuation is a measure of
how much a signal weakens as it travels
through a medium.