Download Optical communication systems

1
Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
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Transmission-Line Basics
 Transmission lines in communication carry:
 Telephone signals,
 Computer data in LANs,
 TV signals in cable TV systems,
 Signals from a transmitter to an antenna or from an
antenna to a receiver.
 Transmission lines are also circuits.
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Transmission-Line Basics
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The two primary requirements of a transmission line
are:
1. The line should introduce minimum attenuation to the
signal.
2. The line should not radiate any of the signal as radio
energy.
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Transmission-Line Basics
Characteristic Impedance
 When the length of transmission line is longer than
several wavelengths at the signal frequency, the two
parallel conductors of the transmission line appear as a
complex impedance.
 An RF generator connected to a considerable length of
transmission line sees an impedance that is a function
of the inductance, resistance, and capacitance in the
circuit—the characteristic or surge impedance (Z0).
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Transmission-Line Basics
Velocity Factor
 The speed of the signal in the transmission line is
slower than the speed of a signal in free space.
 The velocity of propagation of a signal in a cable is less
than the velocity of propagation of light in free space by
a fraction called the velocity factor (VF).
VF = VC/VL
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Transmission-Line Basics
Time Delay
 Because the velocity of propagation of a transmission
line is less than the velocity of propagation in free
space, any line will slow down or delay any signal
applied to it.
 A signal applied at one end of a line appears some time
later at the other end of the line.
 This is called the time delay or transit time.
 A transmission line used specifically for the purpose of
achieving delay is called a delay line.
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Transmission-Line Basics
The effect of the time delay of a transmission line on signals. (a) Sine wave delay
causes a lagging phase shift. (b) Pulse delay.
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Transmission-Line Basics
Transmission-Line Specifications
 Attenuation is directly proportional to cable length and
increases with frequency.
 A transmission line is a low-pass filter whose cutoff
frequency depends on distributed inductance and
capacitance along the line and on length.
 It is important to use larger, low-loss cables for longer
runs despite cost and handling inconvenience.
 A gain antenna can be used to offset cable loss.
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Transmission-Line Basics
Attenuation versus length for RG-58A/U coaxial cable. Note that both scales on the
graph are logarithmic.
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Standing Waves
 If the load on the line is an antenna, the signal is
converted into electromagnetic energy and radiated
into space.
 If the load at the end of the line is an open or a short
circuit or has an impedance other than the
characteristic impedance of the line, the signal is not
fully absorbed by the load.
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Standing Waves
 When a line is not terminated properly, some of the
energy is reflected and moves back up the line,
toward the generator.
 This reflected voltage adds to the forward or incident
generator voltage and forms a composite voltage that
is distributed along the line.
 The pattern of voltage and its related current
constitute what is called a standing wave.
 Standing waves are not desirable.
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Standing Waves
A transmission line must be terminated in its characteristic impedance for
proper operation.
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Standing Waves
Calculating the Standing Wave Ratio
 The magnitude of the standing waves on a transmission line is
determined by
 the ratio of the maximum current to the minimum current,
 or the ratio of the maximum voltage to the minimum voltage,
along the line.
 These ratios are referred to as the standing wave ratio (SWR).
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The Smith Chart
 The mathematics required to design
and analyze transmission lines is
complex, whether the line is a physical
cable connecting a transceiver to an
antenna or is being used as a filter or
impedance-matching network.
 This is because the impedances
involved are complex ones, involving
both resistive and reactive elements.
 The impedances are in the familiar
rectangular form, R + jX.
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The Smith Chart
 The Smith Chart is a sophisticated graph that permits
visual solutions to transmission line calculations.
 Despite the availability of the computing options today,
this format provides a more or less standardized way
of viewing and solving transmission-line and related
problems.
ZO
ZIN
ZL
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The Smith Chart
 The horizontal axis is the pure resistance or zero-
reactance line.
 The point at the far left end of the line represents zero
resistance, and the point at the far right represents
infinite resistance. The resistance circles are centered
on and pass through this pure resistance line.
 The circles are all tangent to one another at the infinite
resistance point, and the centers of all the circles fall
on the resistance line.
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The Smith Chart
 Any point on the outer circle represents a resistance of
0 Ω.
 The R = 1 circle passes through the exact center of
the resistance line and is known as the prime center.
 Values of pure resistance and the characteristic
impedance of transmission line are plotted on this line.
 The linear scales printed at the bottom of Smith charts
are used to find the SWR, dB loss, and reflection
coefficient.
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The Smith Chart
The Smith chart.
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Optical
Communication
© 2008 The McGraw-Hill Companies
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Optical Principles
 Optical communication systems use light to
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transmit information from one place to another.
Light is a type of electromagnetic radiation like radio
waves.
Today, infrared light is being used increasingly as the
carrier for information in communication systems.
The transmission medium is either free space or a
light-carrying cable called a fiber-optic cable.
Because the frequency of light is extremely high, it can
accommodate very high rates of data transmission
with excellent reliability.
© 2008 The McGraw-Hill Companies
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Optical Principles
Light
 Light, radio waves, and microwaves are all forms of
electromagnetic radiation.
 Light frequencies fall between microwaves and x-rays.
 The optical spectrum is made up of infrared, visible,
and ultraviolet light.
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Optical Principles
The optical spectrum. (a) Electromagnetic frequency spectrum showing the optical
spectrum.
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Optical Principles
The optical spectrum. (b) Optical spectrum details.
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Optical Principles
Light
 Light waves are very short and are usually expressed in
nanometers or micrometers.
 Visible light is in the 400- to 700-nm range.
 Another unit of measure for light wavelength is the
angstrom (Ǻ). One angstrom is equal to 10-10 m.
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Optical Principles
Light: Speed of Light
 Light waves travel in a straight line as microwaves do.
 The speed of light is approximately 300,000,000 m/s,
or about 186,000 mi/s, in free space (in air or a
vacuum).
 The speed of light depends upon the medium through
which the light passes.
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Optical Principles
Physical Optics
 Physical optics refers to the ways that light can be
processed.
 Light can be processed or manipulated in many ways.
 Lenses are widely used to focus, enlarge, or decrease
the size of light waves from some source.
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Optical Principles
Physical Optics: Reflection
 The simplest way of manipulating light is to reflect it.
 When light rays strike a reflective surface, the light
waves are thrown back or reflected.
 By using mirrors, the direction of a light beam can be
changed.
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Optical Principles
Physical Optics: Reflection
 The law of reflection states that if the light ray strikes a
mirror at some angle A from the normal, the reflected
light ray will leave the mirror at the same angle B to the
normal.
 In other words, the angle of incidence is equal to the
angle of reflection.
 A light ray from the light source is called an incident
ray.
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Optical Principles
n=c/v
Sin A/Sin C=(n2/n1)
Illustrating reflection and refraction at the interface of two optical materials.
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Optical Principles
Physical Optics: Refraction
 The direction of the light ray can also be changed by
refraction, which is the bending of a light ray that
occurs when the light rays pass from one medium to
another.
 Refraction occurs when light passes through
transparent material such as air, water, and glass.
 Refraction takes place at the point where two different
substances come together.
 Refraction occurs because light travels at different
speeds in different materials.
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Optical Principles
Examples of the effect of
refraction.
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Optical Principles
Physical Optics: Refraction
 The amount of refraction of the light of a material is
usually expressed in terms of the index of refraction n.
 This is the ratio of the speed of light in air to the speed
of light in the substance.
 It is also a function of the light wavelength.
© 2008 The McGraw-Hill Companies
Optical
Communication Systems
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 Optical communication systems use light as the carrier
of the information to be transmitted.
 The medium may be free space as with radio waves
or a special light “pipe” or waveguide known as fiberoptic cable.
 Using light as a transmission medium provides vastly
increased bandwidths.
© 2008 The McGraw-Hill Companies
Optical
Communication Systems
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Light Wave Communication in Free Space
 An optical communication system consists of:
 A light source modulated by the signal to be
transmitted.
 A photodetector to pick up the light and convert it
back into an electrical signal.
 An amplifier.
 A demodulator to recover the original information
signal.
© 2008 The McGraw-Hill Companies
Optical
Communication Systems
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Free-space optical communication system.
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Optical
Communication Systems
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Light Wave Communication in Free Space: Light
Sources
 A transmitter is a light source.
 Other common light sources are light-emitting diodes
(LEDs) and lasers.
 These sources can follow electrical signal changes as
fast as 10 GHz or more.
 Lasers generate monochromatic, or single-frequency,
light that is fully coherent; that is, all the light waves are
lined up in sync with one another and as a result
produce a very narrow and intense light beam.
© 2008 The McGraw-Hill Companies
Optical
Communication Systems
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Light Wave Communication in Free Space: Modulator
 A modulator is used to vary the intensity of the light
beam in accordance with the modulating baseband
signal.
 Amplitude modulation, also referred to as intensity
modulation, is used where the information or
intelligence signal controls the brightness of the light.
 A modulator for analog signals can be a power
transistor in series with the light source and its dc power
supply.
© 2008 The McGraw-Hill Companies
Optical
Communication Systems
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A simple light transmitter with series amplitude modulator. Analog signals:
transistor varies its conduction and acts as a variable resistance. Pulse signals:
Transistor acts as a saturated on/off switch.
© 2008 The McGraw-Hill Companies
Optical
Communication Systems
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Light Wave Communication in Free Space: Receiver
 The modulated light wave is picked up by a
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photodetector.
This usually a photodiode or transistor whose
conduction is varied by the light.
The small signal is amplified and then demodulated to
recover the originally transmitted signal.
Light beam communication has become far more
practical with the invention of the laser.
Lasers can penetrate through atmospheric obstacles,
making light beam communication more reliable over
long distances.
© 2008 The McGraw-Hill Companies
Optical
Communication Systems
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Fiber-Optic Communication System
 Fiber-optic cables many miles long can be constructed
and interconnected for the purpose of transmitting
information.
 Fiber-optic cables have immense information-carrying
capacity (wide bandwidth).
 Many thousands of signals can be carried on a light
beam through a fiber-optic cable.
© 2008 The McGraw-Hill Companies
Optical
Communication Systems
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Fiber-Optic Communication System
 The information signal to be transmitted may be voice,
video, or computer data.
 Information must be first converted to a form compatible
with the communication medium, usually by converting
analog signals to digital pulses.
 These digital pulses are then used to flash a light
source off and on very rapidly.
 The light beam pulses are then fed into a fiber-optic
cable, which can transmit them over long distances.
© 2008 The McGraw-Hill Companies
Optical
Communication Systems
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Fiber-Optic Communication System
 At the receiving end, a light-sensitive device known as a
photocell, or light detector, is used to detect the light
pulses.
 The photocell converts the light pulses into an electrical
signal.
 The electrical signals are amplified and reshaped back
into digital form.
 They are fed to a decoder, such as a D/A converter,
where the original voice or video is recovered.
© 2008 The McGraw-Hill Companies
Optical
Communication Systems
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Basic elements of a fiber-optic communication system.
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Optical
Communication Systems
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Applications of Fiber Optics
 The primary use of fiber optics is in long-distance
telephone systems and cable TV systems.
 Fiber-optic networks also form the core or backbone of
the Internet.
 Fiber-optic communication systems are used to
interconnect computers in networks within a large
building, to carry control signals in airplanes and in
ships, and in TV systems because of the wide
bandwidth.
© 2008 The McGraw-Hill Companies
Optical
Communication Systems
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Benefits of fiber-optic cables over conventional electrical cables.
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Fiber-Optic Cables
 A fiber-optic cable is thin glass or plastic cable that
acts as a light “pipe.”
 Fiber cables have a circular cross section with a
diameter of only a fraction of an inch.
 A light source is placed at the end of the fiber, and
light passes through it and exits at the other end of the
cable.
 Light propagates through the fiber based upon the
laws of optics.
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Fiber-Optic Cables
Fiber-Optic Cable Construction
 Fiber-optic cables come in a variety of sizes, shapes,
and types.
 The portion of a fiber-optic cable that carries the light is
made from either glass, sometimes called silica, or
plastic.
 Plastic fiber-optic cables are less expensive and more
flexible than glass, but the optical characteristics of
glass are superior.
 The glass or plastic optical fiber is contained within an
outer cladding.
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Fiber-Optic Cables
Fiber-Optic Cable Construction
 The fiber, which is called the core, is usually
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surrounded by a protective cladding.
In addition to protecting the fiber core from nicks and
scratches, the cladding gives strength.
Plastic-clad silica (PCS) cable is a glass core with a
plastic cladding.
Over the cladding is usually a plastic jacket similar to
the outer insulation on an electrical cable.
Fiber-optic cables are also available in flat ribbon form.
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Fiber-Optic Cables
Basic construction of a fiber-optic cable.
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Fiber-Optic Cables
Typical layers in a fiber-optic cable.
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Fiber-Optic Cables
Types of Fiber-Optic Cables
 There are two ways of classifying fiber-optic cables.
 The first method is by the index of refraction, which
varies across the cross section of the cable.
 The second method of classification is by mode,
which refers to the various paths the light rays can
take in passing through the fiber.
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Fiber-Optic Cables
Types of Fiber-Optic Cables
 The two ways to define the index of refraction variation
across a cable are the step index and the graded
index.
 Step index refers to the fact that there is a sharply
defined step in the index of refraction where the fiber
core and cladding interface.
 With the graded index cable, the index of refraction of
the core is not constant. It varies smoothly and
continuously over the diameter of the core.
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Fiber-Optic Cables
A step index cable cross
section.
Graded index cable cross section.
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Fiber-Optic Cables
Types of Fiber-Optic Cables: Cable Mode
 Mode refers to the number of paths for light rays in the
cable.
 There are two classifications: single mode and
multimode.
 In single mode, light follows a single path through
the core.
 In multimode, the light takes many paths.
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Fiber-Optic Cables
Types of Fiber-Optic Cables
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In practice, there are three commonly used types of
fiber-optic cable:
1. Multimode step index
2. Single-mode step index
3. Multimode graded index
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Fiber-Optic Cables
Types of Fiber-Optic Cables: Multimode Step Index
Cable
 The multimode step index fiber cable is probably the
most common and widely used type.
 It is the easiest to make and therefore the least
expensive.
 It is widely used for short to medium distances at
relatively low pulse frequencies.
 The main advantage of a multimode stepped index fiber
is its large size.
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Fiber-Optic Cables
A multimode step index cable.
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Fiber-Optic Cables
Types of Fiber-Optic Cables: Single-Mode Step Index
Cable
 A single-mode or monomode step index fiber cable
eliminates modal dispersion by making the core so
small that the total number of modes or paths through
the core is minimized.
 Typical core sizes are 2 to 15 μm.
 The pulse repetition rate can be high and the maximum
amount of information can be carried in this type cable.
 They are preferred for long-distance transmission and
maximum information content.
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Fiber-Optic Cables
Types of Fiber-Optic Cables: Single-Mode Step Index
Cable
 This type of cable is extremely small, difficult to make,
and therefore very expensive.
 It is also more difficult to handle.
 Splicing and making interconnections are more difficult.
 For proper operation, an expensive, super-intense light
source such as a laser must be used.
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Fiber-Optic Cables
Single-mode step index cable.
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Fiber-Optic Cables
Types of Fiber-Optic Cables: Multimode Graded Index
Cable
 Multimode graded index fiber cables have several
modes, or paths, of transmission through the cable, but
they are much more orderly and predictable.
 These cables can be used at very high pulse rates and
a considerable amount of information can be carried.
 This type of cable is much wider in diameter, with core
sizes in the 50- to 100-μm range.
 It is easier to splice and interconnect, and cheaper, less
intense light sources can be used.
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Fiber-Optic Cables
A multimode graded index cable.
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Fiber-Optic Cables
Fiber-Optic Cable Specifications
 The most important specifications of a fiber-optic cable
are:
 Size
 Attenuation
 Bandwidth
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Fiber-Optic Cables
Fiber-Optic Cable Specifications: Cable Size
 Fiber-optic cable comes in a variety of sizes and
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configurations.
Size is normally specified as the diameter of the core,
and cladding is given in micrometers (μm).
Cables come in two common varieties, simplex and
duplex.
Simplex cable is a single-fiber core cable.
In a common duplex cable, two cables are combined
within a single outer cladding.
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Fiber-Optic Cables
Fiber-Optic Cable Specifications: Attenuation
 The most important specification of a fiber-optic cable is
its attenuation.
 Attenuation refers to the loss of light energy as the light
pulse travels from one end of the cable to the other.
 Absorption refers to how light energy is converted to
heat in the core material because of the impurity of the
glass or plastic.
 Scattering refers to the light lost due to light waves
entering at the wrong angle and being lost in the
cladding because of refraction.
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Fiber-Optic Cables
Fiber-Optic Cable Specifications: Bandwidth
 The bandwidth of a fiber-optic cable determines the
maximum speed of the data pulses the cable can
handle.
 As the length of the cable is increased, the bandwidth
decreases in proportion.
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Fiber-Optic Cables
Fiber-Optic Cable Specifications: Frequency Range
 Most fiber-optic cable operates over a relatively wide
light frequency range, although it is normally optimized
for a narrow range of light frequencies.
 The most commonly used light frequencies are 850,
1310, and 1550 nm.
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Fiber-Optic Cables
Connectors and Splicing
 When long fiber-optic cables are needed, two or more
cables can be spliced together.
 A variety of connectors are available that provide a
convenient way to splice cables and attach them to
transmitters, receivers, and repeaters.
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Fiber-Optic Cables
Connectors and Splicing
 Connectors are special mechanical assemblies that
allow fiber-optic cables to be connected to one another.
 Most fiber-optic connectors either snap or twist together
or screw together with threaded ends.
 Connectors ensure precise alignment of the cables to
ensure maximum light transfer between cables.
 Dozens of different kinds of connectors are available for
different applications. The two most common connector
designations are ST (bayonet connectors) and SMA.
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Fiber-Optic Cables
Connectors and Splicing
 Splicing fiber-optic cable means permanently attaching
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the end of one cable to another.
This is usually done without a connector.
The first step is to cut the cable, called cleaving the
cable, so that it is perfectly square on the end.
The two cables to be spliced are then permanently
bonded together by heating them instantaneously to
high temperatures so that they fuse or melt together.
Special tools and machines must be used in cleaving
and splicing to ensure clean cuts and perfect alignment.
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Fiber-Optic Cables
Details of a fiber cable
connector.
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Wavelength-Division Multiplexing
 Data is most easily multiplexed on fiber-optic cable by
using time-division multiplexing (TDM).
 Developments in optical components make it possible
to use frequency-division multiplexing (FDM) on fiberoptic cable (called wavelength-division multiplexing, or
WDM), which permits multiple channels of data to
operate over the cable’s light-wave bandwidth.
 WDM has been widely used in radio, TV, and
telephone systems.
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