Download Principles of Electronic Communication Systems

1
Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
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Chapter 19
Optical Communication
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Topics Covered in Chapter 19
 19-1: Optical Principles
 19-2: Optical Communication Systems
 19-3: Fiber-Optic Cables
 19-4: Optical Transmitters and Receivers
 19-5: Wavelength-Division Multiplexing
 19-6: Passive Optical Networks
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19-1: 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.
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19-1: 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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19-1: Optical Principles
Figure 19-1: The optical spectrum. (a) Electromagnetic frequency spectrum showing
the optical spectrum.
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19-1: Optical Principles
Figure 19-1: The optical spectrum. (b) Optical spectrum details.
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19-1: 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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19-1: 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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19-1: 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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19-1: 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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19-1: 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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19-1: Optical Principles
Figure 19-2: Illustrating reflection and refraction at the interface of two optical
materials.
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19-1: 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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19-1: Optical Principles
Figure 19-3: Examples of the
effect of refraction.
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19-1: 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.
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19-2: 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.
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19-2: 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.
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19-2: Optical
Communication Systems
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Figure 19-6: Free-space optical communication system.
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19-2: 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.
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19-2: 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.
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19-2: Optical
Communication Systems
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Figure 19-7: 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.
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19-2: 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.
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19-2: 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.
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19-2: 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.
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19-2: 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.
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19-2: Optical
Communication Systems
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Figure 19-8: Basic elements of a fiber-optic communication system.
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19-2: 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.
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19-2: Optical
Communication Systems
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Figure 19-10: Benefits of fiber-optic cables over conventional
electrical cables.
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19-3: 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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19-3: 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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19-3: 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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19-3: Fiber-Optic Cables
Figure 19-12: Basic construction of a fiber-optic cable.
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19-3: Fiber-Optic Cables
Figure 19-13: Typical layers in a fiber-optic cable.
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19-3: 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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19-3: 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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19-3: Fiber-Optic Cables
Figure 19-15: A step index cable cross
section.
Figure 19-16: Graded index cable
cross section.
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19-3: 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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19-3: 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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19-3: 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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19-3: Fiber-Optic Cables
Figure 19-17: A multimode step index cable.
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19-3: 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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19-3: 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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19-3: Fiber-Optic Cables
Figure 19-19: Single-mode step index cable.
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19-3: 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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19-3: Fiber-Optic Cables
Figure 19-20: A multimode graded index cable.
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19-3: Fiber-Optic Cables
Fiber-Optic Cable Specifications
 The most important specifications of a fiber-optic cable
are:
 Size
 Attenuation
 Bandwidth
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19-3: 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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19-3: 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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19-3: 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.
 The bandwidth is normally stated in terms of
megahertz-kilometers (MHz-km).
 A common 62.5/125-μm cable has a bandwidth in the
100- to 300-MHz∙km range.
 As the length of the cable is increased, the bandwidth
decreases in proportion.
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19-3: 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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19-3: 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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19-3: 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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19-3: 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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19-3: Fiber-Optic Cables
Figure 19-26: Details of a fiber
cable connector.
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19-4: Optical Transmitters
and Receivers
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 In an optical communication system, transmission
begins with the transmitter, which consists of a carrier
generator and a modulator.
 The carrier is a light beam that is modulated by turning
it on and off with digital pulses.
 The basic transmitter is essentially a light source.
 The receiver is a light or photodetector that converts
the received light back into an electrical signal.
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19-4: Optical Transmitters
and Receivers
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Light Sources
 Conventional light sources such as incandescent lamps
cannot be used in fiber-optic systems because they are
too slow.
 The two most commonly used light sources are lightemitting diodes (LEDs) and semiconductor lasers.
 A light-emitting diode is a PN-junction semiconductor
device that emits light when forward-biased.
 Semiconductor lasers emit coherent monochromatic
light.
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19-4: Optical Transmitters
and Receivers
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Light Transmitters: LED Transmitter
 An LED light transmitter consists of the LED and its
associated driving circuitry.
 The binary data pulses are applied to a logic gate which
operates a transistor switch that turns the LED off and
on.
 Most LEDs are capable of generating power levels up to
approximately several thousand microwatts.
 LED transmitters are good for only short distances.
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19-4: Optical Transmitters
and Receivers
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Figure 19-30: Optical transmitter circuit using an LED.
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19-4: Optical Transmitters
and Receivers
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Light Transmitters: Laser Transmitter
 Most of the circuitry is contained in a single IC.
 A multiplexer either passes the input data directly to the
laser driver transistors or selects data that is clocked via
a flip-flop and an external differential clock signal.
 Enable/disable signals turn the laser off or on.
 The laser diode connects to the driver by several
resistors and a capacitor that set the current and
switching response.
 Most laser packages also contain a photodiode that
monitors the laser light output and provides feedback to
the APC circuit.
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19-4: Optical Transmitters
and Receivers
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Figure 19-31: A typical laser driver circuit. (Courtesy Vitesse Semiconductor Corp.)
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19-4: Optical Transmitters
and Receivers
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Light Detectors
 The receiver part of the optical communication system
consists of a detector that senses the light pulses and
converts them into an electrical signal.
 This signal is amplified and shaped into the original
serial digital data.
 The most critical component is the light sensor.
 The most widely used light sensor is a photodiode. It is
a silicon PN-junction diode that is sensitive to light.
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19-4: Optical Transmitters
and Receivers
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Light Detectors
 The phototransistor amplifies the small leakage
current into a larger, more useful output.
 PIN diodes are more sensitive than the PN-junction
photodiode.
 The avalanche photodiode (APD) is widely used and
is the fastest and most sensitive photodiode, but it is
expensive and its circuitry is complex.
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19-4: Optical Transmitters
and Receivers
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Figure 19-35: Structure of a PIN photodiode.
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19-4: Optical Transmitters
and Receivers
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 A typical light receiver circuit is an IC using an
external PIN or APD photodiode. It can operate at
rates to 3.125 Gbps.
 Optical transceivers or transponders are
assemblies called optical modules into which both
the light transmitter and light receiver are packaged to
form a single module.
 These modules form the interface between the optical
transmission medium and the electrical interface to the
computer or other networking equipment.
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19-4: Optical Transmitters
and Receivers
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Power Budget
 A power budget, sometimes called a flux budget, is
an accounting of all the attenuation and gains in a fiberoptic system.
 There are numerous sources of losses in a fiber-optic
cable system:
 Cable losses
 Connections between cable and light source and
photodetector.
 Connectors
 Splices
 Cable bends
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19-4: Optical Transmitters
and Receivers
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Power Budget
 To calculate the power budget:
 First, calculate all the losses; add all the decibel loss
factors.
 Also add a 4-dB contingency factor.
 Calculate the power gain needed to overcome the
loss:
dB = 10 log Pt / Pr
where Pt is the transmitted power and Pr is the
received power.
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and Receivers
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Power Budget: Regeneration and Amplification

There are several ways to overcome the attenuation
experienced by a signal as it travels over fiber-optic
cable.
1. Use newer types of cable that inherently have lower
losses and fewer dispersion effects.
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and Receivers
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Power Budget: Regeneration and Amplification
2. Use regeneration.
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Regeneration is the process of converting the weak optical
signal to its electrical equivalent, then amplifying and
reshaping it electronically, and retransmitting it on another
laser.
This process is generally known as optical-electrical- optical
(OEO) conversion.
3. Use an optical amplifier (the best option).
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Optical amplifiers boost signal level without OEO conversion.
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and Receivers
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Figure 19-39: An erbium-doped fiber amplifier.
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19-5: Wavelength-Division
Multiplexing
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 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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19-5: Wavelength-Division
Multiplexing
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 The first coarse WDM (CWDM) systems used two
channels operating on 1310 and 1550 nm. Later, four
channels of data were multiplexed.
 Dense wavelength-division multiplexing (DWDM)
refers to the use of 8, 16, 32, 64, or more data
channels on a single fiber.
 Arrayed waveguide grating (AWG) is an array of
optical waveguides of different lengths made with
silica on a silicon chip. It can be used for both
multiplexing and demultiplexing.
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19-6: Passive Optical Networks
 The primary applications for fiber-optic networks are in
wide-area networks such as long-distance telephone
service and the Internet backbone.
 As speeds have increased and prices have declined,
fiber-optic technology has been adopted into MANs,
storage-area networks (SANs), and LANs.
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19-6: Passive Optical Networks
 A newer and growing fiber-optic system is the passive
optical network (PON), a type of MAN technology.
 This technology is also referred to as fiber to the
home (FTTH). Similar terms are fiber to the
premises or fiber to the curb, designated as FTTP or
FTTC.
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19-6: Passive Optical Networks
The PON Concept
 Most optical networking uses active components to
perform optical-to-electrical and electrical-to-optical
(OEO) conversions during transmission and reception.
This is an expensive and problematic structure.
 One solution to this problem is to use a passive optical
network.
 The term passive implies no OEO repeaters, amplifiers,
or any other device that uses power.
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19-6: Passive Optical Networks
The PON Concept
 In a PON, the transmitter sends the signal out over the
network cable, and a receiver at the destination picks it
up.
 There are no intervening repeaters or amplifiers. Only
passive optical devices such as splitters and combiners
are used.
 By using low-attenuation fiber-optic cable, powerful
lasers, and sensitive receivers, it is possible to achieve
distances of up to about 20 km without intervening
active equipment.
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19-6: Passive Optical Networks
Figure 19-42: A passive optical network (PON) used as a high-speed Internet
connection and for TV distribution in fiber to the home systems.
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