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.
© 2008 The McGraw-Hill Companies
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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).
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
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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).
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
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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.
© 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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Benefits of fiber-optic cables over conventional electrical cables.
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Fiber-Optic Cables
Fiber-Optic Cable Specifications
 The most important specifications of a fiber-optic cable
are:
 Size
 Attenuation
 Bandwidth
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies
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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.
© 2008 The McGraw-Hill Companies