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Joseph C. Palais 1.1 1

Chapter 1

Fiber Optic

Communications

Subject:Opto Electronics

Engr.Abid Hussain

Chohan

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Section 1.1

IntroductionHISTORY

Early communications used light signals.

Hand signals used light (from the sun or the moon)

as an information carrier.

The modulator was the hand of the sender.

The detector was the eye of the receiver.

The processor was the brain of the receiver.

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History

Early Optical Communications Smoke signals

Blinker lights

Photophone: invented by Alexander Graham Bell

Properties of early optical communications systems:

1) slow data rate

2) poor integrity, high error rate

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Modern Optical Communications

1960: The first laser was constructed.

1960-1970 Time Period:

Many laser applications were proposed.One proposal was for an optical

communications link that operated line-of-sight

though the atmosphere.

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Modern Optical Communications

Problems

A clear atmosphere was required for efficient

transmission.

The unguided atmospheric system needed an

unobstructed line-of-site.

Solution

In 1970, a low-loss, glass fiber waveguide was

developed at Corning.

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Section 1.2

Basic Communications System

Transmitter

Information

ChannelReceiver

Messages are sent from the transmitter through

the information channel to the receiver.

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INFORMATION CHANNELS

Unguided Channels (Atmosphere)Radio Broadcast

Television Broadcast

Wireless

Satellite

Guided Channels

Conducting Wires

WaveguidesFibers

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INFORMATION CHANNELS

Applications For Guided Channels

Applications include:

cable television

telephone

data links [i.e., local area networks (LANs)]

The triple play:

Voice

Video

Data

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GENERAL OPTICAL SYSTEM

Message

Origin

Carrier

Source

Coupler

Modulator

Coupler

Detector

Processor

Destination

Optic

Domain

Electric Domain

Transmitter Receiver

Channel

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COMPONENTS OF A GENERAL

OPTICAL SYSTEMAnalog(continuous)

Time

Digital (discrete)

1 0 1 0 1

Time

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OPTIC SYSTEM

1.2.3 Carrier Source: Generates the lightwave

on which the informationis carried.

Common devices are:

laser diode (LD)

light emitting diode (LED)

The carrier source is intensity modulated. As theinput current changes, the output optical power

changes in the same way.

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OPTIC SYSTEMThe output power of the light source is proportional

to the input current.

Electrical Domain Optical DomainCarrier Source

ELECTRICAL

CURRENT, Iin

OPTICALPOWER, POUT

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GENERAL FIBER OPTIC SYSTEM

Intensity modulation: The transfer characteristics of

the ideal light source is

Optic

Power

(P)

Input current (I)

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Example:

0 0 tt

I P

The above graphs indicate that the optic

power is directly proportional to the input

current.

Input

Current

Output

Optical Power

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1.2.4 Coupler: Couples light from the source to the

fiber channel. The efficiency may not be high.

Why?

Answer:

1) Fibers are small (50 m diameters or less forsome fibers).

2) The fiber acceptance angle is small.

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Light

Source

Fiber

Acceptance

angle

Light ray

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1.2.5 Information channel: Glass (or plastic)

fibers that are the transmission medium.

Desirable properties:

1) Low attenuation (losses limit path lengths)

2) Low pulse (waveform) distortion

Pulses spread out as they propagate down the

fiber as indicated on the next slide.

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GENERAL FIBER OPTIC SYSTEMPulse Distortion (Pulse Spread)

Input

Power

Waveform after a short travel distance

Waveform after further travel

t

t

t

Power

Power

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Solution to pulse spreading:

The pulses need to be spread out more at the

transmitter, so they do not overlap with each other

at the receiver. This means sending fewer pulses

per second. That is, transmitting at a lower data

rate.

Conclusion: Distortion limits the allowed data rate.

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Receiver Coupler: Transfers the optic power

from the fiber to the photodetector.

Fiber

Photodetector

Electric

CurrentOptic Power

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1.2.6 Photodetector: Converts optical power

to electric current. Ideally, the current is a

replica of the current used to modulate the

light source.

Optical Power

(Pin)

Electric

Current(Iout)

Photodetector

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Transfer Characteristic for Photodetector

Optic Power (P)

Outputcurrent

(i)

Output current is

proportional to the

input optical

power.

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Processing: Electrical domain processing consists

of the following.

1.2.7 Signal Processing

1) Amplification

2) Filtering to improve signal quality

3) Decision making circuitry for digital signals

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Signal Quality Measures

Signal to noise ratio - measure for analog signals

Bit error rate - measure for digital

1.2.8 Message Destination

Devices such as output speakers, telephone

sets, video monitors, and computers.

1.2.9 Some Numbers

Important number in fiber optics

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Common UnitsTable

Units Symbol Measure

meter m lengthKilogram kg massSecond s timeCoulomb C chargeJoule J energy

Watt W power Hertz Hz frequencyNewton N ForceAmpere A CurrentKelvin K Temperature

Celsius C

TemperatureFarad F CapacitanceOhm ResistanceVolt V VoltageRadian r Angle

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IMPORTANT CONSTANTS

Description Value Symbol

Velocity of light 3*108m/s c

Planck constant 6.626*10-34J*s h

Electron charge -1.6*10-19 C -e

Boltzmann constant 1.38*10-23 J/K k

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PREFIXES

Prefix Symbol Multiplication Factor

tera T 1012

giga G 109

mega M 106

kilo k 103

centi c 10-2

milli m 10-3

micro 10-6nano n 10

-9

pico p 10-12

femto f 10-15

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COMMON UNITS IN FIBER OPTICS

Common units for calculation involving

lengths are:

nanometermn1m9

10

micronormicrometerm1m610

Optical wavelengths are usually in the above units.

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Bandwidths Of Common Systems

Common Analog Systems

Type Bandwidth CommentsVoice 4 kHz Telephone

Music 10 kHz AM radio broadcast

Music

TV

200 kHz

6 MHz

FM radio broadcast

Television broadcast

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Bandwidths Of Common Systems

Common Digital Systems

Type BW Comments

Voice 64 kbps Telephone

Ethernet 10 Mbps Xerox LAN

FDDI 100 Mbps Fiber distributed datainterface

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DIGITIZING VOICE MESSAGES

One 8-bit number consists of 8 time slots, each

containing a binary 0 or 1. There are 256 possible

combinations, since 28

= 256. Thus, a sequenceof 8 bits (combinations of zeroes and ones) will

represent the amplitude of each sample.

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DIGITIZING VOICE MESSAGES

Calculate the data rate:

8000(samples/s) x 8 (bits/sample) = 64,000 b/s

Therefore, there are 64,000 b/s transmitted for a

digitized voice message.

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MULTIPLEXING

We can mult ip lexnumerous messages by

transmitting at high transmission rates and

interleaving the bits from separate messages.

This is referred to as t ime div is ion mu lt ip lex ing

(TDM). The messages share the transmission

line. The receiver separates the individual

messages.

The field commander was here

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MULTIPLEXING

Examples of time division multiplexing.

Example 1: The T1 transmission level

operates at a standard rate = 1.544 Mb/s. Since

61.544 10

24.0664,000

Thus, 24 voice channels can be simultaneously

transmitted along a T1 transmission line.

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DIGITIZING VOICE SIGNALS

Examples of time division multiplexing.

Example 2: The T3 transmission level

operates at a standard rate = 44.7 Mb/s.

A T3 transmission normally carries up to 672

voice channels.

44.7x106/64,000 = 698

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ANALOG SIGNAL QUALITY

A measure of the quality of an analog signal is

the signal-to-noise ratio (SNR). It is the signal

power divided by the noise power.

S/N = (Signal Power)/(Noise Power)

For analog television signals, SNR > 10,000 is

required for decent viewing.

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DIGITAL SIGNAL QUALITY

The measure of quality for a digital system is the

bit error rate (BER). The BER is the fraction of

errors contained in a signal.

Example: A BER = 10-9means that there is one

error for every 109bits.

Digital systems require good bit error rates.

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1.2.10 Computing Power Levels in Decibels

The decibel scale is useful for analysis and design

of fiber components and systems.

P1

P2

Component

(System)

210

1

10log P

dBP

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THE DECIBEL SCALE

210

1

10log PdBP

This equation represents the decibel gain or loss

of the component (system). If there is loss the

decibel level will be negative, but if there is gain

then the decibel level will be positive.

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DECIBEL SCALE

The decibel scale is used to compare the ratio of

two power levels.

Example: Suppose P2/P1= 0.5. Then

dB = 10 log (0.5) = -3 dBExample: If P2/P1= 1, then

dB = 10 log 1 = 0 dB

Example: If P2/P1> 1, then dB is positiveExample: If P2/P1< 1, then dB is negative

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DECIBEL SCALE FOR CASCADED

ELEMENTS

The dB scale is useful for analyzing a system of

cascaded elements.

P1P

4

Element1 Element 2 Element 3

1

2

2

3

3

4

1

4

P

P

P

P

P

P

P

P P2 P3

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ELEMENTS

Thus

123

1

210

2

310

3

410

1

2

2

3

3

410

1

410

log10log10log10

log10log10

dBdBdBdB

P

P

P

P

P

PdB

P

P

P

P

P

P

P

PdB

system

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ELEMENTS

The dB of the cascaded elements are simply

added together. This illustrates the great

advantage of the decibel scale.

If the element has a loss, a negative sign

is placed in front of the dB value in the

preceding equation.

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DECIBEL ABSOLUTE POWER

SCALE

The decibel scale can be used to denote absolute

power if a reference power is specified. If the

reference power is set to 1 mW, we have the dBm

scale defined by

dBm = 10 log P

where P is in milliwatts. This is read as dB relativeto a milliwatt.

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SCALE

Example: If P = 2 mW, then

dBm = 10 log 2 = 3 dBm

Signs on the result are important. The dBm for

powers above one milliwatt (P > 1) will be

positive. The dBm for powers below a milliwatt

(P < 1) will be negative.

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SCALEThe dBscale is defined as:

dB= 10 log Pwhere P is in microwatts. This is read as dB

relative to a microwatt.


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SCALE

dBm1 dBm2

dBm2= dBm

1+ dB

x

210 10 2 10 1

1

2 1

2 1

10log 10log 10log

,

x x

x

x

PdB dB P P

P

dB dBm dBm

dBm dBm dB

Proof

dBxP1 P2

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Section 1 3

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Section 1.3

Wave Nature of LightSometimes light behaves as a wave and sometimes

light behaves as a particle. We will look at both

behaviors.

1.3.1 Wave Nature of Light

Light is an electromagnetic wave that satisfies

Maxwells Equations.

The electromagnetic spectrum is a range offrequencies classified into groups.

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Wave Nature of Light

1016 1015 1014 1013 1012 1011 1010 109 108 107 106 105 104 103 102 101 1

Ultraviolet

Visible

InfraredMillimeterwaves

Microwaves

Radio

Power

Frequency (Hz)

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WAVE NATURE OF LIGHT

Most of the frequencies in fiber optic systems are

In the Infrared.

Wavelength and frequency are related by:

f

vwherevis the velocity of the wave in the medium.

In free space,

v = c= 3 x108 m/s

(1.3)

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All materials slow down the light waves, so v < c in

all materials. If the material is changed, then the

wave velocity changes along with the wavelength.

The frequency remains constant, only the velocity

and the wavelength will change.

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WAVE NATURE OF LIGHTMost fiber optic communications systems operate

in the infrared region of the spectrum.

0.2 0.3 0.4 0.5 0.6 0.7 1.0 1.5 2.0

Wavelength (m)

Ultraviolet Visible Infrared

Optical Spectrum (Partial)

Blue Red

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Example: If is 0.85 m, find the frequency. Inthis example the medium is not specified, so let

us assume that the medium is free space.

814

6

3 10 /3.53 10

0.85 10

c m sf Hz

m

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The per iod o f osc i l lat ionT is defined as the time

it takes for the wave to complete one cycle. The

period for the frequency in the preceding

example is then

14

14

1 1

0.28 103.53 10T sf Hz

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WAVE NATURE OF LIGHTFiber optic systems must have low loss. For

glass low loss occurs at several wavelengths.

The major operation regions (w indows)are:

Wavelength Window

0.8-0.9 m First Window~1.300 m Second Window~1.550 m Third Window~1.600 m Fourth Window

These are all in the infrared.

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1.3.2 Particle Nature of Light

Light is made up particles called photons. Each

photon has energy:

p

c

W hf h h= 6.626 x 10-34J s (Plancks constant)

Shorter wavelength (higher frequency) waves have

greater photon energy.

(1.4)

PARTICLE NATURE OF

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PARTICLE NATURE OF

LIGHTExample: How many photons are delivered eachsecond for a wave with average power P = 1Wat wavelength = 0.8 m?The solution appears on the next slide.

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PARTICLE NATURE OF LIGHT

8

34

6

19

6 6

612

19

3 106.626 10

0.8 10

2.48 10

10 1 10

104.03 10

2.48 10

p

p

p

mc sW h J sm

JW

photon

W Pt Joules

W Jphotons

JW

photon

The photon energy is:

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C O G

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PARTICLE NATURE OF LIGHT

The electron volt (eV) is another useful unit for

analysis of fiber optic systems. The electron volt

is defined as the energy acquired by an electron

accelerated across a 1 volt potential difference.

The particle nature of light is used in explaining

the light sources and light detector later in the

text.

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Section 1 4

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Section 1.4

Advantages of Glass Fibers

Cost: The cost of fiber is very low, because glass is plentiful

and cheap.

Weight: Fiber cables are smaller than conducting cables

and weigh much less.

Strength: Fibers are strong and flexible, enabling them to go

around corners. Glass fibers have a very high tensile

strength.

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ADVANTAGES OF GLASS FIBERS

High informatioHigh information

capacity: Glass fiber can carry

more signals then a conducting

cable.

G S O G SS S

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ADVANTAGES OF GLASS FIBERS

Low Loss: As the modulation frequency

increases, coaxial cable losses increase at a

faster rate than do fiber losses.

Copper Fiberer

dB

loss(1kmleng

th)

Modulation Frequency(Hz)

4

73 dB

f 3-dB

3 dB OPTICAL BANDWIDTH

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3-dB OPTICAL BANDWIDTH

As light propagates down a fiber at low modulation

frequencies, its loss remains constant. At higher

modulation frequencies the signal power begins to

decrease.

The 3-dB opt ical bandwid this the frequency at

which the optical signal power is reduced by one

half. It is illustrated on the previous slide.

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0 1 2 3 4 5 6 70

0.5

1

1.5

2

2.5

3

3.5

4

time

opticalpowe

r

The 3dB

optical bandwidth

fa

f3-dB

is half the frequency and magnitude of fa

Average power

fc

OPTICAL 3-dB BANDWIDTH

fa

f3-dB

fc

TIME

OPTICALPOWER

OPTICAL 3 dB BANDWIDTH

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OPTICAL 3-dB BANDWIDTH

The previous slide illustrates the 3-dB optical bandwidth.

cd Ba fff 3

The signal attenuation is caused by the spreading of the

wave (pulse spreading) as it propagates down the fiber,

resulting in a lower peak power.

COMPARISON OF A FIBER CABLE

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WITH A WIRE CABLE

A wire cable consisting of 900 twisted pairs has adiameter of 70 mm. Each wire pair carries 24 voice

channels. The cable capacity is then:

24 x 900 = 21,600 voice calls

Compare this capacity to that of a T3 fiber cable containing

144 fibers having a diameter of 12.7 mm. At the T3 rate there

are 672 voice channels per fiber.


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WITH A WIRE CABLE

The fiber cable has total capacity:

672 x 144 = 96,768 voice calls

This is 4.5 times the capacity of the wire cable. In addition,

the fiber cross sectional area is 1/30th that of a wire cable.

The represents a great savings in space required for

installation.

ADVANTAGES OF FIBERS

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A. Fibers are insulators as opposed to conducting wires.

1) No current

2) No radiation from the sides of the fiber

3) No coupling between adjacent fibers


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B. Fibers reject interference. The following forms of

interference have no affect on fiber systems.

1) RFI: radio frequency interference from TV, radio, radar,

or other electronic signals.

2) EMI: electromagnetic interference from lighting,

sparking, or electromagnetic radiation.

3) EMP: electromagnetic pulse due to nuclear events.


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C. Fibers can be used near high power transmission

lines because of their insulation properties,

whereas wire systems would pick up a large amount

of noise.

D. Security: Fiber systems are difficult to tap.

E. Compatibility: Fibers are compatible with

conventional electronics.

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F. Corrosion resistant: Fiber, as opposed to wire systems,

resist corrosion.

G. Large temperature range: Glass melts at high

temperatures.

Section 1.5

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Applications of Fiber Systems1) Cable TV: Frequency-division-multiplexing (FDM) is

used to transmit television signals over fiber systems. Each

television channel has a bandwidth of 6 MHz.

Po

wer

0 f (Hz)6 MHz

TV Baseband Signal

FREQUENCY DIVISION

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MULTIPLEXING

Power

0 f (Hz)

6 MHz

TV Channel

fo

The television signal can be modulated onto a sub-

carrier frequency fo. The baseband signal has been

shifted upwards in frequency.

sub-carrier

FREQUENCY DIVISION

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MULTIPLEXING (FDM)

Electrical

0 f (Hz)

6 MHz

1

f1

Several television channels can be multiplexed onto different

sub-carrier frequencies and the result used to modulate a

light source.

f3f2 f4

6 MHz 6 MHz 6 MHz

2 3 4

Sub-Carrier Frequencies

Channels

FREQUENCY DIVISION

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MULTIPLEXING (FDM)

At the receiver the television signals are

demultiplexed by filters. Several tens of television

channels can be transmitted by frequency-

division-multiplexing (FDM) on a single fiber.

APPLICATIONS OF FIBER SYSTEMS

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2) Telephone trunk lines: In 1976-77 large-scale

fiber lines were first deployed to connect telephone

exchanges at 45 Mb/s. Fiber data rates are now

beyond 10 Gb/s.

3) Transatlantic cables: In 1989 transatlantic

cables were introduced that could handle 40,000

voice calls. Now all the major oceans and seas have

fiber cables lying beneath them.

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Joseph C. Palais 83

APPLICATIONS OF FIBER

SYSTEMS

4) Wired city: Due to increased demand, fiber-to-

the-home and fiber-to-the-curb systems are

available to bring higher data rates for video and

internet applications to businesses and to the

home.

5) Communications along electric railways: Fibersare not affected by electrical interference from the

railways.


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6) Communications along high voltage lines: Fibers

are not affected by interference from these lines.

7) Other video applications such as remote

monitoring and surveillance.

8) Local area networks (LANs), computers, file

systems: e.g., Ethernet.

9) Military applications such as tactical command

post telecommunications and fiber-guided

missiles.


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Sensors: Measurements of temperature, pressure, velocity,

motion and more.

The following is an example for a hydrophone sound

sensor.

Fluid

Fixed Fiber

Light Source Detector

speaker

sound wave

MonitorFree Fiber


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The hydrophone works on the principle of externalmodulation. The light is modulated by motion of

the fibers, causing misalignments in the

transmission system, resulting in a change inpower received.

S ti 1 6

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Section 1.6

Summary and Discussion

DECISIONS FACED BY DESIGNERS

Fiber or metal cable

Full-duplex or half-duplex two-way communications

Modulation format (analog or digital)

Multiplexing schemes

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Summary and Discussion

Wavelength of operation

Type of light source

Type of fiber

Choice of all other components

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