1 intro optcomm sys

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CHAPTER 1

INTRODUCTION TO OPTICALINTRODUCTION TO OPTICAL

COMMUNICATION SYSTEMSCOMMUNICATION SYSTEMS


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DEFINITION OF FIBER OPTICS

Fiber optics uses light to send information (data). Fiber optics

is the branch of optical technology concerned with the

transmission of radiant power (light energy) through fibers.

Figure 1.1: Fiber optic cable


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1.2 FIBER OPTIC DATA LINKS

A fiber optic data link sends input data through fiber opticcomponents and provides this data as output information. It has

the following three basic functions:

To convert an electrical input signal to an optical signal

To send the optical signal over an optical f iber

To convert the optical signal back to an electrical signal

A fiber optic data link consists of three parts - transmitter,optical fiber, and receiver.


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Figure 1.2: Parts of a fiber optic data link


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The transmitter converts the input signal to an optical signalsuitable for transmission. The transmitter consists of two parts,

an interface circuit and a source drive circuit. The transmitter's

drive circuit converts the electrical signals to an optical signal. It

does this by varying the current flow through the light source.The two types of optical sources are light-emitting diodes

(LEDs) and laser diodes.

The optical source launches the optical signal into the fiber. Theoptical signal will become progressively weakened and distorted

because of scattering, absorption, and dispersion mechanisms

in the fiber waveguides.


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The receiver converts the optical signal exiting the fiber back

into an electrical signal. The receiver consists of two parts, theoptical detector and the signal-conditioning circuits.An optical

detector detects the optical signal. The signal-conditioning

circuit conditions the detector output so that the receiver

output matches the original input to the transmitter.

The receiver should amplify and process the optical signal

without introducing noise or signal distortion. Noise is any

disturbance that obscures or reduces the quality of the signal.Noise effects and limitations of the signal-conditioning circuits

cause the distort ion of the receiver's electrical output signal.


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An optical detector can be either a semiconductor positive-

intrinsic-negative (PIN) diode or an avalanche photodiode (APD).

A PIN diode changes its electrical conductivity according to the

intensity and wavelength of light. The PIN diode consists of an

intrinsic region between p-type and n-type semiconductor

material.

A fiber optic data link also includes passive components other

than an optical fiber. Passive components used to make fiber

connections affect the performance of the data link. These

components can also prevent the link from operating.

Fiber optic components used to make the optical connectionsinclude optical splices, connectors, and couplers.


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Figure 1.3:


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1.3 HISTORICAL DEVELOPMENT OF FIBER

Year Development

1880 Graham Bell's photophone - modulation of sunlight with a

diaphragm giving speech transmission over a distance of 200m

1960 Invention of laser, for unguided transmission

1966 Kao demonstrated transmission of light through optical fiber, but

the attenuation was then 1000 dB/Km compared to coaxial cable

with attenuation of 5 dB/km.

1973 Laser lifetime improved to 1000 hours.

1976 Attenuation of fiber reduced to 5 dB/km.1977 Laser lifetime extended to 7000 hours using AIGaAs operating at

0.8 urn and 0.9 um. Other materials emitting light at 1.1um to

1.6um was found.

1988 Laser lifetime extended between 25 to 100 years (in the lab).


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At present the minimum attenuation for glass fibers isreported to be 0.2 dB/km. There are 1550 nm systems

operating with repeater spacing of 150 km.

Alcatel has unrepeatered systems of 2.5 Gb/s and 622

Mb/s spanning distance of 511 km and 531 kmrespectively.

WDM systems operating at 1.28 Tbit/s (128 wavelengths

at 10.7 Gbit/s (Lucent Technology) is possible today

1.3 HISTORICAL DEVELOPMENT OF FIBER


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1.4 FIBER OPTIC SYSTEMS

System design has centered on long-haul communications and

the subscriber-loop plant. The subscriber-loop plant is the partof a system that connects a subscriber to the nearest switching

center. Cable television is an example.

Long-haul systems require single mode optical fibers with veryhigh performance. Single mode fibers tend to have lower loss

and produce less signal distortion.

In contrast, short-distance and military systems tend to use onlymultimode technology. Example of short-distance systems

includes local area networks (LANs). Short-distance and military

systems have many connections. The larger fiber core and

higher fiber numerical aperture (NA) of multimode fibers reducelosses at these connections.


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In military and subscriber-loop applications, system design

and parts selection are related. Designers consider trade-

offs in the following areas:

Fiber properties

Types of connections

Optical sources

Detector types


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Fiber optic systems have many attractive features that aresuperior to electrical systems. These include improved system

performance, immunity to electrical noise, signal security, and

improved safety and electrical isolation.

Other advantages include reduced size and weight,

environmental protection, and overall system economy. Table

1 details the main advantages of fiber optic systems.

1.5 ADVANTAGES AND DISADVANTAGES OF

FIBER OPTICS


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System Performance Greatly increased bandwidth

and capacity

Lower signal attenuation (loss)

Immunity to Electrical Noise

Immune to noise

(electromagnetic interference

(EMI) and radio-frequency

interference (RFI)

No crosstalk

Lower bit error rates

Signal Security

Difficult to tap

Nonconductive (does not

radiate signals)Electrical

Isolation

No common ground required

Freedom from short circuit

and sparks

Size and Weight

Reduced size and weightcables

Environmental Protection

Resistant to radiation and

corrosion Resistant to temperature

variations

Improved ruggedness and

flexibility Less restrictive in harsh

environments

Overall System Economy

Low per-channel cost Lower installation cost

Table 1: Advantages of Fiber Optics


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1.5 ADVANTAGES OF OPTICAL

COMMUNICATION SYSTEM

Enormous potential bandwidth

The optical carrier frequency in the range 1013 to

1016 Hz yields a far greater potential transmission

bandwidth than metallic cable systems (i.e. coaxialcable bandwidth up to around 500 MHz) or even

millimeter wave radio systems (i.e. systems

currently operating with modulation bandwidths of

700 MHz).

Potential BW - 50 Tbit/s


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Immunity to interference and crosstalk

Optical fibers are free from electromagnetic interference(EMI) and radio frequency interference (RFI). Hence the

operation of an optical fiber communication system is

unaffected by transmission through an electrically noisy

environment and the fiber cable requires no shielding fromEMI. The fiber cable is also not susceptible to lightning

strikes if used overhead rather than underground.

Moreover, it is fairly easy to ensure that there is no optical

interference between fibers and hence, unlikecommunication using electrical conductors, crosstalk is

negligible, even when many fibers are cabled together.


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Small size and weight

Optical fibers have very small diameters which are often

no greater than the diameter of a human hair. Hence, evenwhen such fibers are covered with protective coatings

they are far smaller and much lighter than corresponding

copper cables. This is a tremendous boom towards the

alleviation of duct congestion in cities, as well as allowingfor an expansion of signal transmission within mobiles

such as aircraft, satellites and even ships.

RG-19/U : 22.6 dB/km at 100 MHz - 1110 kg/km

Silica Fiber : 5 dB/km - 6 kg/km


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Electrical Isolation

Optical fibers which are fabricated from glass orsometimes a plastic polymer are electrical insulators and

therefore, unlike their metallic counterparts, they do not

exhibit earth loop and interface problems. Furthermore,

this property makes optical fiber transmission ideallysuited for communication in electrically hazardous

environments as the fibers create no arcing or spark

hazard at abrasions or short circuits.


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Signal security

The light from optical fibers does not radiate significantly

and therefore they provide a high degree of signalsecurity. Unlike copper cables, a transmitted optical signal

cannot be obtained from a fiber in a non-invasive manner

(i.e. without drawing optical power from the fiber).

Therefore, in theory, any attempt to acquire a messagesignal transmitted optically may be detected. This feature

is obviously attractive for military, banking and general

data transmission (i.e. computer network) applications.


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Low transmission loss

The development of optical fibers has resulted in the

production of optical fiber cables which exhibit very lowattenuation or transmission loss in comparison with the

best copper conductors. Fibers have been fabricated with

losses as low as 0.2 dB km -1 . It facilitates the

implementation of communication links with extremelywide repeater spacing (long transmission distances

without intermediate electronics), thus reducing both

system cost and complexity.

Silica Fiber 100 MHz 4 dB/km

RG-19/U 500 MHz 14 dB/km


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Ruggedness and flexibil ity

Although protective coatings are essential, optical fibersmay be manufactured with very high tensile strengths.

Perhaps surprisingly for a glassy substance, the fibers

may also be bent to quite small radii or twisted without

damage. Furthermore, cable structures have beendeveloped which have proved flexible, compact and

extremely rugged. These optical f iber cables are generally

superior in terms of storage, transportation, handling and

installation than corresponding copper cables whilstexhibiting at least comparable strength and durability.


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System reliability and ease of maintenance

These features primarily stem from the low loss property

of optical fiber cables which reduces the requirement forintermediate repeaters or line amplifiers to boost the

transmitted signal strength. Hence with fewer repeaters,

system reliability is generally enhanced in comparison

with conventional electrical conductor systems.Furthermore, the reliability of the optical components is

no longer a problem with predicted lifetimes of 20-30

years now quite common. Both these factors also tend

to reduce maintenance time and costs.


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Potential low cost

The glass which generally provides the optical fibertransmission medium is made from sand - not a scarce

resource. So, in comparison with copper conductors,

optical fibers offer the potential for low cost line

communication. Overall system cost when utilizing opticalsystem are substantially less than those for equivalent

electrical line systems because of the low loss and

wideband properties of optical transmission.

The main advantages:

Large BW and Low loss


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APPLICATIONS

1.6.1 Voice

1. Telephone trunk for high data

Inter-office

Inter-city

Transoceanic

2. Subscriber service

Fiber-to-the home (FTTH)

Broadband services(multimedia, video, etc.)

1.6.2 Video

1. Broadcast Television

Live events

TV mini cameras

2. CATVSource-to-head end trunk

lines

Distributions

Subscriber taps3. Surveil lance

4. Remote monitoring

5. Fiber-guided missile

6. FTTH


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1.6.3 Data

1. Computers

CPU to peripherals

CPU to CPU

2. Interoffice data links3. Local Area Network (LAN)

4. Fiber-to-the home

5. Aircraft and ship wiring

reduced weight6. Satellite ground stations

1.6.4 Sensors

1. Gyroscope

2. Hydrophone - acoustic

measurements in water

3. Position4. Temperature

5. Electric and Magnetic

Fields

6. Medical

1 intro optcomm sys

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