Optical Communications Essentials.pdf

Source: Optical Communications Essentials

Basic Concepts of

Communication Systems

Chapter

Ever since ancient times, people continuously have devised new techniques and

technologies for communicating their ideas, needs, and desires to others. Thus,

many forms of increasingly complex communication systems have appeared

over the years. The basic motivations behind each new one were to improve the

transmission fidelity so that fewer errors occur in the received message, to

increase the transmission capacity of a communication link so that more infor-

mation could be sent, or to increase the transmission distance between relay sta-

tions so that messages can be sent farther without the need to restore the signal

fidelity periodically along its path.

Prior to the nineteenth century, all communication systems operated at a very

low information rate and involved only optical or acoustical means, such as signal

lamps or horns. One of the earliest known optical transmission links, for exam-

ple, was the use of a fire signal by the Greeks in the eighth century B . C . for send-

ing alarms, calls for help, or announcements of certain events. Improvements of

these systems were not pursued very actively because of technology limitations at

the time. For example, the speed of sending information over the communication

link was limited since the transmission rate depended on how fast the senders

could move their hands, the receiver was the human eye, line-of-sight transmis-

sion paths were required, and atmospheric effects such as fog and rain made the

transmission path unreliable. Thus it turned out to be faster, more efficient, and

more dependable to send messages by a courier over the road network.

The invention of the telegraph by Samuel F. B. Morse in 1838 ushered in a

new epoch in communications—the era of electrical communications. In the ensu-

ing years increasingly sophisticated and more reliable electrical communication

systems with progressively larger information capacities were developed and

deployed. This activity led to the birth of free-space radio, television, microwave,

and satellite links, and high-capacity terrestrial and undersea wire lines for

sending voice and data (and advertisements!) to virtually anywhere in the world.

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Basic Concepts of Communication Systems

Chapter One

However, since the physical characteristics of both free-space and electric

wire-based communication systems impose an upper bound on the transmission

capacities, alternative transmission media were investigated. A natural exten-

sion was the use of optical links. After extensive research and development on

the needed electrooptical components and the glass equivalent of a copper wire,

optical fiber communication systems started to appear in the 1970s. It is this

technology that this book addresses.

To exchange information between any two devices in a communication system,

some type of electric or optical signal which carries this information has to be

transmitted from one device to the other via a communication channel. This

channel could consist of a wire, radio, microwave, satellite, infrared, or optical

fiber link. Each of the media used for such communication channels has unique

performance characteristics associated with it. Regardless of its type, the medium

degrades the fidelity of the transmitted signal because of an imperfect response

to the signal and because of the presence of electrical and/or optical noise and

interference. This can lead to misinterpretations of the signal by the electronics

at the receiving end. To understand the various factors that affect the physical

transfer of information-bearing signals, this chapter gives a basic overview of

fundamental data communication concepts. With that as a basis, the following

chapters will describe how information is transferred using lightwave technology.

1.1. Definitions

We start by giving some concepts and definitions used in data communications

and the possible formats of a signal. The signal format is an important factor in

efficiently and reliably sending information across a network.

A basic item that appears throughout any communications book is the prefix

used in metric units for designating parameters such as length, speed, power

level, and information transfer rate. Although many of these are well known, a

few may be new to some readers. As a handy reference, Table 1.1 lists standard

prefixes, their symbols, and their magnitudes, which range in size from 10 24 to

10 24 . As an example, a distance of 2

10 9

m (meters)

■ Information has to do with the content or interpretation of something such as

spoken words, a still or moving image, the measurement of a physical charac-

teristic, or values of bank accounts or stocks.

■ A message may be considered as the physical manifestation of the information

produced by the source. That is, it can range from a single number or symbol

to a long string of sentences.

■ The word data refers to facts, concepts, or instructions presented as some type

of encoded entities that are used to convey the information. These can include

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2 nm (nanometers).

The three highest and lowest designations are not especially common in com-

munication systems ( yet!), but are included in Table 1.1 for completeness.

Next let us define some terms and concepts that are used in communications.

Basic Concepts of Communication Systems

TABLE 1.1. Metric Prefixes, Their Symbols, and Their Magnitudes

Prefix

Symbol

Decimal

Magnitude

Multiple

yotta

10 24

zetta

10 21

exa

10 18

peta

Quadrillion

10 15

tera

Trillion

10 12

giga

1,000,000,000

Billion

10 9

mega

1,000,000

Million

10 6

kilo

1,000

Thousand

10 3

centi

0.01

Hundredth

10 2

milli

0.001

Thousandth

10 3

micro

0.000001

Millionth

10 6

nano

0.000000001

Billionth

10 9

pico

Trillionth

10 12

femto

Quadrillionth

10 15

atto

10 18

zepto

10 21

yocto

10 24

arrays of integers, lines of text, video frames, digital images, and so on. Although

the words data and message each have a specific definition, these terms often

are used interchangeably in the literature since they represent physical embod-

iments of information.

■ Signals are electromagnetic waves (in encoded electrical or optical formats)

used to transport the data over a physical medium.

A block diagram of an elementary communication link is shown in Fig. 1.1.

The purpose of such a link is to transfer a message from an originating user,

called a source , to another user, called the destination . In this case, let us assume

the end users are two communicating computers attached to different local area

networks (LANs). The output of the information source serves as the message

input to a transmitter. The function of the transmitter is to couple the message

onto a transmission channel in the form of a time-varying signal that matches

the transfer properties of the channel. This process is known as encoding .

As the signal travels through the channel, various imperfect properties of the

channel induce impairments to the signal. These include electrical or optical

noise effects, signal distortions, and signal attenuation. The function of the

receiver is to extract the weakened and distorted signal from the channel,

amplify it, and restore it as closely as possible to its original encoded form before

decoding it and passing it on to the message destination.

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Basic Concepts of Communication Systems

Chapter One

Figure 1.1. Block diagram of a typical communication link connecting

separate LANs.

1.2. Analog Signal Formats

The signals emitted by information sources and the signals sent over a transmis-

sion channel can be classified into two distinct categories according to their phys-

ical characteristics. These two categories encompass analog and digital signals.

An analog signal conveys information through a continuous and smooth vari-

ation in time of a physical quantity such as optical, electrical, or acoustical inten-

sities and frequencies. Well-known analog signals include audio (sound) and video

messages. As examples,

■ An optical signal can vary in color (which is given in terms of its wavelength

or its frequency, as described in Chap. 3), and its intensity may change from

dim to bright.

■ An electric signal can vary in frequency (such as the kHz, MHz, GHz desig-

nations in radio communications), and its intensity can range from low to

high voltages.

■ The intensity of an acoustical signal can range from soft to loud, and its tone

can vary from a low rumble to a very high pitch.

The most fundamental analog signal is the periodic sine wave , shown in

Fig. 1.2. Its three main characteristics are its amplitude, period or frequency,

and phase. The amplitude is the size or magnitude of the waveform. This is

generally designated by the symbol A and is measured in volts , amperes , or

watts , depending on the signal type. The frequency (designated by f ) is the

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Basic Concepts of Communication Systems

Amplitude

Time

1/ f

Period = 1/ f

1/ f

Figure 1.2. Characteristics of a basic sine wave.

Amplitude

Wave 1

A A 1

Time

Amplitude

Wave 2

A A 2

Time

A 1 + A 2

Figure 1.3. Two in-phase

waves will add constructively.

A 1 + A 2

number of cycles per second that the wave undergoes (i.e., the number of times

it oscillates per second), which is expressed in units of hertz (Hz). A hertz refers

to a complete cycle of the wave. The period (generally represented by the sym-

bol T ) is the inverse of the frequency, that is, period

1/ f . The term phase

) describes the position of the waveform relative to

time 0, as illustrated in Fig. 1.3. This is measured in degrees or radians (rad):

180°

rad.

If the crests and troughs of two identical waves occur at the same time, they

are said to be in phase . Similarly, if two points on a wave are separated by whole

measurements of time or of wavelength, they also are said to be in phase. For

example, wave 1 and wave 2 in Fig. 1.3 are in phase. Let wave 1 have an ampli-

tude A 1 and let wave 2 have an amplitude A 2 . If these two waves are added, the

amplitude A of the resulting wave will be the sum: A

A 1

A 2 . This effect is

known as constructive interference .

Figure 1.4 illustrates some phase shifts of a wave relative to time 0. When two

waves differ slightly in their relative positions, they are said to be out of phase .

As an illustration, the wave shown in Fig. 1.4 c is 180° (

rad) out of phase

with the wave shown in Fig. 1.4 a . If these two waves are identical and have

the same amplitudes, then when they are superimposed, they cancel each

other out, which is known as destructive interference . These concepts are of

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(designated by the symbol

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