INTRODUCTION TO FIBER OPTIC SYSTEM

CONTENTS PAGE

1.0 Introduction to Fiber Optic……………………………………………. …..31.1 Light………………………………………………………………………..41.2 Applications of Fiber-Optic………………………………………………..51.3 Fiber-Optic System Block Diagram……………………………………......61.4 Fiber-Optic Cable Construction……………………………………………71.5 Basic Optical Laws…………………………………………………………131.6 Light Ray Propagation in a Fiber-Optic Cable……………………………..191.7 Classification of Fiber………………………………………………………20

1

INTRODUCTION TO FIBER-OPTIC SYSTEM

OBJECTIVES

General Objective :

To understand the concept of the Fiber-Optic System.

Specific Objectives

At the end of the unit you will be able to :

define the terms optical and light.

name six typical communications applications for fiber-optic

cable.

draw a basic block diagram of a fiber-optic communications

system and tell what each part of it does.

explain how light is propagated through the fiber-optic cable.

name the three basic types of fiber-optic cables, and state the

two materials from which they are made.

2

1.0 Introduction to Fiber-Optic

Light wave communication was first considered more than 100 years ago. The

implementation of optical communication using light waveguides was restricted to very

short distance prior to 1970. Corning Glass Company achieved a breakthrough in 1970 by

producing a fused silica (SiO2) fiber with a loss approximately 20 dB/km. The

development of semiconductor light source also started to mature at about that time,

allowing the feasibility of transmission over a few kilometers to be demonstrated. Since

1970, the rate of technological progress has been phenomenal, and optical fibers are now

used in transoceanic service. Besides the long-distance routes, fibers are used in inter

exchange routes, and the subscriber loop in the final link in what will eventually be the

global interconnection chain. Optical fibers are associated with high-capacity

communications. A lot of attention is presently being given to optical fibers to provide a

very extensive broadband ISDN.

Fiber optics is defined as that branch of optics that deals with the

transmission of light through ultrapure fibers of glass, plastic, or some

other form of transparent media. From a decorative standpoint, most of

us are familiar with the fiber optic lamp, which uses bundles of thin

optical fibers illuminated from the base end of the lamp by a light

source.

The light source is made to vary in color, which can be seen at the opposite ends

of the fiber as a tree of illuminating points radiating various colors of the

transmitted light. Although the lamp is used for decorative purposes only, it

serves as an excellent model of how light can be transmitted through the fiber.

3

1.1 Light

Light is a kind of electromagnetic radiation. The

basic characteristic of electromagnetic radiation

is its frequency or wavelength. Light frequencies fall between microwaves and x-

rays, as shown in Figure1.1.

Figure1.1 Electromagnetic Frequency Spectrum

Light frequency spectrum can be divided into three general bands:

1. Infrared : Band of light wavelengths that are too long to be seen by the

human eye.

2. Visible : Band of light wavelengths to which the human eye will respond.

3. Ultraviolet :Band of light wavelengths that are too short to be seen by the

human eye.

Light waves are commonly specified in terms of wavelength instead of frequency.

Units typically used are the nanometer or micrometer ( 1 micrometer = 1micron).

4

1.2 Applications of Fiber-Optic

Fiber-optic communications systems are being used more and more each day.

Their primary use is in long-distance telephone systems and cable TV system.

Fiber-optic cables are no more expensive or complex to install than standard

electrical cables, yet their information-carrying capacity is many times greater.

In all cases, the fiber-optic cables replace conventional coaxial or twisted-pair cables.

Below are some applications which use fiber-optic cables :-

1. TV studio to transmitter interconnection eliminating a microwave radio

link.

2. Closed-circuit TV systems used in buildings for security.

3. Secure communications systems at military bases.

4. Computer networks, wide area and local area.

5. Shipboard communications.

6. Aircraft communications.

7. Aircraft controls.

8. Interconnection of measuring and monitoring instruments in plants and

laboratories.

9. Data acquisition and control signal communications in industrial process

control systems.

10. Nuclear plant instrumentation.

11. College campus communication.

12. Utilities ( electrical , gas, and so on) station communications.

13. Cable TV systems replacing coaxial cable.

5

1.3 Fiber-Optic System Block Diagram

Figure1.2 shows a simplified block diagram of an optical fiber communications

link. The three primary building blocks of the link are the transmitter, the

receiver, and the fiber guide.

Figure1.2 Simplified fiber optic communications block diagram

The transmitter consists of an analog to digital converter (coder), and a light

source. The A/D converter is used to convert continuous analog signals such as

voice or video (TV) signals into a series of digital pulses.

Light pulses

Coder

Fiber

Repeater(long distance)

Light Detector

Decoder

LightSource

Fiber

6

The digital pulses are then used to flash a powerful light source off and on very

rapidly. The light source is either a light-emitting diode (LED) or an injection

laser diode (ILD).The light-beam pulses are then fed into a fiber-optic cable

where they are transmitted over long distances.

The optical fiber consists of a glass or plastic fiber core, a cladding, and a

protective jacket.

Repeaters are used to ensure the signals can be transmitted efficiently when the

two stations are separated far enough from each other.

The receiver includes a light detector or photocell and a decoder. The light

detector is very often either a PIN (p-type-intrinsic-n-type) diode or an APD

(avalanche photodiode). The light detector, acting as the receiving element,

converts the received light pulses back to pulses of electrical current. The

electrical pluses are amplified and reshaped back into digital form that is fed to a

decoder such as a D/A converter, where the original voice or video is recovered.

1.4 Fiber-optic cable construction

An optical fiber is a long, cylindrical, transparent material that confines and

propagates light waves shown in Figure 2.3 and Figure 2.4 . It is comprised of

three layers; the centre core that carries the light, the cladding layer covering the

core that confines the light to the core, and the coating that provides protection

for the cladding. The fiber itself is generally regarded as the core and its cladding.

The material composition of these two layers can be any of the following :

Glass cladding and glass core

Plastic cladding and glass core

Plastic cladding and plastic core

Coating materials include lacquer, silicone, and acrylates.

7

The index of refraction in the silica core is about 1.5 and the cladding is slightly

less, at about 1.48. The index of refraction of air is 1.003. The fiber coating is

normally colored using manufacturer’s standard color codes to facilitate the

identification of fiber. Optical fibers can also be made completely from plastic or

other materials. They are usually less expensive but have higher attenuation (loss)

and limited application.

8

Figure1.3 Typical Fiber Optic Cabling

Figure1.4 Basic construction of a fiber-optic cable

Example1.1

The optical spectrum is made up of three parts. Name them.

Solution to Example1.1

In the optical spectrum, the three parts are infrared, visible and ultraviolet.

9

QUIZ 1A

Answer the following questions.

1.1 The major use of fiber-optic cables is ______________________.

1.2 True or False. Fiber-optic cable has more loss than electric cable over long

distance.

1.3 True or False. Fiber-optic cable is smaller, lighter, and stronger than

electric cable.

1.4 Voice and video signals are converted into ____________ before being

transmitted by a light beam.

Choose the letter which best answers each question.

1.5 The core of a fiber-optic cable is made of ….

a. Air

b. Glass

c. Diamond

1.6 Which of the following is not a common application of fiber-optic

cable?

a. Computer networks

b. Telephone systems

c. Consumer TV

10

1.7 The core of a fiber-optic cable is surrounded by …

a. Cladding

b. Wire braid shield

c. Paper

1.8 Which of the following is not part of the optical spectrum?

a. Infrared

b. Gamma-rays

c. Ultraviolet

1.9 The speed of light in plastic compared to the speed of light in air

is….

a. Less

b. More

c. The same

11

Feedback To QUIZ 1A

1.1 telephone systems

1.2 false

1.3 true

1.4 binary or digital pulses

1.5 c

1.6 a

1.7 b

1.8 a

1.9 a

12

Well done! And now you can proceed to the next lesson

I have done all the questions in Quiz1A successfully.

1.5 Basic Optical Laws

When light traveling in a transparent material meets the surface of another

transparent material two things happen:-

a) some of the light is reflected

b) some of the light is transmitted into the second transparent material

The light which is transmitted usually changes direction when it enters the second

material. This bending of light is called refraction and it depends upon the fact

that light travels at one speed in one material and at a different speed in a different

material. As a result each material has its own Refractive Index which we use to

help us calculate the amount of bending which takes place. Refractive index is

defined as:

n = C Equation 1.1

where

n is the refractive index

C is the speed of light in a vacuum

is the speed of light in the material

13

The indexes of refraction of several common materials are given in Table1.1.

Medium Index of Refraction

Vacuum 1.0

Air 1.0003 ( 1.0 )

Water 1.33

Ethyl alcohol 1.36

Fused quartz 1.46

Glass fiber 1.5-1.9

Diamond 2.0-2.42

Silicon 3.4

Gallium-arsenide 3.6

Index of refraction is based on a wavelength of light emitted from a

sodium flame (5890 Å)

How a light ray reacts when it meets the interface of two transmissive materials

that have different indexes of refraction can be explained with Snell’s law.

Snell’s law simply states

14

Table 1.1 Typical Indexes of Refraction

n1 sin 1 = n2 sin 2 Equation 1.2

where n1 = refractive index of material 1 (unit less)

n2 = refractive index of material 2 (unit less)

1 = angle of incidence (degrees)

2 = angle of refraction (degrees)

A refractive index for Snell’s law is shown in figure 1.5. At the interface, the

incident ray may be refracted toward the normal or away from it, depending on

whether n1 is less than or greater than n2 .

1.5.1 Critical Angle

( n1 n2 ) Refracted Ray

1

(angle of incidence)

( n1 = n2 ) Unrefracted RayNormal

Medium n1

Medium n2

2

2

Incident ray

( n1 n2 )

Figure 1.5 Refractive model for Snell’s law

15

The critical angle is defined as the minimum angle of incidence at which a light

ray may strike the interface of two media and result in an angle of refraction of

90 or greater, as shown in Figure 1.6. This definition pertains only when the

light ray is traveling from a more dense medium into a less dense medium. The

critical angle can be derived from Snell’s law as follows:

n1 sin 1 = n2 sin 2

sin 1 = n2 sin 2

n1

When 2 = 90 which result to sin 2 = 1, then 1 = C , Therefore,

Critical Angle : C = Equation 1.3

Normal

Figure 1.6 Critical Angle Refraction

n2 less dense

1=C

(minimum)

2

n1 more dense

Incident ray

Refracted ray(more to less dense)

16

1.5.2 Total Internal Reflection

The transmitted ray now tries to travel in both materials simultaneously for

various reasons this is physically impossible so there is no transmitted ray and all

the light energy is reflected. This is true for any value of 1, the angle of

incidence is equal to or greater than c Figure1.7 shows the Total Internal

Reflection (TIR).

We can define the two conditions necessary for TIR to occur:

1. The refractive index of the first medium is greater than the refractive index of

the second one.

2. The angle of incidence, 1, is greater than or equal to the critical angle, c

The phenomenon of TIR causes 100% reflection. In no other situation in nature,

where light is reflected, does 100% reflection occur. So TIR is unique and very

useful.

17

n2 less dense

1C

n1 more dense

Incident ray

Normal

Figure 1.7 The Total Internal Reflection

1.5.3 Numerical Aperture

One of the properties of a fiber, which we need to know, is called the Numerical

Aperture. The numerical aperture is defined as:

and Equation 1.5

Also,

Where NA = numerical aperture (unitless)

n1 = refractive index of the glass core (unitless)

n2 = refractive index of the cladding (unitless)

A = acceptance angle (degrees)

It is a measure of the light gathering power of the fiber. It lies between 0 and 1. A

numerical aperture of 0 means that the fiber gathers no light (corresponding to A

= 0o). A numerical aperture of 1 means that the fiber gathers all the light that falls

onto it (corresponding to A = 90o).

18

Total Internal Reflection of Incident Ray

Equation 1.4

1.6 Light Ray Propagation in a Fiber-Optic Cable

The angle A in the Figure1.8 is called the Acceptance Angle. Any light entering

the fiber at less than this angle will meet the cladding at an angle greater than c.

If light meets the inner surface of the cladding (the core-cladding interface) at

greater than or equal to c then TIR occurs. So all the energy in the ray of light is

reflected back into the core and none escapes into the cladding. The ray then

crosses to the other side of the core and because the fiber is more or less straight,

the ray will meet the cladding on the other side at an angle which again causes

TIR. The ray is then reflected back across the core again and the same thing

happens. In this way the light zigzags its way along the fiber. This means that the

light will be transmitted to the end of the fiber.

1.6.1 Mode of Propagation and Index profile

19

Cladding n=1.47

TIR 1

Core n=1.5

Cladding

A

Figure1.8 Propagation of light in a fiber-optic

In fiber-optic terminology, the word mode can be defined as path. If there is only

one path for light to take down the cable, it is called single mode. If there are

more paths that can be used in a fiber-optic cable, it is called multimode.

Where by the index profile of an optical-fiber is a graphical representation of the

value of the refractive index across the fiber. There are two basic types of index

profile: Step and graded. A step-index fiber has a central core with a uniform

refractive index less than that of the central core. In a graded-index fiber there is

no cladding, and the refractive index of the core is non-uniform; it is highest at the

center and decreases gradually with distance toward the outer edge.

1.7 Classification of Fiber

Essentially, fiber-optic can be classified into three types of configurations: single-

mode step index, multi-mode step index and multimode graded index, as shown

in figure 1.9.

20

For short distanceEasy to work with.LANsFor very high pulse

rates

For short distanceEasy to work with.LANsProvides more

bandwidth than (c)Most common and

widely used type

For long distanceDifficult to work with.Phone companies and

CATV companies

Figure 1.9 Core index profiles: (a) single-mode step index; (b) multi-mode step index; (c) multi-

mode graded index

Example1.2

In figure1.5 , let medium 1 be glass and medium 2 be ethyl alcohol. For an angle

of incidence of 30 , determine the angle of refraction .

Solution to Example 1.2

From Table1.1,

n1 (glass) = 1.5

n2 (ethyl alcohol) = 1.36

Rearranging Equation 2.2 and substituting for n1, n2 and 1 gives us

21

The simplest way of manipulating light is to reflect it. The

direction of reflected light can be predicted by applying the law of reflection : the angle of incidence is equal to

HINTS& HELP

Example1.3

Determine the critical angle for the fiber describe in Example1.2.

Solution to Example1.3

Using the Equation 1.2,

C =

C =

Example1.4

Determine the numerical aperture for the fiber describe in Example1.2.

22

Solution to Example 1.4

Using the Equation 1.4,

QUIZ 1B

TEST OUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE

NEXT LESSON…!

Answer the following questions.

1.10 The device that converts the light pulses into an electrical signal is a

_________________.

1.11 Light is a type of _________________ radiation.

Choose the letter which best answers each question.

1.12 Total internal reflection takes place if the incident ray strikes the

interface at an angle with what relationship to the critical angle?

a. Less than

b. Greater than

c. Zero

1.13 The operation of a fiber-optic cable is based on the principle of …

23

a. Reflection

b. Dispersion

c. Absorption

1.14 Which of the following is not a common type of fiber-optic cable?

a. Single-mode step index

b. Multi-mode graded index

c. Single-mode graded index

1.15 The ratio of the speed of light in air to the speeds of light in another

substance is called the

a. Speed factor

b. Index of reflection

c. Index of refraction

1.16 Refraction is the

a. Reflection of light waves

b. Distortion of light waves

c. Bending of light waves

1.17 Which type of fiber-optic cable is the most widely used?

a. Multimode step-index

b. Single-mode step-index

c Multimode graded-index

1.18 Which type of fiber-optic cable is best for very high-speed data?

a. Multimode step-index

24

b. Single-mode step-index

c. Multimode graded-index

Feedback To Quiz1B

1.10 light detector or photocell

1.11 electromagnetic

1.12 c

1.13 b

1.14 a

1.15 b

1.16 c

1.17 c

1.18 a

1.19 c

25

CONGRATULATIONS !!!Now you can proceed to the next activity

KEY FACTS

1. Fiber-optics is a communications technology using transmission of light over

glass or plastic fibers.

2. Total Internal Reflection is a used in fiber-optics, governing how light rays

propagate through a transparent medium by reflecting off its boundaries.

3. The multimode step index fiber cable is widely used at short to medium

distances at relatively low pulse frequencies. This cable is also the easiest to make

and the least expensive of the fiber-optic cable.

26

SELF-ASSESSMENT

You are approaching success. Try all the questions in this self-assessment

section and check your answers with those given in the Feedback on Self-

Assessment given on the next page.

Question 1-1

a. List three general bands in the light frequency spectrum.

Question 1-2

a. Fiber-optic communications system is primary used in

_________________________.

Question 1-3

a. Outline the primary building blocks of a fiber-optic system.

b. Briefly describe the construction of a fiber-optic cable.

Question 1-4

a. State Snell’s law for refraction and outline its significant in fiber-optic

cables.

b. Define the following terms : refractive index, critical angle and

total internal reflection.

c. For a glass (n = 1.5) / quartz (n=1.41) interface and an angle incidence

of 38, determine the angle of refraction, the critical angle and the

numerical aperture for the cable.

d. A glass fiber has an index of refraction of 1.55 is surrounded by water

whose index of refraction is 1.33. Compute the critical angle,c ,

above which total internal reflection occurs in the glass slab.

27

e. A glass fiber has an index of refraction of 1.62. It surrounded by

cladding material having an index of refraction of 1.604. Compute

the critical angle, c.

Feedback To Self-Assessment

Have you tried the questions????? If “YES”, check our answers now.

Answer 1-1

a. Infrared, visible and ultraviolet.

Answer 1-2

a. Long-distance telephone systems and cable TV system.

Answer 1-3

a.

b. It is comprised of three layers ; the centre core that carries the light, the

cladding layer covering the core that confines the light to the core, and the

coating that provides protection for the cladding.

28

Coder Light source

Decoder Light Detector

Fiber Optic Cable

Answer 1-4

a. Snell’s law simply states

n1 sin 1 = n2 sin 2

where n1 = refractive index of material 1 (unit less)

n2 = refractive index of material 2 (unit less)

1 = angle of incidence (degrees)

2 = angle of refraction (degrees)

Snell’s law is applied in TIR concept when :

i. The refractive index of the first medium is greater than the refractive

index of the second one.

ii. The angle of incidence, 1, is greater than or equal to the critical angle, c

b. refractive index : Determines the amount of bending that light

undergoes

when entering a different medium.

critical angle : The minimum angle of incidence at which a

light ray may strike the interface of two media and result in an angle of

refraction of 90 or greater.

total internal reflaction. : The principle of how light rays

propagate through a transparent medium by reflecting off its boundaries.

c. 2 = 41 C = 70 NA = 0.512

d. C = 59

e. C = 82

29

30