Optical Fibre - FACTCO · Fig. 1. Basic fibre optic communication system Since its invention in the...

Optical Fibre Fundamentals, Properties and, Advantages

By

Ahmed Alieldin

Electrical Engineering and Electronics Department,

University of Liverpool, UK

April 2019

Optical Fibre: Fundamentals, Properties and, Advantages 2

1. Introduction

There has always been a demand to increase the capacity of transmission of information, and

scientists and engineers continuously pursue technological routes for achieving this goal. The

technological advances ever since the invention of the laser in 1960 have been indeed

revolutionized the area of telecommunication and networking. The availability of laser presented

communication engineers with a suitable carrier wave capable of carrying enormously large

amount of information compared to radio waves and microwaves.

A typical lightwave communication system, shown in Fig. 1, consists of a lightwave

transmitter, a transmission channel (namely, the optical fibre to carry the modulated beam) and a

receiver. At the heart of a lightwave communication system is the optical fibre, which acts as the

transmission channel carrying the light beam loaded with information.

Fig. 1. Basic fibre optic communication system

Since its invention in the early 1970s, the use of and demand for optical fibre have grown

tremendously. The uses of optical fibre today are quite numerous. With the explosion of

information traffic due to the Internet, electronic commerce, computer networks, multimedia,

voice, data, and video, the need for a transmission medium with the bandwidth capabilities for

handling such vast amounts of information is paramount. Optical fibre, with its comparatively

infinite bandwidth, has proven to be the solution.

In 2019, Sir Charles K. Kao was awarded Nobel Prize for Physics for “groundbreaking

achievements concerning the transmission of light in fibre optics for optical communications ”.


2. Objectives

The objective of this report is to:

Identify the basic component of the fibre optic communication system.

Define optical fibre.

Illustrate the principles of operation of optical fibre, its types and modes.

Determine the main characteristics and features of optical fibre.

Discuss the advantages of optical fibre over metallic conductor transmission lines.

Demonstrate the standard optical fibre cables and connectors.

3. What is an OPTICAL FIBRE?

Fibre is a transparent cylinder made of a dielectric. The basic structure of the optical fibre is

made of four concentric layers as shown in Fig. 2. These four layers can be described as:

Core: this central section, made of silica, is the light transmitting region of the fibre. The

most common material used in fibre cables is fused silica (amorphous SiO2).

Cladding: The first layer around the core. It is also made of silica but not with the same

composition as the core as it should have a lower index of refraction. This creates an

optical waveguide which confines the light in the core by total reflection at the core-

cladding interface.

Coating: It is the first non-optical layer around the cladding. The coating typically

consists of one or more layers of a polymer that protect the silica structure against

physical or environmental damage.

Buffer (not pictured): The buffer is an important feature of the fibre. It is 900 microns

and helps protect the fibre from breaking during installation and termination and is

located outside of the coating

Fig. 2. The basic structure of the optical fibre

One of the key elements that make a dramatic improvement in the optical fibre revolution is

that silica is the primary constituent of sand, which is found in so much abundance on our earth.


4. Principles of Operation

Optical Fibre is a medium for carrying information from one point to another in the form of

light. Unlike the copper form of transmission, optical fibre is not electrical in nature. A basic fibre

optic system consists of a transmitting device that converts an electrical signal into a light signal,

an optical fibre cable that carries the light, and a receiver that accepts the light signal and converts

it back into an electrical signal.

The core of the optical fibre cable is a transparent cylinder of refractive index nf embedded in

a cladding material of refractive index nc as in Fig. 3. Refractive index of a medium is defined as

the ratio of the velocity of light in a vacuum to its velocity in this specified medium.

The light is guided down the core of the fibre by the optical cladding which has a lower

refractive index that traps light in the core through "total internal reflection."

If we consider a ray travelling in the plane containing the optical axis then it will remain

constrained as long as

cos(𝜃𝑝) ≥𝑛𝑐𝑛𝑓

(1)

Fig. 3. The principle of operation of optical fibre cables

The cladding provides medium with lower n and protects from frustrated total internal

reflection e.g. from fibre touching, dust or moisture on the surface.

The complexity of a fibre optic system can range from very simple (i.e., local area network)

to extremely sophisticated and expensive (i.e., long-distance telephone or cable television

trunking). For example, the system shown in Fig. 1 could be built very inexpensively using a

visible LEDs, plastic fibre, a silicon photodetector, and some simple electronic circuitry. The

overall cost could be less than £20. On the other hand, a typical system used for long-distance,

high-bandwidth telecommunication that employs wavelength-division multiplexing, erbium-

doped fibre amplifiers, external modulation using distributed feedback laser (DFB lasers) with

temperature compensation, fibre Bragg gratings, and high-speed infrared photodetectors could cost

tens or even hundreds of thousands of pounds. The basic question is “how much information is to


be sent and how far does it have to go?” With this in mind, we can examine the various components

that make up a fibre optic communication system and the considerations that must be taken into

account in the design of such systems.

5. Types of Optical Fibre

Three basic types of optical fibre cable are used in communication systems:

Step-index multimode.

Step-index single mode.

Graded-index.

This is illustrated in Fig. 4.

Fig. 4. Types of fibre

5.1. Step-index multimode fibre

It has an index of refraction profile that steps from low to high to low as measured from

cladding to core to cladding. Relatively large core diameter characterizes this fibre. The

core/cladding diameter of a typical multimode fibre used for telecommunication is 62.5/125 μm

(about the size of a human hair). The term “multimode” refers to the fact that multiple modes or

paths through the fibre are possible. Step-index multimode fibre is used in applications that require


high bandwidth (< 1 GHz) over relatively short distances (< 3 km) such as a local area network or

a campus network backbone. The major benefits of multimode fibre are: (1) it is relatively easy to

work with; (2) because of its larger core size, light is easily coupled to and from it; (3) it can be

used with both lasers and LEDs as sources; and (4) coupling losses are less than those of the single-

mode fibre. The drawback is that because many modes are allowed to propagate (a function of core

diameter and wavelength) it suffers from modal dispersion. The result of modal dispersion is

bandwidth limitation, which translates into lower data rates.

In a step-index multimode fibre, the number of modes Mn propagating can be approximated

by:

𝑀𝑛 =𝑉2

2 (2)

where V is known as the normalized frequency, or the V-number, which relates the fibre size, the

refractive index, and the wavelength. The V-number is given by

𝑉 =2𝜋𝑎

𝜆× √𝑛1

2 − 𝑛22 (3)

where a is the fibre core radius, λ is the operating wavelength, n1 is the core index, and n2 is the

cladding index.

The analysis of how the V-number is derived is beyond the scope of this report, but it can be

shown that by reducing the diameter of the fibre to a point at which the V-number is less than

2.405, higher-order modes are effectively extinguished and single-mode operation is possible.

5.2. Step-index single mode fibre

It allows for only one path, or mode, for light to travel within the fibre. The core diameter for

a typical single-mode fibre is between 5 μm and 10 μm with a 125-μm cladding. Single-mode

fibres are used in applications in which low signal loss and high data rates are required, such as in

long spans where repeater/amplifier spacing must be maximized. Because single-mode fibre

allows only one mode or ray to propagate (the lowest-order mode), it does not suffer from modal

dispersion like multimode fibre and therefore can be used for higher bandwidth applications.

However, even though single-mode fibre is not affected by modal dispersion, at higher data rates

chromatic dispersion can limit the performance. This problem can be overcome by several

methods. One can transmit at a wavelength in which glass has a fairly constant index of refraction

(~1300 nm), use an optical source such as a DFB laser that has a very narrow output spectrum, use

special dispersion compensating fibre, or use a combination of all these methods. In a nutshell,


single-mode fibre is used in high-bandwidth, long-distance applications such as long-distance

telephone trunk lines, cable TV head-ends, and high-speed local and wide area network (LAN and

WAN) backbones. The major drawback of single-mode fibre is that it is relatively difficult to work

with (i.e., splicing and termination) because of its small core size. Also, single-mode fibre is

typically used only with laser sources because of the high coupling losses associated with LEDs.

5.3. Graded-index fibre

It is a compromise between the large core diameter of multimode fibre and the higher

bandwidth of the single-mode fibre. With the creation of a core whose index of refraction decreases

parabolically from the core centre toward the cladding, light travelling through the centre of the

fibre experiences a higher index than light travelling in the higher modes. This means that the

higher-order modes travel faster than the lower-order modes, which allows them to “catch up” to

the lower-order modes, thus decreasing the amount of modal dispersion, which increases the

bandwidth of the fibre. Table I summarizes the three types of optical fibres.

Table I. Comparison of types of optical fibres

Type

Step-index multimode Step-index single mode Graded-index

Char

acte

rist

ics Easy coupling

Model dispersion

Lower data rate

Short distances

Coupling more difficult

No model dispersion

High data rates

Long distances

Easy coupling

Less model dispersion

Good compromise between

multimode and single-mode

6. Characteristics of Optical Fibre

6.1. Attenuation

The attenuation or transmission loss of optical fibres has proved to be one of the most

important factors in bringing about their wide acceptance in telecommunications. As channel

attenuation largely determined the maximum transmission distance prior to signal restoration,

optical fibre communications became especially attractive when the transmission losses of fibres

were reduced below those of the competing metallic conductors (less than 0.1 dB per km). Signal

attenuation within optical fibres, as with metallic conductors, is usually expressed in the

logarithmic unit of the decibel. In optical fibre communications, the attenuation, which is a


function of the wavelength, is usually expressed in decibels per unit length (i.e. dB/km) as

following:

𝛼𝑑𝐵𝐿 = 10 log10𝑃𝑖𝑃𝑜

(4)

where αdB is the signal attenuation per unit length in decibels which is also referred to as the fibre

loss parameter, L is the fibre length, Pi is the input (transmitted) optical power into the fibre and

P0 is the output (received) power from the fibre.

6.2. Material absorption losses

Material absorption is a loss mechanism related to the material composition and the fabrication

process for the fibre, which results in the dissipation of some of the transmitted optical power as

heat in the waveguide. The absorption of the light may be intrinsic (caused by the interaction with

one or more of the major components of the glass) or extrinsic (caused by impurities within the

glass).

6.3. Bend loss

Optical fibres suffer radiation losses at bends or curves on their paths. This is due to the energy

in the evanescent field at the bend exceeding the velocity of light in the cladding and hence the

guidance mechanism is inhibited, which causes light energy to be radiated from the fibre. An

illustration of this situation is shown in Fig. 5. The part of the mode which is on the outside of the

bend is required to travel faster than that on the inside so that a wavefront perpendicular to the

direction of propagation is maintained. Hence, part of the mode in the cladding needs to travel

faster than the velocity of light in that medium. As this is not possible, the energy associated with

this part of the mode is lost through radiation.

Fig. 5. An illustration of the radiation loss at a fibre bend


6.4. Dispersion

Dispersion, expressed in terms of the symbol Δt, is defined as pulse spreading in an optical

fibre. As a pulse of light propagates through a fibre, elements such as core diameter, refractive

index profile, wavelength, and laser linewidth cause the pulse to broaden. This poses a limitation

on the overall bandwidth of the fibre as demonstrated in Fig. 6. Dispersion Δt can be determined

by:

𝛥𝑡 = (𝛥𝑡𝑜𝑢𝑡 − 𝛥𝑡𝑖𝑛) (5)

Fig. 6. Pulse broadening caused by dispersion

and is measured in time, typically nanoseconds or picoseconds. Total dispersion is a function of

fibre length. The longer the fibre, the more the dispersion. total dispersion is given by

𝛥𝑡𝑡𝑜𝑡𝑎𝑙 = 𝐿 × (𝐷𝑖𝑠𝑝𝑒𝑟𝑠𝑖𝑜𝑛/𝑘𝑚) (6)

The overall effect of dispersion on the performance of a fibre optic system is known as

intersymbol interference which occurs when the pulse spreading caused by dispersion causes the

output pulses of a system to overlap, rendering them undetectable. If an input pulse is caused to

spread such that the rate of change of the input exceeds the dispersion limit of the fibre, the output

data will become indiscernible. This is the main reason for having a high model dispersion in step-

index multimode optical fibres and no model dispersion in step-index single-mode optical fibres

as illustrated in Fig. 7.

Fig. 7. Intersymbol interference

Multimode Single-mode


7. Advantages of Optical Fibres

Communication using an optical carrier wave guided along a glass fibre has a number of

extremely attractive features, several of which were apparent when the technique was originally

conceived. Furthermore, the advances in the technology to date have surpassed even the most

optimistic predictions, creating additional advantages. Hence it is useful to consider the merits and

special features offered by optical fibre communications over more conventional electrical

communications. In this section, we commence with the originally foreseen advantages and then

consider additional features which have become apparent as the technology has been developed.

7.1. Enormous potential bandwidth.

The optical carrier frequency in the range 1013 to 1016 Hz (generally in the near infrared around

1014 Hz or 105 GHz) yields a far greater potential transmission bandwidth than metallic cable

systems (i.e. coaxial cable bandwidth typically around 20 MHz over distances up to a maximum

of 10 km) or even mm-wave radio systems (i.e. systems currently operating with modulation

bandwidths of 700 MHz over a few hundreds of meters). Indeed, by the year 2000, the typical

bandwidth multiplied by length product for an optical fibre link incorporating fibre amplifiers was

5000 GHz km in comparison with the typical bandwidth–length product for the coaxial cable of

around 100 MHz km. Hence at that time, optical fibre was already demonstrating a factor of 50,000

bandwidth improvement over the coaxial cable while also providing this superior information-

carrying capacity over much longer transmission distances.

7.2. Small size and weight.

Optical fibres have very small diameters which are often no greater than the diameter of a

human hair. Hence, even when such fibres are covered with protective coatings they are far smaller

and much lighter than corresponding copper cables. This is a tremendous boon towards the

alleviation of duct congestion in cities, as well as allowing for an expansion of signal transmission

within mobiles such as aircraft, satellites and even ships.

7.3. Electrical isolation.

Optical fibres which are fabricated from glass, or sometimes 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 fibre transmission ideally suited for

communication in electrically hazardous environments as the fibres create no arcing or spark

hazard at abrasions or short circuits.

7.4. Immunity to interference and crosstalk.

Optical fibres form a dielectric waveguide and are therefore free from electromagnetic

interference (EMI), radio-frequency interference (RFI), or switching transients giving

electromagnetic pulses (EMPs). Hence the operation of an optical fibre communication system is

unaffected by transmission through an electrically noisy environment and the fibre cable requires

no shielding from EMI. The fibre 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 fibres and hence, unlike communication using electrical conductors, crosstalk is

negligible, even when many fibres are cabled together.

7.5. Signal security.

The light from optical fibres does not radiate significantly and therefore they provide a high

degree of signal security. Unlike the situation with copper cables, a transmitted optical signal

cannot be obtained from a fibre in a non-invasive manner (i.e. without drawing optical power from

the fibre). Therefore, in theory, any attempt to acquire a message signal transmitted optically may

be detected. This feature is obviously attractive for military, banking and general data transmission

(i.e. computer network) applications.

7.6. Low transmission loss.

The development of optical fibres over the last 20 years has resulted in the production of

optical fibre cables which exhibit very low attenuation or transmission loss in comparison with the

best copper conductors. Fibres have been fabricated with losses as low as 0.1 dB/ km and this

feature has become a major advantage of optical fibre communications (So, 100 km before re-

amplification for 10 times attenuation). It facilitates the implementation of communication links

with an extremely wide optical repeater or amplifier spacing, thus reducing system cost,

complexity and power consumption. Together with the already proven modulation bandwidth

capability of fibre cables, this property has provided a totally compelling case for the adoption of

optical fibre communications in the majority of long-haul telecommunication applications,


replacing not only copper cables, but also satellite communications, as a consequence of the very

noticeable delay incurred for voice transmission when using this latter approach. Fig. 8 represents

the number of repeaters (amplifiers) needed for optical fibre and copper transmission lines over a

long distance. It is clear that optical fibre needs much less number of repeaters to maintain the

same signal-to-noise ratio (SNR) over the same distance compared to copper transmission lines

(typically 10 times amplification every 100 km).

Fig. 8. Repeaters for optical fibre and copper transmission lines

7.7. Ruggedness and flexibility.

Although protective coatings are essential, optical fibres may be manufactured with very high

tensile strengths. Perhaps surprisingly for a glassy substance, the fibres may also be bent to quite

small radii or twisted without damage. Furthermore, cable structures have been developed which

have proved flexible, compact and extremely rugged. Taking the size and weight advantage into

account, these optical fibre cables are generally superior in terms of storage, transportation,

handling and installation to corresponding copper cables, while exhibiting at least comparable

strength and durability.

7.8. System reliability and ease of maintenance.

These features primarily stem from the low-loss property of optical fibre cables which reduces

the requirement for intermediate repeaters or line amplifiers to boost the transmitted signal

strength. Hence with fewer optical repeaters or amplifiers, 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 to 30 years being quite

common. Both of these factors also tend to reduce maintenance time and costs.


7.9. Potential low cost.

The glass which generally provides the optical fibre transmission medium is made from sand

– not a scarce resource. So, in comparison with copper conductors, optical fibres offer the potential

for low-cost line communication. Although over recent years this potential has largely been

realized in the costs of the optical fibre transmission medium which for bulk purchases has become

competitive with copper wires (i.e. twisted pairs), it has not yet been achieved in all the other

component areas associated with optical fibre communications. For example, the costs of high-

performance semiconductor lasers and photodiodes are still relatively high, as well as some of

those concerned with the connection technology (demountable connectors, couplers, etc.).

Overall system costs when utilizing optical fibre communication on long-haul links, however,

are substantially less than those for equivalent electrical line systems because of the low-loss and

wideband properties of the optical transmission medium. As indicated before, the requirement for

intermediate repeaters and the associated electronics is reduced, giving a substantial cost

advantage. Although this cost-benefit gives a net gain for long-haul links, it is not always the case

in short-haul applications where the additional cost incurred, due to the electrical-optical

conversion (and vice versa), may be a deciding factor. Nevertheless, there are other possible cost

advantages in relation to shipping, handling, installation and maintenance.

The reducing costs of optical fibre communications have provided strong competition not only

with electrical line transmission systems but also for microwave and mm-wave radio transmission

systems. Although these systems are reasonably wideband, the relatively short-span ‘line of sight’

transmission necessitates expensive aerial towers at intervals no greater than a few tens of

kilometres. Hence, with the exception of the telecommunication access network due primarily to

current first installed cost constraints, optical fibre has become the dominant transmission medium

within the major industrialized societies.

Table II summarizes the advantages of optical fibre over copper

Table II. Advantages of optical fibre over copper

Copper coaxial cable Optical fibre cable

Multimode Single mode

Bandwidth 100 MHz 500 MHz +100,000 MHz

Attenuation/km @ 1GHz > 45 dB 1 dB 0.2 dB

Cable cost High Low Low

Cable diameter (in.) 1 1/8 1/8

Data security Low Excellent Excellent

EMI immunity OK Excellent Excellent


8. Optical fibre cables and links.

Typical multimode fibres have a core diameter/cladding diameter ratio of 50 μm/125 μm and

62.5 μm/125 μm (although 100 μm/140 μm and other sizes are sometimes used depending on the

application). Single mode fibres have a core/cladding ratio of 9 μm /125 μm at wavelengths of

1300nm and 1550nm as shown in Fig. 9. Fig. 10 portrays a cable with multiple optical fibres.

Fig. 9. Popular optical fibre core/cladding diameter ratios

Fig. 10. Example of the construction of a multi-fibre cable

It is very important to learn how to link two optical fibre. There are two ways of linking two

optical fibre

Fusion Splice: This operation consists in directly linking two fibres by welding with an

electric arc, by aligning best possible both fibre cores. The specific device to make this fusion

is called a fusion splicer (shown in Fig. 11).

The advantages of this method are being fast and relatively simple to make. Also, the light

loss generated by the welding, due to the imperfect alignment of the cores, remains very


weak. The drawbacks are being relatively fragile (in spite of the protection of fusion by a

heat-shrinkable tube) and permanent. It is also necessary to invest in a fusion splicer.

Use of connector: In this case, it is necessary to terminate a connector at each end of the

fibres to be connected. The two fibres can then be connected by connecting the two

connectors together.

The advantage of this method is having a robust connection. Moreover, the type of connector

can be chosen according to the application field of the system. Also, Connection is

removable. It is possible to connect and disconnect two fibres hundreds to thousands of times

without damaging the connectors

Fig. 11. Fibre-optic fusion splicer

9. Summary

Optical Fibre is a medium for carrying information from one point to another in the form of

light. Optical fibre outperforms copper transmission line as it enjoys:

Larger bandwidth

Smaller size and weight

Better electrical isolation

Better immunity to interface

Higher signal security

Lower transmission loss, hence less power consumption

And, more flexibility and reliability

Which make optical fibre an excellent candidate for telecommunication systems.


References

[1] Ghatak, A., and Thyagarajan, K., “An Introduction to Fibre Optics”. Cambridge: Cambridge University

Press, UK, 1998.

[2] C. Breck Hitz; James J. Ewing; Jeff Hecht, "Fibre Lasers," Introduction to Laser Technology, IEEE, 2012.

[3] Senior, John M. “Optical Fibre Communications: Principles and Practice”, Prentice Hall, New York, USA,

1992.

[4] Michael Bass, and Eric W. Van Stryland, “FIBRE OPTICS HANDBOOK Fibre, Devices, and Systems for

Optical Communications”, McGraw-HILL, New York, USA, 2002.

[5] Govind P. Agrawal, ”Fibre-Optic Communication System”, A John Wiley & Sons, Inc., New York, USA,

2002.

Date post:	15-Jul-2020
Category:	Documents
Upload:	others
View:	1 times
Download:	0 times

Optical Fibre - FACTCO · Fig. 1. Basic fibre optic communication system Since its invention in the...

Documents