Optical Fibre Overview

8/14/2019 Optical Fibre Overview

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An Overview of

Optical Fibre

Technology

Zulfiqar Ali Mirani Senior Electronics Engineer

Civil Aviation Authority - Pakistan



Contents

1 Introduction

2 Fibre Theory

3 Fibre Manufacture

4 Splicing

5 System Design

6 Light Sources

7 Fibre Optic Sensors

http://floti.bell.ac.uk/MathsPhysics/introduction.htm


http://floti.bell.ac.uk/MathsPhysics/fibre.htm


http://floti.bell.ac.uk/MathsPhysics/fibreman.htm


http://floti.bell.ac.uk/MathsPhysics/splicing.htm


http://floti.bell.ac.uk/MathsPhysics/system.htm


http://floti.bell.ac.uk/MathsPhysics/light.htm


http://floti.bell.ac.uk/MathsPhysics/optsens.htm











1. Introduction

1.1 What are Optical Fibres ? Optical Fibres are fibres of glass, usually about 120micrometres in diameter, which are used to carrysignals in the form of pulses of light over distances up

to 50 km without the need for repeaters. These signalsmay be coded voice communications or computer data.

1.2 History

Interest in the use of light as a carrier for informationgrew in the 1960's with the advent of the laser as asource of coherent light. Initially the transmissiondistances were very short, but as manufacturing techniques for very pure glass arrived in1970, it became feasible to use optical fibres as a practical transmission medium. At thesame time developments in semi-conductor light sources and detectors meant that by

1980 world wide installation of fibre optic communication systems had been achieved.

1.3 Advantages

Capacity

Optical fibres carry signals with much less energy loss than copper cable and with a muchhigher bandwidth . This means that fibres can carry more channels of information overlonger distances and with fewer repeaters required.

Size and Weight

Optical fibre cables are much lighter and thinner than copper cables with the samebandwidth. This means that much less space is required in underground cabling ducts.Also they are easier for installation engineers to handle.

Security

Optical fibres are much more difficult to tap information from undetected; a greatadvantage for banks and security installations. They are immune to Electromagneticinterference from radio signals, car ignition systems, lightning etc. They can be routedsafely through explosive or flammable atmospheres, for example, in the petrochemical

industries or munitions sites, without any risk of ignition.

Running Costs

The main consideration in choosing fibre when installing domestic cable TV networks isthe electric bill. Although copper coaxial cable can handle the bandwidth requirement overthe short distances of a housing scheme, a copper system consumes far more electricalpower than fibre, simply to carry the signals.



1.4 Disadvantages Price

In spite of the fact that the raw material for making optical fibres, sand, is abundant andcheap, optical fibres are still more expensive per metre than copper. Having said this, onefibre can carry many more signals than a single copper cable and the large transmission

distances mean that fewer expensive repeaters are required.

Special Skills

Optical fibres cannot be joined (spliced) together as a easily as copper cable and requiresadditional training of personnel and expensive precision splicing and measurementequipment

1.4 Areas of Application Telecommunication's

Optical fibres are now the standard point to point cable link between telephonesubstations.

Local Area Networks (LAN's)

Multimode fibre is commonly used as the "backbone" to carry signals between the hubs ofLAN's from where copper coaxial cable takes the data to the desktop. Fibre links to thedesktop, however, are also common.

Cable TV

As mentioned above domestic cable TV networks use optical fibre because of its very lowpower consumption.

CCTV

Closed circuit television security systems use optical fibre because of its inherent security,as well as the other advantages mentioned above.

Optical Fibre Sensors

Many advances have been made in recent years in the use of Optical Fibres as sensors.Gas concentration, chemical concentration, pressure, temperature, and rate of rotationcan all be sensed using optical fibre. Much work in this field is being done all over theworld.

2. Optical Fibre- Theory







2.1 Total Internal Reflection

Reflection and Refraction of Light

When light travelling in a transparent material meets the surface of another transparentmaterial two things happen

1. some of the light is reflected2. some of the light is transmitted into the second transparent material

The light which is transmitted usually changes direction when it enters the secondmaterial. This bending of light is called refraction and it depends upon the fact that lighttravels at one speed in one material and at a different speed in a different material. As aresult each material has its own Refractive Index which we use to help us calculate theamount of bending which takes place. Refractive index is defined as:-

where

n is the refractive index

C is the speed of light in a vacuum

V is the speed of light in the material

Two possible cases exist.

These are:-

1. where light goes from a material with a low refractive index to one with a highrefractive index, OR

2. where light goes from a material with a high refractive index to one with a lowrefractive index.

These two cases are shown in the diagrams below.



Figure 1 n1 < n2 Figure 2 n1 > n2

Total Internal Reflection

In the second case above, q 2 is always greater than q 1 . So, as we increase q 1,eventually q 2 will reach 90o before q 1 does. At this point where q 1 has reached a valuecalled the critical angle (q c ).

The transmitted ray now tries to travel in both materials simultaneously For variousreasons this is physically impossible so there is no transmitted ray and all the light energyis reflected. This is true for any value of q 1, the angle of incidence, equal to or greaterthan q c This phenomenon is called Total Internal Reflection (TIR).

We can define the two conditions neccessary for TIR to occur

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

2. The angle of incidence, q1, is greater than or equal to the critical angle, qc

The phenomenon of TIR causes 100% reflection. In no other situation in nature, wherelight is reflected, does 100% reflection occur. So TIR is unique and very useful.

2.2 Optical Fibres

Structure of Fibre

Optical fibres are very fine fibres of glass. They consist of a glass core, roughly fiftymicrometres in diameter, surrounded by a glass "optical cladding" giving an outsidediameter of about 120 micrometres. They make use of TIR to confine light within the coreof the fibre.



Figure 3 Structure of an Optical Fibre

The Structure of a Fibre

The core has a higher refractive index than the cladding. Although the cladding does notcarry light, it is nevertheless an essential part of the fibre. The cladding is not just a merecovering. It keeps the value of the critical angle constant throughout the whole length ofthe fibre.

Figure 4 Structure of a Fibre

Optical Fibres are optical waveguides. This means that wherever the fibre goes the light,which is confined to the core of the fibre, also goes. So optical fibres can be used to make

light bend round corners

Propagation of light in the fibre

The angle in the Figure 5 is called the Acceptance Angle. Any light entering the fibre atless than this angle will meet the cladding at an angle greater than C . If light meets theinner 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 andnone escapes into the cladding. The ray then crosses to the other side of the core and,because the fibre 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 againand the same thing happens. In this way the light zig zags its way along the fibre. Thismeans that the light will be transmitted to the end of the fibre.

Figure 5 Propagation of Light in a Fibre

In reality the light which enters the fibre is a focused beam, consisting of many millions of"rays" behaving in a similar way. They all zig zag along the core of the fibre, crossing overeach other, and filling up the core with light. A pulse of light travelling along the core of thefibre is really a bundle of these rays.

2.3 Types of Fibres:

There are two main fibre types:

(1) Step index (multimode, single mode)

(2) Graded index (multimode)

Step Index Fibre:

Step index fibre is so calledbecause the refractive index of

the fibre 'steps" up as we movefrom the cladding to the core ofthe fibre. Within the claddingthe refractive index is constant,and within the core of therefractive index is constant.

Multimode

Although it may seem fromwhat we have said about total



internal reflection that any ray of light can travel down the fibre, in fact, because of thewave nature of light, only

certain ray directions can actually travel down the fibre. These are called the "FibreMode". In a multimode fibre many different modes are supported by the fibre. This isshown in the diagram below.

Figure 7 Multimode fibre

Single Mode

Because its core is so narrow Single Mode fibre can support only one mode. This iscalled the "Lowest Order Mode". Single mode fibre has some advantages over multimodefibre which we will deal with later

Figure 8 - Single Mode Fibre

Graded Index Fibre

Graded Index Fibre has a different core structure from single mode and multimode fibre.Whereas in a step-index fibre the refractive index of the core is constant throughout thecore, in a graded index fibre the value of the refractive index changes from the centre of

the core onwards. In fact it has what we call a Quadratic Profile. This means that therefractive index of the core is proportional to the square of the distance from the centre ofthe fibre.

Figure 9 - Graded Index Fibre

Graded index fibre is actually a multimode fibre because it can support more than one

fibre mode. But when we refer to "multimode" fibre we normally mean "step indexmultimode".

2.4 Pulse Spreading

The data which is carried in an optical fibre consists of pulses of light energy followingeach other rapidly. There is a limit to the highest frequency, i.e. how many pulses persecond which can be sent into a fibre and be expected to emerge intact at the other end.



This is because of a phenomenon known as pulse spreading which limits the "Bandwidth"of the fibre.

Figure 11 Pulse Spreading in an Optical Fibre

The pulse sets off down the fibre with an nice square wave shape. As it travels along thefibre it gradually gets wider and the peak intensity decreases.

Cause of Pulse Spreading

The cause of cause spreading is dispersion. This means that some components of thepulse of light travel at different rates along the fibre. there are two forms of dispersion.

1. Chromatic dispersion2. Modal dispersion

Chromatic Dispersion

Chromatic dispersion is the variation of refractive index with the wavelength (or thefrequency) of the light. Another way of saying this is that each wavelength of light travelsthrough the same material at its own particular speed which is different from that of otherwavelengths.

For example, when white light passes through a prism some wavelengths of light bendmore because their refractive index is higher, i.e. they travel slower This is what gives usthe "Spectrum" of white light. The "red' and "orange" light travel slowest and so are bent

most while the "violet" and "blue" travel fastest and so are bent less. All the other colourslie in between.

This means that different wavelengths travelling through an optical fibre also travel atdifferent speeds. This phenomenon is called "Chromatic Dispersion".

Figure 10 Dispersion of Light through a Prism

Modal Dispersion

In an optical fibre there is another type of dispersion called "Multimode Dispersion".



More oblique rays (lower order modes) travel a shorter distance. These correspond torays travelling almost parallel to the centre line of the fibre and reach the end of fibresooner. The more zig-zag rays (higher order modes) take a longer route as they passalong the fibre and so reach the end of the fibre later.

Now:-

Total dispersion = chromatic dispersion + multimode dispersion

Or put simply: for various reasons some components of a pulse of light travelling along anoptical fibre move faster and other components move slower. So, a pulse which starts offas a narrow burst of light gets wider because some components race ahead while othercomponents lag behind, rather like the runners in a marathon race.

Consequences of pulse spreading

Frequency Limit (Bandwidth)

The further the pulse travels in the fibre the worse the spreading gets

Figure 12 - Merging of Pulses in a Fibre.

Pulse spreading limits the maximum frequency of signal which can be sent along a fibre.If signal pulses follow each other too fast then by the time they reach the end fibre theywill have merged together and become indistinguishable. This is unaceptable for digitalsystems which depend on the precise sequence of pulses as a code for information. The

Bandwidth is the highest number of pulses per second, that can be carried by the fibrewithout loss of information due to pulse spreading.

Distance Limit

A given length of fibre, as explained above has a maximum frequency (bandwidth) whichcan be sent along it. If we want to increase the bandwidth for the same type of fibre wecan achieve this by decreasing the length of the fibre. Another way of saying this is thatfor a given data rate there is a maximum distance which the data can be sent.

Bandwidth Distance Product (BDP)

We can combine the two ideas above into a single term called the bandwidth distanceproduct (BDP). It is the bandwidth of a fibre multiplied by the length of the fibre. The BDPis the bandwidth of a kilometre of fibre and is a constant for any particular type of fibre.For example, suppose a particular type of multimode fibre has a BDP of 20 MHz.km,then:-

1 km of the fibre would have a bandwidth of 20 MHz








The typical B.D.P. of the three types of fibres are as follows:-

Multimode 6 - 25 MHz.km

Single Mode 500 - 1500 MHz.km

Graded Index 100 - 1000 MHZ.km

NB: The units of BDP are MHz.km (read as megahertz kilometres). They are not MHz/km(read as megahertz per kilometres). This is because the quantity is a product (of

bandwidth and distance) and not a ratio.

Choice of Fibre

Multimode Fibre

Muitimode fibre is suitable for local area networks (LAN's) because it can carry enoughenergy to support all the subscribers to the network. In a LAN the distances involved,however, are small. Little pulse spreading can take place and so the effects of dispersionare unimportant.

Single Mode Fibre.

Multimode Dispersion is eliminated by using Single Mode fibre. The core is so narrow thatonly one mode can travel. So the amount of pulse spreading in a single mode fibre isgreatly reduced from that of a multimode fibre. Chromatic dispersion however remainseven in a single mode fibre. Thus even in single mode fibre pulse spreading can occur.But chromatic dispersion can be reduced by careful design of the chemical composition ofthe glass.

The energy carried by a single mode fibre, however, is much less than that carried by amultimode fibre. For this reason single mode fibre is made from extremely low loss, very

pure, glass.

Single mode low absorption fibre is ideal for telecommunications because pulsespreading is small.

Graded Index Fibre

In graded index fibre rays of light follow sinusoidal paths. This means that low ordermodes, i.e. oblique rays, stay close to the centre of the fibre, high order modes spendmore time near the edge of core. Low order modes travel in the high index part of thecore and so travel slowly, whereas high order modes spend



predominantly more time in the low index part of the core and so travel faster. This way,although the paths are different lengths, all the modes travel the length of the fibre intandem, i.e., they all reach the end of the fibre at the same time. This eliminatesmultimode dispersion and reduces pulse spreading.

Graded Index fibre has the advantage that it can carry the same amount of energy asmultimode fibre. The disadvantage is that this effect takes place at only one wavelength,

so the light source has to be a laser diode which has a narrow linewidth.

Figure 13 - Ray Paths in Graded Index Fibre

2.5 Attenuation

Attenuation is specified in db.km-1

Where

I out = outgoing intensity (intensity is measured in W.m

-2

)

I in = ingoing intensity (W.m-2)

Attenuation in a fibre is measured using an OTDR (Optical Time-Domain Reflectometer)which looks at the light reflected back long the fibre when a pulse of light is sent down thefibre. Another method is to send light from a continuous source of light and measure thepower emerging at the other end of the fibre.

Optical Time Domain Reflectometer



Causes of Attenuation

The light travelling along a fibre is attenuated, i.e. its intensity decreases as it movesalong the fibre. This happens for 3 main reasons

Atomic absorption of light photons

Scattering of light by flaws and impurities

Reflection of light by splices and connectors

We will look at each of these factors in turn.

Atomic Absorption

The atoms of any material are capable of absorbing specific wavelengths of light becauseof their electron orbital structure. This absorption can be observed if you look into theedge of a pane of glass. The light which emerges has a green colour because so much

red and blue light have been absorbed by the atoms of the glass. In the same way, aslight passes along an optical fibre. more and more light is absorbed by the atoms as itcontinues on its path

Scattering by Flaws and Impurities

This type of scattering is called "Rayleigh Scattering". The amount of Rayleigh Scatteringwhich takes place depends on the relative size of the scattering particle and thewavelength of the light. If the wavelength of the light is large compared to the size of thescattering particle then little light is scattered. If the wavelength of the light is smallcompared to the scattering particles then a lot of light is scattered. So long wavelengths

are preferred in fibre optics because of the lower absorption. Thus 1500 nm is better than1300 nm which is better than 850 nm

Reflection by Splices and Connectors

In a long fibre cable there may be many splices which join the individual lengths of fibretogether. In a Local Area Network there will be many connectors because of the numberof subscribers to the system. At each connector and/or splice some light will be reflectedback along the fibre in the opposite direction. This will happen even for the most perfectsplice or connector. Light reflected backwards does not leave the fibre but is no longerusefully available for the rest of the fibre, i.e. it is no longer part of the ongoing light.

2.6 Numerical Aperture

One of the properties of a fibre which we need to know is called the Numerical Aperture.The numerical aperture is defined as



It is a measure of the light gathering power of the fibre. It lies between 0 and 1. Anumerical aperture of 0 means that the fibre gathers no light (corresponding to qA = 0o). A

numerical aperture of 1 means that the fibre gathers all the light that falls onto it(corresponding to qA = 90o)

3. Fibre Manufacturing

There are two main stages to the manufacture of optical fibres. These are:-

1) the making of the preform

2) the extrusion of the preform

Perform Manufacture

The most common method of making fibre preforms is known as Modified ChemicalVapour Dispersion (MCVD). An outer glass "bait tube" is heated by a traversing burner.Through this tube a mixture of gases is passed at a steady rate, which when heatedundergoes a chemical reaction. The gas mix contains compounds of silicon, metalhalides, oxygen and dopant materials which will determine the refractive index of theglass of the core. The solid end products of the reaction are deposited on the interior ofthe bait tube as "soot". This soot will eventually form the core of the fibre while the baittube will form the cladding. When enough soot has been deposited the gas flow isstopped and the heat is turned up so that the soot melts to form a sintered glass.

Finally the tube is heated up enough to soften the bait tube and the sintered glass so thatthe whole tube collapses to form a solid rod. This is illustrated in the diagram below.

Figure 15 Modified Chemical Vapour Deposition

Extrusion of the Preform

The preform now has the same internal structure as the fibre to be drawn. The preform isheld vertically and passed through an oven which softens its end. This end is nowstretched to form a glass fibre. The interior of the fibre retains the same refractive indexstructure as the preform with the same relative dimensions. The fibre passes through adevice for monitoring its diameter so that the size of the fibre stays within predefined limitsIt then passes through a coater which coats it with a plastic buffer. This part of theprocess is crucial since the strength of the fibre depends on freedom from any surfacecontamination. The fibre must therefore be coated before any contamination such asdust, etc, in the surrounding air can reach it. Finally the fibre is rolled on to a drum for



distribution or for further work on it such as incorporation into a fibre cable. This isillustrated in the diagram on the right.

Figure 16 Extrusion of the Preform

4. Splicing

Optical fibres have to be joined together to make longer lengths of fibre or existing fibrelengths which have been broken have to be repaired. Also the ends of the fibre have tobe fitted with convenient connectors (terminations) to allow them to be easily plugged intoequipment such as power meters, data transmitters, etc. Unlike electrical cables where allthat is needed is to solder lengths of cable together, the process of joining two fibres(splicing) or terminating the end of a fibre is more complex and requires specialequipment.

Splicing is the process of joining the two bare ends of two fibres together. The ends of thefibre must be precisely lined up with each other, otherwise the light will not be able to

pass from one fibre across the gap to the other fibre. There are four main alignmenterrors and any splicing technique is designed to deal with ends of these errors.

Possible alignment errors during splicing

There four alignment errors in splicing optical fibres. These are:-

Lateral,Axial,



Angular,Poor End Finish.

These are illustrated in the diagrams below.

Figure 20 Lateral Misalignment Figure 17 Angular Misalignment

Figure 18 Axial Misalignment Figure 19 Poor End Finnish

There are two main types of splicing:

Fusion Splicing and;Mechanical Splicing

Fusion Splicing

Figure 21 Fusion Splicing



Fusion Splicer

You can see this process by clicking on the icons below.

Alignment Pre-Fuse Fuse

In fusion splicing the ends of the fibres are aligned either manually using micro-manipulators and a microscope system for viewing the splice, or automatically eitherusing cameras or by measuring the light transmitted through the splice and adjusting thepositions of the fibres to optimise the transmission The ends of the fibres are then meltedtogether using a gas flame or more commonly an electric arc.

Near perfect splices can be obtained with losses as low as 0.02 dB (best mechanicalsplice 0.2 dB)

One of the systems in top of the range fusion splicers is called a Profile Alignment System(PAS). This system uses a TV camera to view the splice before it is fused. The image issent to a microcomputer inside the splicer which is programmed to recognise when thecores of the two fibres form a continuous straight line. An adjustment is made to bring thefibres form a continuous straight line. An adjustment is made to bring the fibres intoalignment in that plane. The camera then moves to a new position to view the splice in anorthogonal plane. The same process aligns the fibres in this plane too. The camera then

goes back to the original view and starts to make fine adjustments in that plane. It goes tothe second plane and makes fine adjustments in that plane too. This goes on until thealignment is as close as possible. At this point the arc is fired and the heat form the arcmelts the fibres together locally.

Mechanical Splicing

In mechanical splicing the two fibre ends are held together in a splice. This consists ofsome device usually made of glass which by its internal design automatically brings thetwo fibres into alignment. The openings at each end of the device are usually fluted to



allow the fibres to be guided into the capillary where the alignment takes place. The spliceis fist filled with an optical cement whose refractive index is the same as that of the coreof the fibre. After the fibres have been entered into the splice they are adjusted to give theoptimum transmission of light. At this point they are clamped in position and the wholeassembly is exposed to ultra-violet light which cures the cement.

Figure 22 Mechanical Splice

Mechanical splices are best used for multimode fibre. Some splices now exist which aresuitable SM fibre, but have a loss of 0.1dB. This is five times the loss of the best fusion

splice.

5. System Design

Fibre Optic Systems Design Considerations

In designing an fibre optic system there two main areas of crucial importance to consider.These are:-

Power budgetBandwidth Budget

We have to calculate both of these to see if our system will carry out the task required ofit. But often there are compromises which we must make on the basis of cost. Let's firstconsider each of the above in turn

Power budget

Losses occur at many points in a fibre optic system. We have to ensure that the light

source launches enough power into the fibre to provide enough power at the receiver.The receiver has limited sensitivity.

Transmitter output - Receiver input = Losses + Margin (All calculations are done in dB)

Types of Loss

Light source to fibre coupling lossConnector lossSplice loss



Coupler lossFibre lossFibre to receiver coupling lossMargin

Light source to fibre coupling loss

For LED coupling to 62.5/125 MM (62.5m m core, 125m m cladding, multimode) a typicalloss would be 1 mW to 50m W.

I.e. 13 dB loss

A laser diode can couple several milliwatts of power into 62.5/125 MM fibre typically witha dB loss so small that it's negligible (i.e. 0 dB loss).

These are however, more expensive, and shorter lived than LED's and require specialstabilised power supplies.

Couplers, connectors and splices

Simply multiply either the measured loss or the manufacturers specifications by thenumber of these devices in the system. For small numbers of devices use the maximumloss quoted per device. For large numbers of devices use the average loss quoted perdevice.

Fibre Loss

Multiply the dB.km~1 loss figure for the fibre by the length of the fibre.

So called "transient" losses occur in the first few 100 m of MM fibre coupled to an LED.So for short lengths of fibre the loss/km is greater than the manufacturer's figure.

Fibre / receiver coupling loss

This is not usually a problem since the area of the detector and its numerical aperture arelarger than those of fibre.

Margin

In addition to the above known losses it is usual to allow a margin, in case some of the

losses turn out to be higher than expected, but mainly to compensate for any futuredegradation of the system which may happen with time.

We usually allow between 3 dB and 10 dB margin.

Bandwidth Budget

The bandwidth budget is a series of calculations which allows us to work out whether thefibre system can support the data rate which we require. We do this by calculating theoverall Response Time of the system. This overall time response of a fibre system mustbe less than the bit time of the signal.



Calculation of Response Time of a System

There are a number of coding systems for digital information. The simplest to use, fromthe point of view of calculating response time is the Non Return to Zero coding (NRZ). Foran NRZ coding the bit time is given by

Where R is the rate at which information is being sent (i.e. the number of bits per secondor bit rate). For example, a bit rate of R = 1000 M bit/s the bit time is

Response time of a system is defined as longer of the rise time or the fall time of a bitleaving the system.

A system which can transmit 1 Mbit/s, for example, must have a response time less than1m s, then one bit will be trying to rise while the previous bit is still falling. As a result bitsof information will merge together.

Calculation of overall response time

The overall response time is affected by only 3 individual response times.

transmitterfibrereceiver

Components such as splices and connectors have a negligible effect on response time.The light can pass through them without any delaying effect.

The overall response time is given, in general, by the formula:

Where t is the response time of component So for a fibre system we would use theformula



where

ttx = response time of the transmitter

tf = response time of the fibre

trx = response time of the receiver

suppose

ttx = 2ns

tf = 1ns

trx = 0.5ns

then

t2

=(2x10-9

)2

+(1 x10-9

)2

+(0.5x10-9

)2

= 5.25 x 10-8

= t= 2.29 ns

Thus, if we know the individual response time, we can calculate the response time of thewhole system, and knowing that we can decide whether the system is fast enough for theinformation rate we want it to carry. The response times of the transmitter and thereceiver will be given by the manufacturer. The fibre response time, however, has to becalculated, because it depends on the length of the fibre and therefore is different for

different systems.

Calculation of Fibre Response Time

This is composed of two things

Modal Response TimeChromatic Response Time

Each of these contribute a component to the response time of the fibre in the followingway:

Where

t m = modal response time

t c = chromatic response time



Modal Response Time

The modal response time is given by

where

D m = Modal Dispersion L = Length of fibre and D m is given by:-

where

BDP = Bandwidth distance product

For example, a fibre of bandwidth of say 200 MHz.km has a model dispersion of 5ns.km -1.

Chromatic Dispersion

For chromatic dispersion where response time also depends on the range of wavelengthslaunched into the fibre we use the formula:

where

D c = Chromatic Dispersion

D l = Range of wavelengths of the light launched into the fibre (sometimes called thelinewidth)

L = Length of fibre

For example, 200 m length of 85/125 fibre and chromatic dispersion

100 ps.nm-1.km-1

For 850nm LED, D l = 50nm

then



Notice that D l is converted to nm and L is converted to km. Dc is expressed in the

number of seconds per nanometre per kilometre in this case 100 ps, i.e. 110 x 10

-12

pernm per km, [ps.nm-1.km-1]

Thus the overall response time of the above fibre is given by:-

6. Light Sources

There are two main light sources used in the field of fibre optics.-

Light Emitting diodes (LED's)

Laser Diodes (LD's)

LED's

An LED is a p-n junction diode in a transparent capsule usually with a lens to let the lightescape and to focus it. LED's can be manufactured to operate at 850 nm, 1300 nm, or1500 nm. These wavelengths are all in the infrared region. LED's have a typical responsetime of 8 ns, a linewidth of 40 nm, and an output power of tens of microwatts.

Figure 23 Planar LED



Laser Diodes

A laser diode Is an LED with two important differences

(1) The operating current is much higher in order to produce OPTICAL GAIN

(2) Two of the ends of the LD are cleaved parallel to each other. These ends act asperfectly aligned mirrors which reflect the light back and forth through the "gain medium"in order to get as much amplification as possible

The typical response time of a laser diode Is 0.5 ns. The linewidth is around 2 nm with atypical laser power of 10's of milliwatts. The wavelength of a laser diode can be 850 nm,1300 nm, or 1500 nm.

Figure 25 Laser Diode

7. Fibre Optic Sensors

Fibre Optic System

Telecommunications System

A Telecommunications Link is the simplest of fibre optic systems. It consists basically of atransmitter, a fibre link and a receiver. The transmitter will normally be equipped with alaser diode, usually with an output wavelength of 1300 nm or 1500 nm. The fibre link will



be made of single lengths of Single Mode optical fibre of length 2 km fusion splicedtogether. The link will be able to carry thousands of telephone conversations"simultaneously" by means of TIME DIVISION MULTIPLEXING.

Figure 26 Telecommunications Link

Fibre Optic Sensors

Microbending Sensor

A Microbending Sensor consists of two plates between which passes an optical fibre. Theplates have parallel grooves on their facing surfaces and the grooves from the two platesinterleave with each other. This means that the high point between two grooves on, say,the upper plate lies above a groove on the lower plate. The fibre passing between theplates is therefore bent alternatively up and down.

When a fibre is bent sufficiently the light in the core no longer meets the cladding at anangle equal to or greater than the critical angle. TIR, therefore, does not occur. Thismeans that light escapes into the cladding and doesn't reach the end of the fibre This iscalled "microbending loss". The more the plates press the more loss occurs. A detector atthe end of the fibre can thus measure how much pressure is on the pIates This has amilitary application in submarine detection.

Fibre Optic Gyroscope

The fibre optic gyroscope consists of a long length of fibre wound into a coil. Laser light issent into both ends of the fibre using a beam splitter which reflects 50% of the light andtransmits 50%. Light travelling round the coil clockwise emerges from the end of the fibrewith the same phase as the light travelling in the anticlockwise direction. This is becauseboth have travelled exactly the same distance.

Fibre Optic Gyroscope

If the gyroscope is now rotated, say, in the clockwise direction, then the light travellinground the fibre coil in the clockwise direction will take longer to reach the end of the fibrebecause the end is always moving away from the light. Likewise light travelling in the



anticlockwise direction will take less time to reach the other end because that end ismoving towards the light. This introduces a phase difference between the two emergingbeams of light which is proportional to the rate of rotation of the gyroscope.

Evanescent Wave Sensor

Theory

The model of light travelling through glass in the form of millions of infinitesimally thin raysworks very well as a way of explaining Total Internal Reflection (TIR). As you mightexpect, however, it is not the whole picture.

Light is, in fact, a wave motion. This means that it propagates through space, throughglass, through any transparent medium, in the form of electromagnetic waves which, likeall wave motions, tend to spread out as they travel.

Because of this characteristic of light waves we have to look at TIR at a boundary in alittle more detail. Some of the energy of the light waves in the core of the fibre does

actually penetrate into the cladding for a very short distance. We can think of it asescaping from the core then immediately coming back in again. This thin penetration oflight energy into the cladding is called the Evanescent Wave.

In a single mode fibre in particular there is always a layer" of light energy surrounding thecore whenever light is travelling along the fibre. The energy flow of this evanescent waveis parallel to the surface of the core and in the same direction as the main flow of energywithin the core.

Distribution of Energy for a Guided Wave

Single mode fibres, in particular, not only have to have a core made from low absoprtionglass, but also the cladding has to be made of low absorption glass because theevanescent wave carries a significant proportion of the guided energy. If a lot of thisevanescent energy is absorbed by the cladding then energy will be drawn out of the coreto replace it.

Application to Sensors

In a fibre what is a disadvantage can be used to our advantage in a sensor. We can

design a sensor where energy is absorbed from the evanescent wave in the presence ofcertain chemicals. When the chemicals are not present then the evanescent energy is notabsorbed.



Evanescent Wave Chemical Sensor

In the detector cell shown in the above diagram, a liquid can be poured in to surround thefibre. The fibre is not a standard communications fibre with a core and a cladding. It hasbeen manufactured without a cladding. Thus the sample poured in is in direct contact withthe evanescent wave. Any materials which absorb the particular wavelength of light beingcarried by the fibre will take energy out of the evanescent wave. This in turn drains energy

from the interior of the fibre and the output at the detector is reduced.

Blood Components Meter

By choosing the correct wavelength we can quickly measure the concentrations ofspecific components of blood such as total protein, cholesterol, urea, and uric acid. Whenthe concentration is high the output at the detector is less and vice versa

The concentrations of these chemicals are important to doctors in the diagnosis andmonitoring of certain disease conditions. Fibre optic sensors can give very results quicklywithout the need to send samples away to an analytical laboratory.

Other Materials

A sensor like this can be used to analyse many other liquids including gases.

Optical Fibre Overview

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