What Are Optical Fibres

What are Optical Fibers ?

Optical Fibres are fibres of glass, usually about 120 micrometres in diameter, which are used to carry signals in the form of pulses of light over distances up to 50 km without the need for repeaters. These signals may be coded voice communications or computer data.

History Interest in the use of light as a carrier for information grew in the 1960's with the advent of the laser as a source of coherent light. Initially the transmission distances were very short, but as manufacturing techniques for very pure glass arrived in 1970, it became feasible to use optical fibres as a practical transmission medium. At the same time developments in semi-conductor light sources and detectors meant that by 1980 world wide installation of fibre optic communication systems had been achieved.

Advantages Capacity Optical fibers carry signals with much less energy loss than copper cable and with a much higher bandwidth. This means that fibers can carry more channels of information over longer distances and with fewer repeaters required.

Size and Weight Optical fibre cables are much lighter and thinner than copper cables with the same bandwidth. 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 great advantage for banks and security installations. They are immune to Electromagnetic interference from radio signals, car ignition systems, lightning etc. They can be routed safely 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 is the electric bill. Although copper coaxial cable can handle the bandwidth requirement over the short distances of a housing scheme, a copper system consumes far more electrical power than fibre, simply to carry the signals.

Disadvantages

Price In spite of the fact that the raw material for making optical fibres, sand, is abundant and cheap, optical fibres are still more expensive per metre than copper. Having said this, one fibre 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 fibers cannot be joined (spliced) together as a easily as copper cable and requires additional training of personnel and expensive precision splicing and measurement equipment

Areas of Application

Telecommunication's Optical fibres are now the standard point to point cable link between telephone substations. Local Area Networks (LAN's) Multimode fibre is commonly used as the "backbone" to carry signals between the hubs of LAN's from where copper coaxial cable takes the data to the desktop. Fibre links to the desktop, however, are also common. Cable TV As mentioned above domestic cable TV networks use optical fibre because of its very low power consumption. CCTV Closed circuit television security systems use optical fiber 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 rotation can all be sensed using optical fibre. Much work in this field is being done at the University of Strathclyde.

Reflection and Refraction of Light When light travelling in a transparent material meets the surface of another transparent material two things happen 1. some of the light is reflected 2. 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:-

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 high refractive index, OR 2. where light goes from a material with a high refractive index to one with a low refractive 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, θ2 is always greater than θ1 . So, as we increase θ1, eventually θ o 2 will reach 90 before θ1 does. At this point where θ1 has reached a value called the critical angle (θc ). 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 θ1, the angle of incidence, equal to or greater than θc. 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 the second one. 2. The angle of incidence, θ1, is greater than or equal to the critical angle, θc. 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.

There are two main fiber types: (1) Step index (multimode, single mode) (2) Graded index (multimode)

Step Index Fibre:

Figure 6 - Step Index Fibre

Step index fibre is so called because the refractive index of the fibre 'steps" up as we move from the cladding to the core of the fibre. Within the cladding the refractive index is constant, and within the core of the refractive index is constant.

Multimode Although it may seem from what we have said about total internal reflection that any ray of light can travel down the fibre, in fact, because of the wave nature of light, only certain ray directions can actually travel down the fibre. These are called the "Fibre Mode". In a multimode fibre many different modes are supported by the fibre. This is shown 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 is called the "Lowest Order Mode". Single mode fibre has some advantages over multimode fibre 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 the core, 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 the refractive index of the core is proportional to the square of the distance from the centre of the 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 index multimode".

Pulse Spreading The data which is carried in an optical fibre consists of pulses of light energy following each other rapidly. There is a limit to the highest frequency, i.e. how many pulses per second 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 the fibre 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 the pulse of light travel at different rates along the fibre. there are two forms of dispersion. 1. Chromatic dispersion 2. Modal dispersion Chromatic Dispersion

Chromatic dispersion is the variation of refractive index with the wavelength (or the frequency) of the light. Another way of saying this is that each wavelength of light travels through the same material at its own particular speed which is different from that of other wavelengths. For example, when white light passes through a prism some wavelengths of light bend more because their refractive index is higher, i.e. they travel slower This is what gives us the "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 colours lie in between. This means that different wavelengths travelling through an optical fibre also travel at different 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 to rays travelling almost parallel to the centre line of the fibre and reach the end of fibre sooner. The more zig-zag rays (higher order modes) take a longer route as they pass along the fibre and so reach the end of the fibre later.

Total dispersion = chromatic dispersion + multimode dispersion Or put simply: for various reasons some components of a pulse of light travelling along an optical fibre move faster and other components move slower. So, a pulse which starts off as a narrow burst of light gets wider because some components race ahead while other components 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 they will have merged together and become indistinguishable. This is unaceptable for digital systems 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 fibre without loss of information due to pulse spreading.

Distance Limit

A given length of fibre, as explained above has a maximum frequency (bandwidth) which can be sent along it. If we want to increase the bandwidth for the same type of fibre we can achieve this by decreasing the length of the fibre. Another way of saying this is that for 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 distance product (BDP). It is the bandwidth of a fibre multiplied by the length of the fibre. The BDP is 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

2 km of the fibre would have a bandwidth of 10 MHz 5 km of the fibre would have a bandwidth of 4 MHz 4 km of the fibre would have a bandwidth of 5 MHz 10 km of the fibre would have a bandwidth of 2 MHz 20 km of the fibre would have a bandwidth of 1 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 Multimode fibre is suitable for local area networks (LAN's) because it can carry enough energy 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 dispersion are unimportant. Single Mode Fibre. Multimode Dispersion is eliminated by using Single Mode fibre. The core is so narrow that only one mode can travel. So the amount of pulse spreading in a single mode fibre is greatly reduced from that of a multimode fibre. Chromatic dispersion however remains even 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 of the glass.

The energy carried by a single mode fibre, however, is much less than that carried by a multimode 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 pulse spreading is small. Graded Index Fibre In graded index fibre rays of light follow sinusoidal paths. This means that low order modes, i.e. oblique rays, stay close to the centre of the fibre, high order modes spend more time near the edge of core. Low order modes travel in the high index part of the core 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 in tandem, i.e., they all reach the end of the fibre at the same time. This eliminates multimode dispersion and reduces pulse spreading. Graded Index fibre has the advantage that it can carry the same amount of energy as multimode 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

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 the fibre. Another method is to send light from a continuous source of light and measure the power 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 moves along 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 because of their electron orbital structure. This absorption can be observed if you look into the edge 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, as light passes along an optical fibre. more and more light is absorbed by the atoms as it continues on its path Scattering by Flaws and Impurities

This type of scattering is called "Rayleigh Scattering". The amount of Rayleigh Scattering which takes place depends on the relative size of the scattering particle and the wavelength of the light. If the wavelength of the light is large compared to the size of the scattering particle then little light is scattered. If the wavelength of the light is small compared 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 than 1300 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 fibre together. In a Local Area Network there will be many connectors because of the number of subscribers to the system. At each connector and/or splice some light will be reflected back along the fibre in the opposite direction. This will happen even for the most perfect splice or connector. Light reflected backwards does not leave the fibre but is no longer usefully available for the rest of the fibre, i.e. it is no longer part of the ongoing light.

Fibre Manufacture

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 Chemical Vapour 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 heated undergoes a chemical reaction. The gas mix contains compounds of silicon, metal halides, oxygen and dopant materials which will determine the refractive index of the glass of the core. The solid end products of the reaction are deposited on the interior of the bait tube as "soot". This soot will eventually form the core of the fibre while the bait tube will form the cladding. When enough soot has been deposited the gas flow is stopped 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 that the 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 is held vertically and passed through an oven which softens its end. This end is now stretched to form a glass fibre. The interior of the fibre retains the same refractive index structure as the preform with the same relative dimensions. The fibre passes through a device for monitoring its diameter so that the size of the fibre stays within predefined limits It then passes through a coater which coats it with a plastic buffer. This part of the process is crucial since the strength of the fibre depends on freedom from any surface contamination. The fibre must therefore be coated before any contamination such as dust, 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 is illustrated in the diagram on the right.

Figure 16 Extrusion of the Preform

Splicing Optical fibres have to be joined together to make longer lengths of fibre or existing fibre lengths which have been broken have to be repaired. Also the ends of the fibre have to be

fitted with convenient connectors (terminations) to allow them to be easily plugged into equipment such as power meters, data transmitters, etc. Unlike electrical cables where all that 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 special equipment. Splicing is the process of joining the two bare ends of two fibres together. The ends of the fibre 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 alignment errors 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.

Lateral Misalignment

Axial Misalignment

There are two main types of splicing: Fusion Splicing and; Mechanical Splicing

Angular Misalignment

Poor End Finish

Fusion Splicing

In fusion splicing the ends of the fibres are aligned either manually using micromanipulators and a microscope system for viewing the splice, or automatically either using cameras or by measuring the light transmitted through the splice and adjusting the positions of the fibres to optimise the transmission The ends of the fibres are then melted together 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 mechanical splice 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 is sent to a microcomputer inside the splicer which is programmed to recognise when the cores of the two fibres form a continuous straight line. An adjustment is made to bring the fibres form a continuous straight line. An adjustment is made to bring the fibres into alignment in that plane. The camera then moves to a new position to view the splice in an orthogonal 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 to the second plane and makes fine adjustments in that plane too. This goes on until the alignment is as close as possible. At this point the arc is fired and the heat form the arc melts the fibres together locally.

Mechanical Splicing In mechanical splicing the two fibre ends are held together in a splice. This consists of some device usually made of glass which by its internal design automatically brings the two 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 splice is fist filled with an optical cement whose refractive index is the same as that of the core of the fibre. After the fibres have been entered into the splice they are adjusted to give the optimum transmission of light. At this point they are clamped in position and the whole assembly is exposed to ultra-violet light which cures the cement.

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

Fiber Optic Systems Design Considerations In designing a fiber optic system, there two main areas of crucial importance to consider. These are:Power budget Bandwidth Budget We have to calculate both of these to see if our system will carry out the task required of it. But often there are compromises that we must make on the basis of cost. Let's first consider each of the above in turn

Power budget

Losses occur at many points in a fiber optic system. We have to ensure that the light source launches enough power into the fiber 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 loss Connector loss Splice loss Coupler loss Fibre loss Fibre to receiver coupling loss Margin Light source to fibre coupling loss For LED coupling to 62.5/125 MM (62.5m m core, 125m m cladding, multimode) a typical loss 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 with a 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 special stabilised power supplies. Couplers, connectors and splices Simply multiply either the measured loss or the manufacturers specifications by the number of these devices in the system. For small numbers of devices use the maximum loss quoted per device. For large numbers of devices use the average loss quoted per device. 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 are larger 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 future degradation 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 the fibre system can support the data rate which we require. We do this by calculating the overall Response Time of the system. This overall time response of a fibre system must be 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, from the point of view of calculating response time is the Non Return to Zero coding (NRZ). For an 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 second or 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 bit leaving the system.

A system which can transmit 1 Mbit/s, for example, must have a response time less than 1m s, then one bit will be trying to rise while the previous bit is still falling. As a result bits of information will merge together. Calculation of overall response time The overall response time is affected by only 3 individual response times. transmitter fibre receiver 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 the formula

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 the whole system, and knowing that we can decide whether the system is fast enough for the information rate we want it to carry. The response times of the transmitter and the receiver will be given by the manufacturer. The fibre response time, however, has to be calculated, 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 Time Chromatic Response Time Each of these contribute a component to the response time of the fibre in the following way:

Where tm = modal response time tc = chromatic response time Modal Response Time The modal response time is given by

where Dm = Modal Dispersion L = Length of fibre and Dm 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 wavelengths launched into the fibre we use the formula:

where Dc = Chromatic Dispersion  = Range of wavelengths of the light launched into the fibre (sometimes called the linewidth) 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 per nm per km, [ps.nm-1.km-1] Thus the overall response time of the above fibre is given by:-

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 light escape and to focus it. LED's can be manufactured to operate at 850 nm, 1300 nm, or 1500 nm. These wavelengths are all in the infrared region. LED's have a typical response time 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 as perfectly 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 a typical 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

Fibre Optic System Telecommunications System

A Telecommunications Link is the simplest of fibre optic systems. It consists basically of a transmitter, a fibre link and a receiver. The transmitter will normally be equipped with a laser 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 spliced together. 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. The plates have parallel grooves on their facing surfaces and the grooves from the two plates interleave 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 the plates is therefore bent alternatively up and down. When a fibre is bent sufficiently the light in the core no longer meets the cladding at an angle equal to or greater than the critical angle. TIR, therefore, does not occur. This means that light escapes into the cladding and doesn't reach the end of the fibre This is called "microbending loss". The more the plates press the more loss occurs. A detector at the end of the fibre can thus measure how much pressure is on the pIates This has a military application in submarine detection.

Fibre Optic Gyroscope

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

Fibre Optic Gyroscope If the gyroscope is now rotated, say, in the clockwise direction, then the light travelling round the fibre coil in the clockwise direction will take longer to reach the end of the fibre because 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 is moving towards the light. This introduces a phase difference between the two emerging beams 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 rays works very well as a way of explaining Total Internal Reflection (TIR). As you might expect, however, it is not the whole picture. Light is, in fact, a wave motion. This means that it propagates through space, through glass, through any transparent medium, in the form of electromagnetic waves which, like all 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 a little 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 as escaping from the core then immediately coming back in again. This thin

penetration of light 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 the core whenever light is travelling along the fibre. The energy flow of this evanescent wave is parallel to the surface of the core and in the same direction as the main flow of energy within the core.

Distribution of Energy for a Guided Wave Single mode fibres, in particular, not only have to have a core made from low absoprtion glass, but also the cladding has to be made of low absorption glass because the evanescent wave carries a significant proportion of the guided energy. If a lot of this evanescent energy is absorbed by the cladding then energy will be drawn out of the core to 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 of certain chemicals. When the chemicals are not present then the evanescent energy is not absorbed.

Evanescent Wave Chemical Sensor In the detector cell shown in the above diagram, a liquid can be poured in to surround the fibre. The fibre is not a standard communications fibre with a core and a cladding. It has been manufactured without a cladding. Thus the sample poured in is in direct contact with the evanescent wave. Any materials which absorb the particular wavelength of light being carried 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 of specific components of blood such as total protein, cholesterol, urea, and uric acid. When the 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 and monitoring of certain disease conditions. Fibre optic sensors can give very results quickly without 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.