INTRODUCTION:
Fiber optics (FO) is a science, by which electrical energy is converted into light (or optical energy); that light is transported through optical fibers to some other place, and finally is converted again in electrical energy.
Optical fiber is a solid strand of glass (or in some cases, plastic) that conducts light in much the same way that copper conducts. Light travels through the fiber by reflecting from the fiber’s inner surfaces. Because of the construction of the fiber, light that passes into the glass does not pass back out, but reflects and stays in the fiber. The fiber is very thin and flexible, and therefore, it can be routed around corners and through small openings. Light passing through the fiber can be used for illumination, for sensing changes in temperature or other information, or can be used for sensing, welding, or cutting (for example: the high intensity Light in Laser beams.)
Many polymers can form neat fibers, as long as certain intermolecular forces can occur between chains, holding them together in a crystal-like fashion. Thus, polymer fibers are strong materials with excellent tensile strength, which makes them very useful as textiles.
In fact, optical fibers have polymers in their composition. However, the term optical fiber doesn't tell us anything about polymers, but about two transparent, dielectric tubes or cylinders, one surrounding the
The First Step, which Lead To Its Discovery:
Back in the 1870s, British scientist John Tyndall performed a demonstration in which he showed
light guiding by means of a curved stream of water flowing from an illuminated tank.
Between 1900 and 1930 numerous experiments followed Tyndall's demonstration. It was discovered that bent thin glass rods not only could transmit light, but using a bundle of glass rods (or optical fibers, as they were called later) complete images could be carried as well.
What type of phenomenon was involved in these sensational findings? Well, it was just reflection, total internal reflection, as the walls of the thin fiber acted like mirrors in which the incident light bounced back and forth.
Total Internal Reflection:
You have many examples of total internal reflection in your everyday life. Just look for down asphalt road on a hot, sunny day, and you should see reflection from the road surface, as if the road had become a mirror. Well, it has due to the same property that causes internal reflection.The other example is of diamond. You know why it glitters.It is due to this same property. When light is incident at an angle greater then its critical angle (usually it is 52 degree) light get totally internally reflected and sparkles according to number of reflections it suffers. At some instant when its incident angle decreases its critical angle it comes out of it and stops glittering.
If light hits at a low enough angles, it can't penetrate the surface, and just bounces off. This reflection, governed by Snell's law, is made possible due to the differences between the refractive index of the glass and the air, the latter having the lowest value.
However, the usefulness of light guidance would not be completely appreciated until 1950, when many scientists began to think in its potential applications. They were smart enough to understand that such applications ranged from medicine, enabling the visualization of inaccessible regions of the human body, to communication networks, in replace of metal wires.
But there were a number of drawbacks that arose in those first applications: light couldn't be carried too far, and the transmitted images came pretty poor and distorted.
That was because the fibers had no coating at all surrounding them to increase the difference in refractive index. For that reason, if such fibers got wet, for example, the glass-air interface didn't remain fixed and the way the light reflected inside was changed.
Only in the mid 50s, Dutch Abraham van Heel designed a covering layer surrounding the fiber with another cylinder of glass, of lower refractive index than that in the middle. This way, the total reflection was not affected by water, dirt or other contamination. Finally, a third layer was added, in order to protect the glass fiber from damage, and to make it easier to handle.
BASIC PHENOMENON BEHIND OPTICAL FIBRE:
Consider an ordinary glass of water. We know that if we look through the water at an angle, images will appear distorted. This happens because light actually slows down a little bit when it enters the water, and speeds up again when it moves back into the air again.
Since the light has a slight but measurable width, if it hits the water at an angle, the part of the light that hits the water first will slow down first. The result is that the direction the light is traveling changes, and the path of the light actually bends at the surface of the water.
No matter what angle the light is traveling as it approaches the water, it will take a steeper angle once it actually enters the water. You can see this at any time by looking at a picture or newspaper through a glass of water, and by looking at different angles. Even a straw in a glass of water looks bent, although it really isn't. This phenomenon is called refraction.
The same phenomenon happens with glass, although we don't usually notice it when looking through a window. Nevertheless, light striking the glass at an angle bends as it slows down within the glass, and then bends again as it speeds up when leaving the glass. You can see this phenomenon clearly if you slide a piece of flat glass over the print in a book or newspaper.
Any substance that light can travel through will exhibit this phenomenon to some extent. Glass happens to be a very practical choice for optical fiber because it is reasonably strong, flexible, and has good light transmission characteristics.
Now Question arises,
"How can we use this phenomenon to keep the light inside the glass, especially if we want to bend the glass (with the light still inside) around corners?"
Now, consider looking into a glass of water from below the surface of the water. If you look up through the bottom of the glass, you will see a somewhat distorted view of the ceiling or whatever is above the glass. However, if you look in from the side of the glass and observe the underside of the top surface, you will begin to note an interesting and useful effect.
If you are looking up from a steep angle, the light you see entered the top surface of the water at a shallower angle, as shown on the left. However. As you look at the underside of the top surface from a shallower angle, as shown on the right, you will find a point at which light can't enter the top surface at a yet shallower angle. At this point, the top surface of the water looks like a perfect mirror, even though you know it isn't.
Now, the light you see is reflected from the surface, rather than being refracted through it. This effect persists for all angles shallower than the critical angle at which the phenomenon first appears. As you might expect, the same phenomenon is exhibited by glass or any other material through which light might pass.
Consider a single glass fiber, such as the one shown in an enlarged view The actual fiber is so thin that light entering one end will experience the "mirror effect" of this discussion every time it touches the wall of the fiber. As a result, the light will travel from one end of the fiber to the other, bouncing back and forth between the walls of the fiber.
This is the basic concept of optical fibers, and it correctly describes the fundamental operation of all such fibers.
INDEX OF REFRACTION
It is the ratio of the speed of light in a vacuum to the speed of light in the specified medium. This is expressed as a positive real number greater than 1.
The speed of light in a vacuum such as open space is just slightly less than 300,000,000 meters per second (current measurements place it at very nearly 299,792,500 meters per second, give or take up to 200 m/s). For practical calculations, we'll just use the approximation.
The speed of light in water is 225,000,000 meters per second. Therefore, the index of refraction of water is:
300,000,000 = 1.33
225,000,000
In typical glass such as is used for optical fibers, the speed of light might well be about 200,000,000 m/s. The index of refraction for such glass would be:
300,000,000 = 1.50
200,000,000
In a real-world, practical fiber, this might describe the cladding, while the core would have a refractive index of perhaps 1.55 to 1.60. Such a combination is quite effective in keeping the bulk of the light energy inside the core, so that it can accomplish something when it reaches its destination.
CONSTRUCTION
An optical fiber contains three layers:
1. A core made of highly pure glass with a high refractive index for the light to travel,
2. A middle layer of glass with a lower refractive index known as the cladding which protects the core glass from scratches and other surface imperfections, and
3. Outer polymer jacket to protect the fiber from damage.
For glass optical fibers, the diameter of the core ranges between 10-600 microns, the cladding thickness is between 125-630 microns, and that of the jacket varies between 250-1040 microns. For POF all diameters range between 750-2000 microns. As can be seen, one of the main differences between glass and plastic optical fibers is their diameter. This makes POF easier to handle.
The material used for currently commercialized fibers (core and cladding) include pure glass (SiO2), plastic, or a combination of both. The use of one or the other material will be determined by such factors as quality and economics.
Plastic optical fibers (POF) have the advantage of being made of cheaper materials than glass and to operate in the visible range of the spectrum. However, they show a high loss, and for that reason their applications are confined to short distance transmission. In spite of this, POF is widely used for medical and industrial instruments, and currently research is carried out about using POF as a replacement of copper wiring for data transmission in automobiles.
If you use silica glass for the core, it must be high purity in order to allow the light to be transmitted along the core with minimal loss
.
Q1 How can silica glass be obtained since most glass is made from sand?
For the first question, there is a chemical reaction that can be used to make glass instead of melting sand. You start from SiCl4 and O2 in their gas state, and use heat or a catalyst to make the reaction go:
SiCl4 + O2 ----------> SiO2 + 2 Cl2
Q2 How can the refractive index of the core and cladding be varied to give the best performance?
For the second, knowing that the refractive index of the glass core must be higher than the cladding; the procedure is quite simple as well. Adding small amounts of something to a given substance often results in a change or improvement of its properties. For instance the sulphur added to plain rubber in the vulcanization process changes its properties.
In this case we add a bit of germanium (as a germanium tetrachloride gas) to pure silica glass. The germanium, which has 18 more electrons than silicon, acts as a dopant. Consequently, the refractive index of the core glass is increased, although the attenuation is not affected. Likewise, you can add a bit of boron or fluorine to reduce the refractive index of the cladding glass. Both increase the difference in refractive index, which is the key requirement for good light transmission.
Because the refractive index of the core glass is greater than that of the cladding, light traveling in the core glass will remain in the core class due to total internal reflection as long as the light strikes the core/cladding interface at an angle greater than the critical angle.
The total internal reflection phenomenon, as well as the high purity of the core glass, enables light to travel long distances with little loss of intensity.
MODIFICATON IN THE FIELD OF OPTICAL FIBER
Unfortunately, it is not possible to use fibers of the basic construction for any practical application. The reason for this has to do with the physical realities of the phenomenon of reflection within the fiber, and how the parameters involved will change under different conditions.
The basic fact governing the reflection of light within the fiber has to do with the speed of light inside the fiber, and the speed of light in the medium just outside the fiber. Every possible material through which light can pass has a characteristic called the refractive index, which is a measure of the speed of light through that material as compared to the speed of light in open space. .
One of the requirements of an optical fiber is that its diameter remains constant throughout its length. Any change in the thickness of the fiber will affect the way light reflects from the inner walls of the fiber. In some cases, this could even mean that the reflected light could exceed the critical angle required for total reflection, and so be lost through the walls of the fiber.
Unfortunately, the same effect will be noticed if the characteristics of the medium outside the fiber should change. For example, if the fiber gets wet (as it would in rain, fog, or some underground situations), the characteristics of the boundary between the inside and the outside of the fiber will change, and hence the effective shape of the fiber will change, and will keep changing as drops of water move along the surface of the fiber.
The question now is,
"How can we make the fiber so the boundary layer is permanently fixed and precisely predictable?"
The easiest way to ensure that the boundary between the inside of the fiber and the outside of the fiber remains constant and unchanging no matter what is to create a permanent boundary of known characteristics. The practical approach is to surround the glass fiber with another layer of glass, while making sure that the speed of light in the outer layer remains faster than the speed of light in the inner fiber. The result is shown here.
In this figure, the original fiber is now the core of a two-layer construct. The diameter of the core is kept constant, at approximately 50 to 60 µm (micrometers, at one time designated "microns") and its surface is kept as perfectly smooth as possible. The outer layer, known as cladding, is bonded at all points to the surface of the core.
To the outside world, this construction is effectively one solid piece of glass, even though it is constructed of two different types of glass. Thus, it is impervious to water, dirt, and other materials. If the outer surface gets wet, that makes no difference because it still doesn't affect the boundary between the core and the cladding. The whole composite fiber may be covered with rubber or plastic for easier handling and visibility.
This type of optical fiber is known as a multi-mode step-index fiber, because of the fixed and definite boundary, or step, between the core and the cladding, as well as the fact that light traveling through the fiber may assume any of several possible electromagnetic "modes." This is the first successful type of optical fiber that was developed. Since then, more advanced types of optical fibers, such as graded-index and single-mode fibers have been.
Earlier light get trapped inside the core of the optical fiber because the speed of light in the cladding is higher than the speed of light in the core.
The obvious next questions are,
"How much faster?" and,
"How can we make it different for different parts of the glass fiber?"
The answer to both questions lies in the various additives that may be used in the glass-making process. Glass is primarily made from molten silica (sand), although flint and quartz may also be used, and are used for specific purposes. However, if you just melt ordinary sand, the resulting glass is likely to be green or brown, due to the presence of iron in the mix. To eliminate that color, manganese is added.
Most manufactured glass is actually soda-lime glass. Glass known as "crystal" is a potassium silicate mix that also includes lead oxide. Hence this is sometimes called "lead glass." Glass that must be resistant to chemicals and high temperatures has a fair amount of borax added. All additives will have an effect on the melting temperature of the glass, as well as on the density of (and therefore the speed of light inside) the glass.
DESIGNING OF FIBER CABLES
In the first example on the right we have a multi-layered system in which the fiber is firstly surrounded by a buffer tube. This buffer tube is usually a layer of silicone or epoxy, which is softer than the external jacket, and has no optical function. It keeps the fiber from "microbends" due to physical contact with the other components of the cable. As relatively fragile materials, fibers need some mechanical reinforcement. Many materials in form of strands or filaments can play this role. One of them is fiberglass, Of course, since fiberglass is a stiff material, another layer of polyurethane is added, to provide cushioning. If your fiber cable needs a good tensile strength as well as electrical insulation, which is highly desirable, you can place a layer of Kevlar®. Usually Kevlar® is arranged into the cable in the form of filaments. Finally, you must think that your fiber cable may be placed in a number of different environments: in the air, into the water, or under ground. Therefore an external protective jacket becomes essential. PVC and polyurethane are the most used materials for that purpose. : PVC is a better material than polyurethane when considering its resistance to: water, flame, acids, alkalis, hydrocarbons, and alcohol.
Conversely, polyurethane has certain advantages over PVC when dealing with its resistance to: abrasion, nuclear radiation, and low temperatures. So while selecting material it’s very important to go through the properties of material before actually to fabricate it.
From figure we have an exterior fiber cable with a buffer jacket, which can be constituted by two sheaths: one of silicone, and the other of Hytrel, extruded on silicone. Hytrel is a neat polyether-ester block copolymer, a thermoplastic elastomer with optimum water resistance. Between two layers of Kevlar filaments, a moisture barrier is inserted. This moisture barrier can be made of plastic (often polyethylene), metal (particularly aluminium), or both. Lastly, a PVC coating ensures the utility of this fiber cable to be installed in free air.
Right fig is a slight modification of the second. Only this time an underground fiber cable is shown. Since it requires a further protection against humidity, two sheaths have been inserted between a Kevlar layer: A moisture barrier and a metallic shield.
LOSSES DURING PROPAGATION OF DATA
The biggest problem with any optical fiber has to do with losses in one form or another. Some losses, such as the absorption of some light energy by the glass itself, cannot be avoided. But there are other kinds of problems that can be reduced by careful design of the fiber.
If a narrow pulse of light (light of brief duration, such as might be produced by a flashbulb) is applied to one end of an optical fiber, the pulse of light at the far end will have lower amplitude and a longer duration, as shown here. In this image, a narrow pulse of light enters the fiber from the left, and a wider, weaker pulse leaves the fiber at the right:
This phenomenon is called pulse dispersion or pulse spreading, and has three basic causes. These are:
Modal Dispersion:
In real not all light travels as the basic wave. It has many different modes, characterized by different relationships between the electric and magnetic fields that make up the total electromagnetic wave. The higher, more complex propagation modes take longer to travel a given distance in any medium, and they require a wider path than the simpler, basic mode.
The larger the diameter of an optical fiber's core, the more different propagation modes it can support, and the more pronounced the modal dispersion effect will be. On the other hand, if we make the core small enough, we can block all except the basic mode, and minimize this effect. Such a fiber is called a single-mode fiber.
Material Dispersion:
The velocity of propagation through the core is not the same for all colors (or wavelengths) of light. "White" light actually contains all visible colors, and a narrow white pulse entering the fiber will produce a series of overlapping pulses of different colors at the far end.
The solution to this problem is to use an LED or laser diode as the light source, so that all of the light passing through the fiber is at very nearly the same wavelength.
Wave-guide Delay Distortion:
This is a phenomenon observed first with microwaves traveling through wave-guides. An optical fiber is essentially a wave guide for light waves, and exhibits the same behavior, each propagating mode within the fiber experiences a slight dispersion effect simply because of the confining wave guide, which cannot behave as if it were open space. This dispersion effect is quite small but cannot be eliminated. It can pretty much be ignored, unless you are trying to push the communications speed of the optical fiber to its limits.
The basic problem caused by any dispersion effect is that it limits the rate at which data may be transmitted through the fiber. If the light amplitude is modulated at too high a rate, dispersion tends to level out the changes so that the light at the far end of the fiber is of nearly constant amplitude. The end result is that the modulations become indecipherable, and all data is lost.
ADVANTAGES OF OPTICAL FIBER
Fiber optic transmission systems – a fiber optic transmitter and receiver, connected by fiber optic cable – offer a wide range of benefits not offered by traditional copper wire or coaxial cable. These include:
1. The ability to carry much more information and deliver it with greater fidelity than either copper wire or coaxial cable.
2. Fiber optic cable can support much higher data rates, and at greater distances, than coaxial cable, making it ideal for transmission of serial digital data.
3. The fiber is totally immune to virtually all kinds of interference, including lightning, and will not conduct electricity. It can therefore come in direct contact with high voltage electrical equipment and power lines. It will also not create ground loops of any kind.
4. As the basic fiber is made of glass, it will not corrode and is unaffected by most chemicals. It can be buried directly in most kinds of soil or exposed to most corrosive atmospheres in chemical plants without significant concern.
5. Since the only carrier in the fiber is light, there is no possibility of a spark from a broken fiber. Even in the most explosive of atmospheres, there is no fire hazard, and no danger of electrical shock to personnel repairing broken fibers.
6. Fiber optic cables are virtually unaffected by outdoor atmospheric conditions, allowing them to be lashed directly to telephone poles or existing electrical cables without concern for extraneous signal pickup.
7. A fiber optic cable, even one that contains many fibers, is usually much smaller and lighter in weight than a wire or coaxial cable with similar information carrying capacity. It is easier to handle and install, and uses less duct space. (It can frequently be installed without ducts.)
8. Fiber optic cable is ideal for secure communications systems because it is very difficult to tap but very easy to monitor. In addition, there is absolutely no electrical radiation from a fiber.
SAFETY PRECAUTIONS
Two kinds of hazards can occur when working with optical fiber: Glass shards and optical radiation. Aside from other common laboratory hazards such as electrical shock and chemical exposure, these two hazards are most prominent. The use of glass optical fiber means that the possibility of glass shards or silvers is always strong, especially when fibers are cut, cleaved or broken. These silvers are dangerous because they are often difficult to see and can easily embed themselves into the skin so that the potential for cuts, contamination of the eyes, or even swallowing is increased.
To reduce the hazard of glass shards, care should be taken when handling the fiber, especially if it is not a part of cable package. Always wear safety goggles to protect your eyes and carefully wash your hands after working in the lab or the field.
Optical radiation hazards (especially from lasers) can cause damage to the eyes and is especially dangerous when working with invisible infrared light used with fiber. The Center for Disease and Radiological health (CDRH) and the American National Standards Institute have established guidelines and regulations for working with lasers and other optical hazards. These guidelines stipulate a labeling system that warns the user of possible
Dangers and necessary precautions. Using specialized protective goggles, shutters, and other systems can limit or deny exposure to hazardous emissions.
CONCLUSION
Optical fiber is the plumbing of the Internet age. And
In today’s information age, the need for fast, accurate, and high volume communication is paramount. Electronics and computers have permeated society so that they touch every aspect of our daily lives in some ways. Barcode scanners read the prices of our grocery items, optical disks play our music, computers dispense money from automatic tellers, and robots controlled by computers assemble our manufactured items. This increase in electronic methods for manipulating, interpreting, and communicating information has revealed the limits of traditional communication technologies such as copper wire and radio waves. Optical fibers have been developed to overcome these disadvantages.
Optical fiber is a riverbed along with the huge stream of data generated by modern technology flows. The bed is wide, deep, and capable of handling all the data we can pour into it without overflowing its banks. Originally, fiber was used only for long-distance telephone lines where the need for its greater capacity outweighed its expense and difficulties. As the technology developed and its cost dropped, the fiber became viable for ship and airplane systems, medium-and short-haul telephone lines, LANs and even cable TV. This widespread use of optical fiber has occurred because of its advantages over electrical signals in copper wire.
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