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Fiber Optics

.. on requires great deal of changes in current networks and systems. This requires a lot of time and effort which the management is not willing to sacrifice. People are comfortable with what they have and don’t want to change. Although most problems regarding program changing can be solved, the solutions to it will take much longer than expected.

Thus, any new program has to be a big improvement over the old one to justify a significant change (although the great improvement usually means that the old program does not work). Another fundamental problem in fiber optic LANs is the change in technology. The hardware and software to make LAN run efficiently add up to an expensive package. If many terminals in a building must be in constant touch with each other and a variety of other hardware, such as printers and storage devices, LAN will be cost efficient. However, if the real need is to keep the terminals in touch with a mainframe computer, it would be cheaper to run cables between them and the mainframe. If the terminals need to talk to each other, ordinary telephone lines could very well be used as telephone lines are much cheaper than fiber optics. 3) Economic Evaluation The major practical problem with fiber optics is that it usually costs more than ordinary wires.

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All costs elements involved in economic evaluation can be grouped into two main classes; which are investment costs and operation costs. The investment costs usually includes expenditures related to acquiring and owning properties and plants, in this case changing wires to fiber optic cables. All investment costs should be considered, such as those incurred for equipment and materials (also including storage and handling costs), engineering costs and miscellaneous costs. Operation costs include the usage of fiber optics and the wear and tear of it. The higher costs of fiber is often not by itself. Fiber optic cables are much cheaper than coaxial cables.

The main difference comes when all the other components of fiber optics add up, such as transmitters, receivers, couplers and connectors. Fiber systems require separate transmitters and receivers because they cannot directly use the electrical output of computer devices; that signal must be converted into optical form and then converted back into electrical form. Fiber optic connectors and couplers are more expensive than any other electrical components. These costs are the ones that add up and form the major disadvantage of fiber optics. Conclusion: Fiber optic transmission has found a vast array of applications in computer systems.

Some design considerations depend largely on the application. For certain terminal to terminal application, crucial factors including maximising transmission speed and distance and minimising fiber and splice loss. By contrast, connector loss becomes important in local area networks that operate within buildings. In other systems, it is important to minimise the cost of cable, with the intention of reducing the cost of terminal equipment. These system considerations make design and construction of practical fiber optic systems a difficult task.

Guidelines appropriate for one system is usually not suitable for another system. There are a number of essential points about fiber optics that have been mentioned throughout this report. As we move towards a more sophisticated and modern future, the uses of fiber optics are going to grow in all computer systems as well as telecommunication networks. Modern information systems handle ever-increasing data loads which strain the data throughput ability of information systems. Designers have made significant progress in increasing processor speeds, however progress in the design of high-speed interconnection networks has lagged so much so that the most significant bottleneck in today’s information systems is the low speed of communications between integrated chips. These low speed communications networks consume increasing amounts of power in an effort to keep up with the faster processors.

The slow communications speed is brought on by the small bandwidth available to existing communications networks based on the propagation of electrical signals through metallic lines. Optical interconnections offer several advantages over metallic interconnections, they include: higher bandwidth; higher interconnection densities; lower crosstalk; crosstalk which is independent of data rate; inherent parallelism; immunity from electromagnetic interference and ground loops; the ability to exploit the third dimension; lower clock and signal skew; and a higher fan-in/fan-out capability. These advantages mean that optical interconnections have the potential to exhibit higher data rate communication, higher densities of interconnections with lower crosstalk, and lower power consumption. The shortest interconnections however, will remain electrical ones, due in part to the inverse relationship between electrical interconnection length and power consumption, and to a length independent minimum latency time inherent to optical interconnections caused by the time delays required for electrical to optical to electrical conversion. Agrawal, G.P.

(1992). Fiber-optic communication systems. New York: Wiley. This source provides details pertaining to my research. It provides details regarding the selection of fiber parameters. It says about the process by which the fiber parameters are selected.

It tells about the impact of the parameters on factors like cost of fiber, fiber attenuation, ease of cabling, and connection loss. This factors helps in determining the type of fiber cables we should use. Bonadedo, N.H. (1995). Fiber Optics theory and practice.

New York: McGraw- Hill. This source provides details about the input-output characteristics of the fiber. It provides details about attenuation, as it is one of the important features. This feature helps in determining the loss of light energy when a light pulse propagates down the fiber. Buck, J.A.

(1992). Fundamentals of optical fibers. New York: Wiley. This source provides details about the input-output properties of fibers. This information is helpful in learning how fibers can be used for carrying light over long distances.

The source provides regarding the distances that can be spanned without using amplifiers. Cai, M. (2000). Single-mode fiber cables. Optics Letters, 25(19), 1430-2.

This source provides details about the propagation models of fiber optics. The information about the propagation of light signals in optical fibers is provided by the source. We can know about the fields that exist within the fiber. Chanclou, P. (2001). High return loss at the end face of fiber. Applied Optics, 40(4), 458- 60.

The details regarding the geometry of the fiber is provided by this source. We study details about the physical size of the fibers. The details regarding about the core and cladding region, the materials used for these, and how they varies for multi mode and single mode fibers. By this we can use suitable fibers basing on the applications. Clarkson, C. (2000).

Fiber shines light on many industries. Laser Focus World, 36(8), 197-200. This source provides details about the advantages associated with the use of fiber optics. It provides details about the data security, immunity to electromagnetic interference, the ease of installation of fiber cables and the high bandwidth associated with them. Damien, B. (2001). Intelligent tools increase recovery.

Harts’s E, 74(1), 45-7. This source provides details regarding the applications of fiber optics. We learn from this source about the implementation of fibers in the field of medicine, lasers, industrial uses and commercial uses. This source helps me in determining which type of fiber is used in a particular application. Erdogan, T. (2000).

High speed fiber optics. Poptronics 1(11), 13-14. This source tells about differences between the coaxial cables and fiber optic cables. This source explains why fiber optic cables are preferred in favor of coaxial cables for many applications. By this information we can learn how fiber optic cables differ in performance from ordinary coaxial cables.

Jayo, Yi. (2000). Beam and fiber optics. Optical Physics, 47(11), 1821-7. This source provides details about the various measurements regarding the fibers. The various measurements associated with the fiber cables are optical characterization, quality control measurements, attenuation measurements, bandwidth measurements etc.

These measurements help us to understand the working of the fiber optic cables better. Karen, B. (2000). Data highway to the stars. IEEE Review, 47(2), 15-18.

This source provides details about the economics associated with the use of fiber optics. We get know details about the costs associated with installing optical cables and the cost breakdown. Larry, P.H. (2000).Blazing Data. Lightning Design & Application, 30(10), 24. This source provide details regarding the multi mode fibers.

We study details about the features of multi mode fiber cables like their diameter, their construction and how they differ from single mode fiber cables. Saleim, M. (2000). Single-mode fiber cables. Optics Letters, 25(19), 1430-2. This source provides details regarding the single mode fiber optic cables. Here we study about the details of single mode fiber cables like their make, the path of light inside the core etc.

Samuel, R. (1994). Fundamentals of fiber optics. New York: Wiley. This source provides details about the basics of fiber optic cables. We study about the basic parts that make up the fiber optics like the transmitters, receivers, fiber cables design etc. This gives me good knowledge about the fiber optics, which will be important in my research work. Tom, N.H.

(1995). Fiber Optics theory and practice. New York: McGraw- Hill. This source gives details about the coaxial cables , their properties their uses etc. This study helps in determining the types of cables that should for a particular application.

We have a choice to select between the fiber cable and coaxial cable. Wiseman, C. (2000). Fiber shines light on many industries. Laser Focus World, 36(8), 197-200. This source provides details about the details about the implementation of fiber optics and where they are used. Computers and Internet.

Fiber Optics

.. ight- or left-handed symmetry group. At the transition temperature the tetrahedral framework of beta-quartz twists, resulting in the symmetry of alpha-quartz; atoms move from special space group positions to more general positions. At temperatures above 867 C (1,593 F), beta-quartz changes into tridymite, but the transformation is very slow because bond breaking takes place to form a more open structure. At very high pressures alpha-quartz transforms into coesite and at still higher pressures, stishovite.

Such phases have been observed in impact craters. Quartz is piezoelectric: a crystal develops positive and negative charges on alternate prism edges when it is subjected to pressure or tension. The charges are proportional to the change in pressure. Because of its piezoelectric property, a quartz plate can be used as a pressure gauge, as in depth-sounding apparatus. Just as compression and tension produce opposite charges, the converse effect is that alternating opposite charges will cause alternating expansion and contraction. A section cut from a quartz crystal with definite orientation and dimensions have a natural frequency of this expansion and contraction (ie. vibration) that is very high measured in millions of vibrations per second.

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Properly cut plates of quartz are used for frequency control in radios, televisions, and other electronic communications equipment and for crystal-controlled clocks and watches. 3.3 WHAT IS ENDOSCOPIC PHOTOGRAPHY? With the use of modern light -weight single lens reflex cameras employing either automatic exposure control or through-the-lens metering, good half or whole frame 35mm colour photographs can be taken. Distal cameras (intragastric cameras), producing 5mm or 6mm colour pictures and electronic distal flash, are also available in some fibre-endoscopes. Endoscopic photography is the available equipment and the best method of obtaining the best possible colour photographs. It is possible to obtain high-quality colour transparencies of bowel lesions. These are generally employed for patient records, teaching and research.

They are not usually employed for diagnosis since visual inspection and biopsy will already have been performed. An exception is in so called gastro-camera diagnosis where miniature photographs are taken from within the stomach as an aid to the detection of early gastric cancer. Endoscopic cine-photography is useful for recording motility, endoscopic techniques, and unusual lesions. It can be also be used to make teaching films. Close circuit colour television endoscopy is already in routine use in some centres of Japan, the United States and Europe and will undoubtedly find a wider use, especially for teaching and training. This equipment is naturally very costly but cheaper equipment can be anticipated.

4. ENDOSCOPIC PHOTOGRAPHY ELEMENTS 4.1 FIELD FLATTENER In lens design, it is desirable that the image coincide with the Gaussian image plane so that the whole field may be in focus simultaneously. In this case, the Petzval sum of the optical system must be zero or, at most, be a small residual to compensate for the secondary effects of higher-order astigmatism and oblique spherical aberration. When the third-order astigmatism coefficient is zero, it is well-known that the sagittal and tangential image surfaces coincide with the Petzval surface. The curved fields of such an astigmatic lens system can be flattened by using a bundle of fibers.

The shape and curvature of the entrance end of the bundle is determined by the image surface of the lens system that precedes it. The other end of the fiber bundle may be flat if the system is to be used for direct observation or photography, as shown in Fig. 4.1.However, when an image is field flattened in this manner, there is an interaction between the lens distortion coefficient and a distortion term introduced on field flattening. Distortion term shows the exit pupil of a lens system through which a principal ray passes at an inclination U and intersects the Petzval surface at the point P and the Gaussian image plane at the point Q. Since the principal ray does not intersect the Gaussian plane when a field flattener is used but is intercepted by a fiber at the Petzval surface, the effective image size is changed by an amount OQ = ?h.

And ?h = hG – h where hG is the Gausiian image height and h is the intersection height of the principal ray at the Gaussian image plane. There are several methods available for the production of a field flattener. In one of these methods, the fibers are ground and polished along the curve desired according to the Fresnel element, and then the entrance ends of the fibers are displaced to lie on the curved image surface. Obviously, this method suffers from technological limitations and is acceptable only when low-resolutison field flatteners are required. A second method consisting of lapping the field flattener in against a metallic master. In the third, most promising method, a Fresnel surface is produced at the curved surface of the fiber assembly with a master, employing an epoxy of the type used for making diffraction grating replicas. 4.2 CONICAL CONDENSER A conical fiber bundle is placed at the focal end of a lens system to increase the photographic speed of the system by utilizing the flux-condensing property of a cone.

However, the condensing ratio of a glass-coated glass cone is determined by the ratio f- ratio and the field angle of the preceding image forming system, as well as the refractive indices of the fiber core and coating materials. If we make some simplifying assumptions of a meridional ray propagation in a cone with axial length many times greater than its diameter. For cones located off-axis at the image plane and with bend sides, there are obvious deviations. Figure 4.2 shows an image transmitted by a conical fiber bundle having a 2,5 : 1 ratio. 4.3 DISTORTION CORRECTOR It is possible to fabricate fiber bundles with the capability of correcting for pin-cushion and barrel distortion. It is also possible to evolve techniques for fabricating fiber bundles to compensate for the distortion term introduced in large-angle line scan systems and S-shaped distortion of the type introduced in electron-optical systems.

Figure 4.3 shows images transmitted through two fiber plates, demonstrating the correction capability for pin-cushion and barrel distortion. Such fused fiber assemblies are fabricated by subjecting to well defined thermal and pressure gradients. As another intersting example of the application of a combination of field flattener and distortion corrector, we shall cite the problem of a wide-angle spot scan systems in which a severe distortion term proportional to the field angle is introduced because of a change in spot size. In such a system, it is also desirable to use a curved image fieldto facilitate the mechanical synchronization of the two scanning functions of the data-acqusition and print-out systems. 4.4 FOCON RESOLUTION Of importance in the determination of the overall performance of a lens-fiber optics combination is the angular resolution (Rang) of an image-forming system of a aperture diameter, D, which, according to classical theory, is given by the formula: Rang = D/1.22? By inserting the value of the focal ratio (F), it is possible to determine the linear resolution (Rang), which is given by the following expression; Rlin = 1/1.22F? On the other hand, the linear displacement between two points which can be resolved by static fiber optics is between 2d + 3t and d + 2t, where d is the fiber diameter and t (? 0.5 ?) is the spacing between them.

The resolution is then given by the reciprocal of this quantity. Waveguide effects and evanescent wave coupling between the fibers can be avoided if the fiber diameter is greater than or equal to ?? when the fiber numerical aperture is close to unity. Such a fiber will propagate approximately 20 modes of wavelength, ?. Thus the optimum static resolution that can be obtained with fibers is approximately 1/ ?? + 2t. Consequently, for ? = 0.5 ?, a maximum static resolution of 220 to 350 lines / mm can be expected with high resolution fiber optics.

Of course, dynamic scanning can be used to improve the resolution. Thus the highest linear resolution obtainable with a fiber bundle is considered to be equivalent to that of a diffraction-limited f/4 lens. Figure 4.4 shows a curve of the resolution of fiber conical condenser used in conjunction with diffraction-limited lenses of a given f-number. Each curve corresponds to a conical condenser of ? = a2/a1 (no2 n2)1/2, where a1/a2 is the cone ratio, and no and n are the refractive indices of the fiber core and coating, respectively. 5.

ENDOSCOPIC PHOTOGRAPHY TECHNIQUES 5.1 COLOUR PHOTOGRAPHY WITH FIBRE-OPTIC ENDOSCOPES This technique is the one of employed in great majority of endoscopic examinations. Photographs are taken through the endoscope by a camera placed on the eyepiece. This means that whatever the operator sees will be recorded photographically. The disadvantages of this method are that the fibre-matrix is also photographed. In addition, any imperfections in the operators view, such as poor focus or bad picture composition, will be reflected in the photograph. To this extent the problems are similar to those of conventional photography, but otherwise there are few similarities.

When employing a proximal camera for endoscopic photography the following points should be remembered. 1. A single lens reflex (SLR) camera must be employed. 2. Through the lens exposure metering (TTL metering) must be employed, unless there is automatic exposure control of the light source output.

3. A medium focal length lens, eg 70-105 mm or telephoto lens, may be required with some endoscopes and must be focussed at infinity. 4. The camera lens must be focussed at infinity. 5.

Photography must be carried out at aperture if a camera lens is employed. 6. It may not with some endoscopes be necessary to use a camera lens. 7. It is not usually possible to vary the ligthing.

8. High speed film is usually necessary and must be of the correct type. 5.2 CINE ENDOSCOPY Although cine endoscopy is employed routinely by some authorities to record lesions, motility , etc, it is usually reserved for occasional use in teaching because of the cost equipping with suitable cameras and films. Suitable cine cameras include: Super-8 Kodak M-30 with power-operated zoom lens (from f/1.9) and Beaulieu R-16 B medical camera (16 mm). The Beaulieu R-16 B Euratom camera is undergoing evaluation at present. It houses an automatic light control system in place of the lens turret consisting of a graded neutral density filter wheel coupled to the exposure meter.

This wheel is adjusted by a small servo motor so that the light reaching the film remains constant. This novel form of light control provides and alternative to the iris diaphragm which, as we have already seen, is not possible with endoscopy photography. At the present, however, this camera is nut fully tested. Probably the best currently available system is the standard 16 mm Beaulieu R-16 B medical camera, employing a suitable adaptor supplied by the manufacturer for their endoscopes. 5.3 CLOSED CIRCUIT COLOUR TELEVISION ENDOSCOPY In a number of Japanese centers and in some centers in the USA and Europe, closed circuit colour television endoscopy is employed for demonstration and teaching. The results, as might be expected, are variable, but it is possible, by employing the best available equipment to produce excellent television images with good colour reproduction. Television technology is highly developed, nevertheless it will be useful to discuss the items that make up an effective system for endoscopy and to point out the weak links.

A succesful system for use in gastro-intestinal endoscopy would consist of: a colour television camera; a flexible optical coupling between the television camera and the endoscope; a light control system; colour television monitor(s); a fibre-optic endoscope, and a suitable light source. 5.4 GASTRO-CAMERA EXAMINATION Gastro-camera examination of the stomach is an investigation in which a flexible tube is passed into the stomach and multiple colour photographs taken employing a miniature camera and flash lamp mounted distally on the tube. This method was developed by the Japanese in 1950 in an attempt to diagnose gastric cancer, a disease that accounts for more deaths in Japan than any other form of cancer. Diagnosis is based on a complete photographic survey of the stomach, followed by careful inspection of the transparencies. Suspicious areas are noted and the patient called back for full fibre-endoscopy and biopsy, or alternatively surgical biopsy. The term gastro-camera is understood to include blind gastro cameras which do not have visual control and visually controlled instruments with image blundles.

With the blind gastro-cameras the tip of the instrument is positioned by observing the light from it through the abdominal wall. Clearly this must take place in darkened room. 6. CONCLUSION Fibre-optic endoscopy has established itself as an important diagnostic tool in the investigation and management of disease of the gastric-intestinal tract. Considerable advances have been made in the design and construction of fibre-optic endoscopes and their support systems, over the past ten years. It is unlikely that development will take place at the same pace over the next decade.

We are now entering a phase of consolidation during which objective evaluation of each area of endoscopy will take place as the techniques become more widely used. Advances will be made in producing serviceable instruments and local servicing facilities are likely to be increased and streamlinid. Fibre bundle technology will probably not strive to produce smaller fibres since the limit has already been nearly reached. Design will probably concentrate on reliability, and cheaper meth-pds of production. Endoscope support systems, such as light sources, will probably improve with the development of more powerful, cooler and reliable lamps. The great advantage of flexibility provides the key to the use of optical communication within as well as outside medicine.

As a result of this technology medical fibre-optics are likely to receive the benefit of cheaper more dispensible fibre-bundles. These are, at present, the most expensive items in a Fibre endoscope. Bibliography 1) Kapany, N.S., Fiber Optics, Academic Press, New York, 1967 2) Buck, J.A., Fundamentals of Optical Fibers, Wiley-Interscience Publication, New York, 1995 3) Salmon, P.R., Fibre Optic Endoscopy, Pitman Medical Publishing, New York, 1974 4) 5)


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