Fiber Optics Fiber optics produced by special methods from silica glass and quartz which replaced copper wire is very useful in telecommunications, long distance telephone lines and in examining internal parts of the body (endoscopy). Equipment for photography is available with all current fiber-optic endoscopes. Through a process known as total internal reflection, light rays beamed into the fiber can propagate within the core for great distances with remarkably little attenuation or reduction in intensity. In general, the methods of fiber production fall into three categories; (a) the extrusion method for synthetic fibers; (b) hot drawing of fibers from molten bulk material through an orifice; and (c) drawing of uncoated, coated and multiple fibers from assemblies of rods and tubes fed through a hollow cylindrical furnace. Three forms of fiber optics components have been proposed for the improvement of the image quality, field angle and photographic speed of various types of optical systems. These fiber optics elements, in the form of a field flattener, a conical condenser and distortion corrector, can be used separately or combined into a single unit called a “Focon”.
BOGAZII NIVERSITESI MAKINA MHENDISLIGI DEPARTMANI MALZEME DERSI DNEM PROJESI YAZ OKULU 2000 ZET Gnmzde bak?r tellerin yerini alan silikon cam?ndan ve kristalinden retilen fiber optikler, telekomnikasyonda, uzun mesafeli telefon hatlar?nda ve insan vcudunun i k?s?mlar?n? inceleyen endoskopilerde kullan?lmaktad?r. Fotograf ekipmanlar?nda da btn fiber-optik endoskoplara kullan?lmaktad?r. Tam i yans?ma olarak bilinen islem yoluyla, fiberin iinde toplanan ?s?k ?s?nlar?, uzun mesafeler boyunca siddetinde kk bir azalma ve bozulmayla yol alabilmektedir. Genellikle, fiber retimleri kategoridedir; Sentetik fiber retiminde d?s?na ?karma methodu; Erimis dkme maddelerden ag?zlar?na dogru olusan fiberlerin s?cak izimleriyle, kaplanm?s,kaplanmam?s veya kar?s?k fiberlerin izimleriyle. esit olan fiber optik paralar?; grnt kalitesini, esitli optik sistemlerdeki alan a?s? ve fotografik h?zlar? gelistirmek iin dsnlmstr. Bu fiber optik elemanlar?; alan dzlestirici, konik yogunlast?r?c? ve sapma dzenleyici sekillerindedir ve ayr? veya “Focon” ad? verilen nite iin birlesmis olarak kullan?labilirler. LIST OF FIGURES Figure 2.1 Photograph of the earliest bundle of uncoated aligned fibers Page 7 Figure 3.1 Core of a step index fiber Page 8 Figure 3.2 Schematic diagram of a typical fiber drawing Page 9 Figure 3.3 Preform manufacturing apparatus used in Silica-Quartz Page 11 Figure 3.4 Comparison of static,dynamic and spitial filtering imagery Page 12 Figure 4.1 Field flattener system of photography Page 13 Figure 4.2 Showing the image transmission through a conical fiber bundle Page 14 Figure 4.3 Fiber optics distortion correctors Page 14 Figure 4.4 Limiting resolution of Focon system Page 15 Figure 5.1 Single lens reflex camera Page 16 TABLE OF CONTENTS 1. INTRODUCTION 2.
HISTORY OF FIBER OPTICS 3. WHAT IS FIBER OPTICS? 3.1 WHAT IS SILICA? 3.2 WHAT IS QUARTZ? 3.3 WHAT IS ENDOSCOPIC PHOTOGRAPHY? 4. ENDOSCOPIC PHOTOGRAPHY ELEMENTS 4.1 FIELD FLATTENER 4.2 CONICAL CONDENSER 4.3 DISTORTION CORRECTOR 4.4 FOCON RESOLUTION 5. ENDOSCOPIC PHOTOGRAPHY TECHNIQUES 5.1 COLOUR PHOTOGRAPHY WITH FIBRE-OPTIC ENDOSCOPES 5.2 CINE- ENDOSCOPY 5.3 CLOSED CIRCUIT COLOUR TELEVISION ENDOSCOPY 5.4 GASTRO-CAMERA EXAMINATION 6. CONCLUSION 7. REFERENCES 8. APPENDIX 1.
INTRODUCTION The technology of fiber drawing for nonoptical applications is old and fairly standard. Very-small-diameter glass and quartz fibers were made as early as by Faraday. In the early stages of the production of glass fibers on an industrial scale, the main application of the fibers was envisaged in the textile industry. More recently, they have been used for insulation against sound, heat and electricity. Presently, very fine fibers are being made of materials such as glass, quartz, nylon, polystyrene, polymethylcrylate. Of these, glasses, quartz and plastics are preferred for optical use because of their higher visible light transmission, longer thermal working range, better surface characteristics and mechanical strength. Furthermore, it has been shown that glass fibers can have greater tensile strength than can be expected from the bulk material.
2. HISTORY OF FIBER OPTICS The conduction of light along transparent cylinders by multiple total internal reflections is a fairly old and well known phenomenon. It is entirely possible that grecian and other ancient glassblowers observed and used this phenomenon in fabricating their decorative glassware. In fact, the basic techniques used by the old Venetian glassblowers for making millifiore form an important aspect of present-day fiber optics technology. However, the earliest recorded scientific demonstration of this phenomenon was given by John Tyndall in 1870.
In demostration Thyndall used an illuminated vessel of water and showed that, when a stream of water was allowed to flow through a hole in the side of the vessel, light was conducted along the curved path of the stream. In 1951 when A.C.S. van Heel in Holland and H.H. Hopkins and N.S. Kapany studied on the transmission of images along an aligned bundle of flexible glass fibers.
But it was the year 1956 that Kapany first applied the term fiber optics to this field and described its principle and various of possible applications. Kapany defines fiber optics as the art of the guidance of light, in the ultraviolet, visible and infrared regions of the spectrum, along transparent fibers through predetermined paths. Between 1957 and 1960 Potter, Reynolds, Reiffel and Kapany investigated the use of scintillating fibers for tracking high energy particles. Potter also investigated the theory of skew ray propagation along fibers in some detail. One of the biggest application area of fiber optics is in medicine. Hirschowitz have been working on the developement of fiber optics gastroduodenal endoscopes and Kapany have been researching fiber optics in gastrocopy, bronchoscopy, retroscopy and cyctoscopy. Kapany, Drougard and Ohzu have made basic studies on image transfer characteristics of fiber assemblies.
3. WHAT IS OPTICAL FIBRES? Optical fibres are glass or plastic waveguides for transmitting visible or infrared signals. Since plastic fibres have high attenuation and are used only in limited applications, they will not be considered here. Glass fibres are frequently thinner than human hair and are generally used with LEDs or semiconductor lasers that emit in the infrared region. For wavelengths near 0.8 to 0.9 m, gallium arsenide-aluminum gallium arsenide (GaAs-AlxGa1 – xAs) sources are used, and, for those of 1.3 and 1.55 m, indium phosphide-gallium indium arsenide phosphide (InP-GaxIn1 – xAsyP1 – y) sources are employed.
As noted earlier, optical fibres consist of a glass core region that is surrounded by glass cladding. The core region has a larger refractive index than the cladding, so that the light is confined to that region as it propagates along the fibre. Fibre core diameters ranges between 1 and 100 m, while cladding diameters are between 100 and 300 m. Fibres with a larger core diameter are called multimode fibres, because more than one electromagnetic-field configuration can propagate through such a fibre. A single-mode fibre has a small core diameter, and the difference in refractive index between the core and cladding is smaller than for the multimode fibre. Only one electromagnetic-field configuration propagates through a single-mode fibre.
Such fibres have the lowest losses and are the most widely used, because they permit longer transmission distances. They have a constant refractive index in the core with a diameter between 1 and 10 m. The index in the cladding layer decreases by roughly 0.1 to 0.3 percent. This type of fibre is called a step-index fibre. The multimode fibres may be step-index fibres with diameters between 40 and 100 m. The refractive index step between the core and cladding is approximately 0.8 to 3 percent. In a graded-index fibre, the core refractive index varies as a function of radial distance.
In such a fibre, a ray in the centre of the core travels more slowly than one near the edge, because the speed of propagation v is related to refractive index n as v = c/n, where c is the speed of light. The ray near the edge has a longer zigzag path than the ray in the centre. The transit times of the rays are thus equalized. Both single-mode and multimode fibres are made of silica glass. The refractive indexes of the silica are varied with dopants such as germanium dioxide (GeO2), phosphoric oxide (P2O5), and boric oxide (B2O3).
Vapour-phase growth reactions are used to obtain the “preform” rod, which is then drawn into optical fibres. For example, a GeO2-SiO2 film may be deposited inside a silica tube. In this case, the GeO2 increases the core refractive index. In another method, preforms for low-loss, single-mode fibres are made by first depositing a low-index borosilicate layer on the inner surface of the silica tube and then depositing a silica layer or inserting a pure fused silica rod before collapsing the preform. The preform is then drawn into the optical fibre and covered with a polymer coating. There are a number of factors that contribute to attenuation in an optical fibre.
Rayleigh scattering is caused by microscopic variations in the refractive index of a fibre and is proportional to 4. Absorption by hydroxyl (OH) ions increases the absorption and gives the minim in loss at 1.3 and 1.55 m. At longer wavelengths; absorption by the atomic vibrations in the silicon-oxygen atoms rapidly increases the loss. Single-mode fibres commercially available for communications systems have losses as low as 0.2 decibel per kilometre. The low fibre loss permits increased repeater spacing and lower system cost. High-bit-rate digital systems without repeaters have been demonstrated for fibre lengths of more than 100 kilometres.
Fibre splicing techniques have been developed so that repairs can be made in the field with losses of only 0.1 to 0.3 decibel. A variety of optical connectors are used, providing both ease of use and low loss of only a few tenths of a decibel. Fibres are combined into many different kinds of cables, which can be laid both in the ground and under the sea. 3.1 WHAT IS SILICA? Of the various glass families of commercial interest, most are based on silica, or silicon dioxide (SiO2), a mineral that is found in great abundance in nature–particularly in quartz and beach sands. Glass made exclusively of silica is known as silica glass, or vitreous silica. (It is also called fused quartz if derived from the melting of quartz crystals.) Silica glass is used where high service temperature, very high thermal shock resistance, high chemical durability, very low electrical conductivity, and good ultraviolet transparency are desired.
However, for most glass products, such as containers, windows, and lightbulbs, the primary criteria are low cost and good durability, and the glasses that best meet these criteria are based on the soda-lime-silica system. After silica, the many “soda-lime” glasses have as their primary constituents soda, or sodium oxide (Na2O; usually derived from sodium carbonate, or soda ash), and lime, or calcium oxide (CaO; commonly derived from roasted limestone). To this basic formula other ingredients may be added in order to obtain varying properties. For instance, by adding sodium fluoride or calcium fluoride, a translucent but not transparent product known as opal glass can be obtained. Another silica-based variation is borosilicate glass, which is used where high thermal shock resistance and high chemical durability are desired–as in chemical glassware and automobile headlamps.
“Crystal” tableware was made of glass containing high amounts of lead oxide (PbO), which imparted to the product a high refractive index (hence the brilliance), a high elastic modulus (hence the sonority, or “ring”), and a long working range of temperatures. Lead oxide is also a major component in glass solders or in sealing glasses with low firing temperatures. 3.2 WHAT IS QUARTZ? Quartz has attracted attention from the earliest times; water – clear crystals were known to the ancient Greeks as krystallos – hence the name crystal, or more commonly rock crystal, applied to this variety. The name quartz is an old German word of uncertain origin first used by Georgius Agricola in 1530. Quartz has great economic importance. Many varieties are gemstones, including amethyst, citrine, smoky quartz, and rose quartz. Sandstone, composed mainly of quartz, is an important building stone.
Large amounts of quartz sand (also known as silica sand) are used in the manufacture of glass and ceramics and for foundry molds in metal casting. Crushed quartz is used as an abrasive in sandpaper, silica sand is employed in sandblasting, and sandstone is still used whole to make whetstones, millstones, and grindstones. Silica glass (also called fused quartz) is used in optics to transmit ultraviolet light. Tubing and various vessels of fused quartz have important laboratory applications, and quartz fibres are employed in extremely sensitive weighing devices. Quartz is the second most abundant mineral in the Earth’s crust after feldspar. It occurs in nearly all-acid igneous, metamorphic, and sedimentary rocks. It is an essential mineral in such silica-rich felsic rocks as granites, granodiorites, and rhyolites.
It is highly resistant to weathering and tends to concentrate in sandstones and other detrital rocks. Secondary quartz serves as a cement in sedimentary rocks of this kind, forming overgrowths on detrital grains. Microcrystalline varieties of silica known as chert, flint, agate, and jasper consist of a fine network of quartz. Metamorphism of quartz-bearing igneous and sedimentary rocks typically increases the amount of quartz and its grain size. Quartz exists in two forms: (1) alpha-, or low, quartz, which is stable up to 573 C (1,063 F), and (2) beta-, or high, quartz, stable above 573 C. The two are closely related, with only small movements of their constituent atoms during the alpha-beta transition.
The structure of beta-quartz is hexagonal, with either a left- or right-handed symmetry group equally populated in crystals. The structure of alpha-quartz is trigonal, again with either a r …