Essay, Research Paper: Fiber Optics


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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
YAZ OKULU 2000 ÖZET Günümüzde bakır tellerin yerini alan silikon camından
ve kristalinden üretilen fiber optikler, telekomünikasyonda, uzun mesafeli
telefon hatlarında ve insan vücudunun iç kısımlarını
inceleyen endoskopilerde kullanılmaktadır. Fotoğraf ekipmanlarında
da bütün fiber-optik endoskoplara kullanılmaktadır. Tam iç yansıma
olarak bilinen işlem yoluyla, fiberin içinde toplanan ışık
ışınları, uzun mesafeler boyunca şiddetinde küçük
bir azalma ve bozulmayla yol alabilmektedir. Genellikle, fiber üretimleri üç
kategoridedir; Sentetik fiber üretiminde dışına çıkarma
methodu; Erimiş dökme maddelerden ağızlarına doğru oluşan
fiberlerin sıcak çizimleriyle, kaplanmış,kaplanmamış
veya karışık fiberlerin çizimleriyle. Üç çeşit olan
fiber optik parçaları; görüntü kalitesini, çeşitli optik
sistemlerdeki alan açısı ve fotografik hızları geliştirmek
için düşünülmüştür. Bu fiber optik elemanları; alan düzleştirici,
konik yoğunlaştırıcı ve sapma düzenleyici şekillerindedir
ve ayrı veya “Focon” adı verilen ünite için birleşmiş
olarak kullanılabilirler. 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.
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 573º C (1,063º F), and (2)
beta-, or high, quartz, stable above 573º 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 right- 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 867º C (1,593º
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.
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
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’ = δh. And δh = 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-resolutişon 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/1.22λ 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 =
1/1.22Fλ 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 μ) 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 πλ when the fiber numerical
aperture is close to unity. Such a fiber will propagate approximately 20 modes
of wavelength, λ. Thus the optimum static resolution that can be obtained
with fibers is approximately 1/ πλ + 2t. Consequently, for λ =
0.5 μ, 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 φ = a2/a1 (no2 – n’2)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 operator’s 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.Bibliography1) 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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