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From an optical point of view, in order to obtain information about an object, at least three conditions must be met, namely: (a) the object must be bright, ie to emit light directly or indirectly, (b) the light coming from the object must be transmitted to the place of detection without too great a loss and (c) the amount of light reaching the place of detection is large enough. We notice that the medium through which the optical information is transmitted is of essential importance so that the transmitted optical signal is not "mutilated" or distorted.
Even in cases where we are only interested in simply observing objects, classical optical devices and devices either become too complicated or cannot solve a certain problem. Let's take just one example: those who work in the medical field are interested in having fast and safe methods of exploring certain internal parts or internal organs of the human body. Classic methods, based on the use of incandescent lamps, are not only cumbersome and involve little lighting, but also present risks due to the use of electrical connections. All these difficulties are eliminated if the lighting is done from the outside by means of a thin optical fiber.
But optical fibers are already widely used in communications or image transmission techniques. This possibility is facilitated by the electromagnetic nature of light, the frequency of light waves being much higher than that of radio waves. In a more general context, optical fibers are a field of integrated optics, and the progress that will be made in integrated optics will depend very much on the progress that will be made in the field of optical fibers.
As a field of optics, which emerged exclusively from the most diverse practical needs, optical fibers experienced a rapid development after 1950 as a result of obtaining the first high-performance optical fibers. The principle of operation of optical fibers is similar, in many ways, to the principle of transmitting light through a transparent glass rod. Theoretically, light can be transmitted through such an optical glass rod, if the refractive index of the glass is higher than the refractive index of the air. From a practical point of view, however, the inhomogeneities of composition and processing, as well as the impurities on the surface of the material imply very large losses of light along the path of light. On the other hand, the electromagnetic nature of light radiation shows that light losses and parasitic phenomena can occur that drastically limit the possibilities of practical use of optical fibers.
Regardless of the fields in which they are used, optical fibers are light guides used to transmit information with low energy losses from one place to another. We will analyze the transmission of light radiation through optical fibers from the point of view of geometric optics and from the point of view of wave optics.
SIMPLE OPTICAL FIBER
By simple optical fiber we mean a transparent optical medium, of long length, with symmetrical circular cross section and variable or radially variable refractive index, separated from another material with constant and smaller refractive index, so that the separation surface is produces total reflection of light radiation without loss. After the radial variation of the refractive index of the transparent material, called the optical fiber core, we distinguish several types of optical fibers. The fiber optic coating also has the role of protecting against impurities the separation surface between the core and the coating, where the phenomenon of total reflection occurs. The technology for obtaining optical fibers is presented by Tader and Spulber (1985).
CONSIDERATIONS OF GEOMETRIC OPTICS
The propagation of light radiation through optical fiber can be analyzed from the point of view of geometric optics when the diameter of the core of the optical fiber is large compared to the wavelength of light radiation (diffraction effects are neglected). If the diameter of the fiber optic core is of the same order of magnitude as the wavelength of the light radiation, the analysis must be made in corrugated optics. In this section we will consider that the conditions of applicability of geometric optics are fulfilled.
In the language of geometric optics, the light radiation incident at the separation limit between the fiber core (with refractive index n1) and the protective coating (with refractive index n2, n1> n2) will be fully reflected and therefore will propagate without loss of along the optical fiber, if the angle of incidence θ is greater than or equal to the limit angle l (0> l), where the limit angle is given by the relation
sin l=n2/n1=1/n21 (8.1)
Let be a cylindrical optical fiber with cross section, circular radius R0 and refractive index n1 = constant, surrounded by a protective medium with refractive index n2 = constant and let SI be a ray of light, which intersects the axis of symmetry of the fiber, incidence on the flat surface of the optical fiber, perpendicular to the axis of symmetry, under the angle of incidence i. After suffering the refraction at the flat surface under the angle of refraction r, given by the relation
r=arc sin (n0/n1 sin i) (8.2)
where n0 is the refractive index of the medium from which light enters the fiber, the ray of light reaches the separation surface between the fiber core and the protective medium under the angle of incidence θ given by the relation
θ=π/2-r (8.3)
According to relations (8.1) - (8.3), the total reflection condition in point I 'is given by the relation
sin θ=cos r=(1-sin²r)½=(1-n²0 /n²1 sin²i)½>n2/n1 (8.4a)
or
(n1²-n2²)½≡sin imax>sin i (8.4b)
This means that any ray of light, incident on the flat surface of the optical fiber below the angle of incidence and smaller than the angle of the image, given by the relation (8.4b), will be trapped in the optical fiber (trapped ray). The maximum refractive angle for a sunken radius is given by the relation
sin rmax= n0 sin imax= (1- n2²)½ (8.4c)
n1 n1²
The numerical aperture (A.N.) of the optical fiber is
A.N.= n0 sin imax= (n1²-n2²)½
The distance from the axis of symmetry to the successive paths traveled by the radius inside the optical fiber is a constant size, denoted by dc. Also, the angle of incidence θ inside the fiber remains constant, being given by the relation
cos θ= sin r cosγ= n0 sin i cos γ
n1
where sin γ = dc / R0. Depending on the angle of incidence at the entrance, i, the condition of trapping the ray of light is written n0 sin i <A.N. sec γ. Incident rays that do not intersect the axis of symmetry of the optical fiber determine a virtual numerical aperture (A.N.V.) that can be calculated using the relation
A.N.V.= n0 sin imax= (n1²-n2²)½ sec γ
Since not all such rays are trapped by the optical fiber, even if the condition i <imax is met, the effective numerical aperture (A.N.E.) is calculated using
(A.N.E.)²= n0²- 2 {[(n0²-n1²+n2²)]½+[n0²-2(n1²-n2²)]arccos[(n1²-n2²)½/n0]}
π
for which all light rays were taken into account, regardless of whether or not it intersects the axis of symmetry, and the optical fiber was considered perfectly cylindrical.
When the flat surface of the optical fiber, through which light enters, is the oblique phase of the axis of symmetry, the cone of the trapped rays will also be oblique, at the exit of the fiber, towards the axis of symmetry.
If the optical fiber is conical, the angle of incidence of a beam trapped inside the fiber changes along it, the light ray being able to even return to the input surface. The trapping condition of a ray of light that intersects the axis of symmetry of the conical fiber is given by the relation
n0 sin i= n1 sin r= n1R2 sin rx< (n1²-n2²)½ R2
R1 R1
where R1 is the radius of the inlet surface, and R2 is the radius of the outlet surface of the conical fiber. The numerical aperture of the conical optical fiber is less than R1 / R2 times the numerical aperture of the cylindrical optical fiber. Obtaining as high a spatial concentration of light as possible can be achieved by tapering the optical fiber, but this is accompanied by an increase in the angular divergence of the light beam. We can increase the surface illuminated by the beam by decreasing the convergence angle of the cone.
By bending the optical fiber certain rays of light initially trapped can pass radially. In practice, the radii of curvature are so large that radiative losses are negligible, which provides a great advantage to optical fibers as light guides. The bending of the optical fiber destroys the axial symmetry. The effect of curvature is most pronounced on the light rays in the plane of curvature that intersect the axis; therefore, for the beginning we will consider only such rays. The ray of light entering the optical fiber at point I '' is refracted below the angle of refraction r, and the angle of incidence θ1 at point I '', obtained by applying the sine theorem in triangle I ''
sin θ1=Rc-R0 sin I„I`O=Rc-R0 cos r
Rc+R0 Rc+Ro
The angle of incidence for the next point of incidence, I „, will be θ2 = π-r
2
and the path traveled by the ray of light between two successive reflections will be
d= I`I„= (Rc+R0) sin β
cos r
The obsolete result highlights the fact that in the case of optical fibers with refractive indices that differ very little from each other, even the small curves of the optical fiber destroy the trapping effect of light rays.
CONSIDERATIONS BASED ON ELECTROMAGNETIC OPTICS
Many phenomena that occur when guiding light through optical fibers cannot be addressed in geometric optics; electromagnetic optics must be used to explain them. Similar to rectangular waveguides, optical fibers with a circular cross section can support several modes. Qualitatively, the modes can be described in relation to the radial variation of the field with maxima or minima on the symmetry axis and with additional maxima along the radius of the core. The latter are denoted by the letter m. Stationary modes are characterized by a field that decreases monotonously outside the fiber optic core.
Simultaneously with the radial variation, an azimuthal variation can appear; the field can rotate cyclically near the circumference. The length of the circumference must correspond to an integer number of cycles. If the light is linearly polarized (PL), the different modes are characterized by symbolic notations of the form PLlm.
The attenuation of the light beam during propagation along the optical fiber is mainly due to the following causes:
-reflection at the input surface in the optical fiber;
-spreading and absorption in the fiber optic material;
-complete incomplete reflection at the core-layer separation limit.
The attenuation is high at the beginning of the optical fiber after which only the remaining trapped modes propagate in the fiber.
Simple fiber optic already has multiple practical applications. It can be used as a small device in various devices. It is also used to transport radiative energy for local heating purposes. For example, in conjunction with a 100W single fiber optic incandescent lamp it was used to weld connections in electronic devices.
When high densities of radiant energy are involved, transmitted, as in the case of coupling the optical fiber with a power laser, the solarization effect of the material limits the scope of the optical fiber. For example, for a power density of 15kW / cm², an ordinary optical fiber, 1.5 m long, reduces its transmittance in 7 minutes from 0.53 to 0.25, due to solarization. The use of optical materials with superior properties allowed the obtaining of optical fibers in which the solarization effect, under the specified conditions, determines a reduction of the transmittance in one hour of only 10%.
FIBER OPTIC CABLES
Although simple optical fiber has great flexibility, due to the fact that the energy and amount of information transmitted through the fiber are limited, cables made of several simple optical fibers are used.
Fiber optic cables are of two types:
incoherent cables or light guides, which are used when the signal transmitted by a single optical fiber of the cable is not correlated with the signals transmitted to the other simple fibers of the cable; in such cables it is not important the relative position of the different simple fibers that make up the cable;
coherent cables, used in particular for the transmission of images; in such cables the relative position of the various simple fibers that enter into their component is of vital importance.
INCONSISTENT CABLES: The primary function of incoherent cables is to transmit light from one place to another. Their advantages over other optical devices that can fulfill the same role are flexibility, high efficiency, compactness and the possibility of modeling the cross section of the light beam. Flexibility allows you to guide the light along complicated paths without the need to use mirrors or prisms. High efficiency can have higher values than one. With the help of optical cables you can change both the shape of the cross section of a light beam and the number of transmitted beams; a single light beam may be divided into several separate light beams, or several light beams may be combined into a single light beam.
The arrangement structure of simple optical fibers in a cable can be either hexagonal or square. In a hexagonal assembly, the optical fibers occupy a fraction equal to π / 2√3 = 0.9069 of the surface of a network element, if the thickness of the protective material state is not taken into account, and occupy a fraction equal to o, 9069 R0 / R1 if the thickness of the protective layer is also considered, R1 being the radius of the cross section corresponding to the protective layer. In a square arrangement the fraction is π / 4 = 0.785, which determines that the transmission of these cables is lower than that of the cables with hexagonal arrangement of 2 / √3 = 1.115 times.
The diameter of the glass optical fibers used to make the cables can reach up to 0.15 mm without reducing the flexibility of the cable too much. If plastic optical fibers are used, the maximum diameter can be about 1.5 mm. By bending (bending) the cables, the most requested are the external optical fibers. Such stresses reduce the cable transmittance. In the case of transmitting glass cables, it stabilizes at a value about 1% or 2% lower than the initial one after approximately 100 stresses, while for plastic fiber cables the transmittance continues to decrease with increasing number of stresses.
The temperature up to which the glass cables are used depends on the material of the protective layer and the material used to join the fibers and can be up to 4ooºC, and the maximum temperature at which the plastic cables can be used is imposed by the plastic material used to obtain the fibers. .
COHERENT CABLES
Because each simple optical fiber, a component of the cable, can carry a certain amount of energy, corresponding to a certain element of the object's surface, independently and without the influence of neighboring fibers, coherent cables serve to transmit images from one place to another.
The optical fiber is extracted from the furnace on a drum, but taking care to position the successive turns of the helix next to each other without overlapping. After the desired width has been obtained, a new layer is deposited by reversing the direction of spiraling of the fiber, the number of layers depending on the number of fibers that must make up the cable. After the desired number of layers has been achieved, the fibers on the drum are cut parallel to the axis of the drum. The process does not allow obtaining thinner fibers of about 20 μm, which is why the cable is reheated and stretched to obtain fibers with diameters of about 5 μm.
The optical fiber is extracted from the furnace on a drum, but taking care to position the successive turns of the helix next to each other without overlapping. After the desired width has been obtained, a new layer is deposited by reversing the direction of spiraling of the fiber, the number of layers depending on the number of fibers that must make up the cable. After the desired number of layers has been achieved, the fibers on the drum are cut parallel to the axis of the drum. The process does not allow obtaining thinner fibers of about 20 μm, which is why the cable is reheated and stretched to obtain fibers with diameters of about 5 μm.
In general, the two types of optical cables, coherent and incoherent, have the same optical properties, although from some points of view differences may occur. For example, using insulation to prevent light from passing from one optical fiber to another makes the numerical aperture of coherent optical cables smaller due to increased attenuation of light rays that are more inclined to the axis. In addition, the effective propagation function becomes of special interest.
The insulation between the fibers is not perfect, so that parasitic light can appear in the fibers. When the illumination of the cable entry surface is maintained in the cone of light with the semicircle at the top i <imax, the stray light may be due to one of the following causes:
light penetrates through the material between the fiber core;
deviation from total internal reflection;
scattering light in the fiber or on its surface;
cable bending.
Any construction defect of the optical fibers can lead to image distortion. These distortions include dark spots due to torn or broken fibers and image distortions due to incorrect alignment of the fibers in the cable. In most cases, the deviations from the axial alignment cause a lateral displacement of the image.
CABLE APPLICATIONS
When used in lighting technology, fiber optics have several advantages over conventional systems, advantages that will be presented below:
a. Optical fibers allow the separation of the light source from the surface to be illuminated, a fact of essential importance especially in medical optical devices introduced into the body for visual inspection of various internal organs. The classical methods of observation based on the use of the incandescent lamp greatly complicate the optical system, do not allow obtaining sufficient illumination and present risks from the point of view of electrical connections. All these difficulties are removed if the lighting is done with a thin optical fiber.
b. Optical cables allow miniaturization, a crucial issue in applications that involve the use of multiple light sources.
c. Optical fibers can be used to illuminate measuring and control instruments. For example, an optical system may incorporate several instruments that, from a classical point of view, are illuminated separately using incandescent bulbs. Using a fiber optic cable illuminated by a single light source can divide the light beam into several beams, each of which is used to illuminate an instrument.
d. The method of coupling or disconnecting the different electrical connections, based on the use of optical fibers, ensures a high protection and acquires an increasing extension.
e. Controlling light sources located in hard-to-reach places opens up a wide range of applications for optical cables.
f. It is known that extended light sources have a low illumination efficiency of small surfaces, especially when they are rectangular slots of optical devices. The use of optical cables whose cross section varies continuously from circular to elongated shape has a potential advantage.
g. Optical fibers can be used to obtain multichannel correlators, the beams from different places can be summed as a single signal.
When used in communications systems, fiber optics offer multiple advantages over conventional systems. However, such applications must take into account not only the possibilities of distorting the transmitted signals but also the possibilities of destroying the optical fiber cables in time, especially due to the fragility of the glass fibers. The protection of the optical cables must be ensured against abrasion and contamination, against the tension at tension, and against the voltage due to bending. The protective layers, used to ensure the conditions imposed by the safe use of optical cables, can occupy an important part of the entire volume of the cable. Since the function of a communications system is to transmit information, such systems must be appreciated and compared in relation to the information capacity of a channel. From this point of view, the size of the information capacity is related to the decrease of the pulse scattering, due to both the material dispersion and the modal dispersion, and to the increase of the transmission power under a convenient signal / noise ratio.
As for the possibility of using coherent optical cables to transmit images from one place to another, we must start from the fact that it is impossible to place the optical fiber in contact with the object. The method is to form the image of the object on the input side of the cable using classical means. It is often necessary to enlarge the image formed on the output side of the cable, also using classical means. The coherent lens-cable combination of optical-ocular fibers is known as the fibroscope.
Fibroscopes already have multiple applications in both medicine and industry, especially for the control of internal surfaces to which access by classical means is not possible.
There are still many applications of optical fibers for obtaining life-size images, for making converter tubes with a swept beam or in ultrafast photography. The advances made so far in the field of optical fibers and those that will be obtained further open the way for the development of a new top field of optics, integrated optics.
OPTICAL FIBER
A technology that uses glass (or plastic) wires (fibers) to transmit data. A fiber optic cable consists of several glass wires, each of which is capable of transmitting messages at speeds close to the speed of light.
Fiber optics have several advantages over traditional metal communication lines:
fiber optic cables have a much higher bandwidth than metal cables; this means that they can carry a lot of data;
fiber optic cables are less susceptible to interference than metal cables;
fiber optic cables are much thinner and lighter than metal wires;
data can be transmitted digitally (the natural form of data on computers) instead of being transmitted analogically.
The main disadvantage of fiber optics is the high price of cable installation. In addition, they are much more fragile than metal wires and are harder to branch.
Fiber optics is a technology especially for local-area networks. Moreover, traditional telephone companies are gradually replacing telephone lines with fiber optic cables. In the future, almost all communications will use fiber optics.
Optical fibers are long and flexible cylinders with a diameter of 10-100μm, through which light rays propagate through multiple total internal reflections on the lateral surface of the fiber; there are also gradient optical fibers, characterized by the fact that the refractive index is maximum in the center of the fiber gradually decreases towards its periphery so that the total reflection of light is more complicated than in the case of simple optical fibers.
WHAT ARE OPTICAL FIBERS?
Optical fibers are thin, long strips of very fine glass the diameter of human hair.
They are arranged in bundles called optical cables and are used to transmit light signals over long distances.
If you look closely at a single fiber optic, you will see that it has the following parts:
core - the thin center of the fiber where light circulates;
the coating - the outer optical material that surrounds the core and reflects light back into it;
protective environment - plastic coating that protects the fiber from damage and moisture.
Hundreds or thousands of these optical fibers are arranged in bundles in the optical cable. The sheaves are protected by the outer sheath of the cable called clothing.
Optical fibers are of two types:
simple fibers - used to transmit a signal over the fiber (used for telephone and cable TV);
– fibre multiple – folosite sa transmiti mai multe semnale pe aceeasi fibra (folosite la retelele de calculatoare).
Fibrele simple au miezul foarte subtire (cam 3,5∙10-4 inci sau 9 microni in diametru) si transmit lumina laser inflarosu.
Fibrele multiple au miezul mai mare (cam 2,5∙10-3 inci sau 62,5 microni in diametru) si transmit lumina inflarosie de la o dioda luminoasa (LED). Unele fibre optice sunt facute din plastic. Acestea au un miez mai mare (0,04 inci sau 1 mm diametrul) si transmit lumina rosie din LED-uri.
Suppose you want to light a flashlight in a long, straight hallway. Simply point the flashlight at the hol- light flowing in straight lines, so it's no problem. What if the hall has a curve? Put a mirror in the corner to reflect the light. What if the hall had many curves? You could dress the walls in mirrors and direct the light so that it bounces from one wall to another in the hallway. This is exactly what happens in an optical fiber.
Diagram about the internal reflection of an optical fiber
Light in a fiber optic cable travels through the core (hallway) constantly bouncing off the casing (mirror walls), a principle called total internal reflection. Because the coating does not absorb any light from the core, the light wave can travel long distances. However, some of the light signals degrade into fiber, mainly due to impurities in the glass. How much the signal is damaged depends on the purity of the glass and the wavelength of the transmitted light. The best optical fibers do not damage the signal, less than 10% / km at 1550nm.
To understand how optical fibers are used in communication systems let's look at an example from a movie from World War II, where 2 ships in a fleet have to communicate with each other without radio signals or on rough seas. . The captain of a ship sends a message to a sailor on deck. The sailor translates the message into MORSE code (deck and lines) and uses a light signal (a powerful lamp with covers) to send the message to the other ship. The sailor on the other ship sees the MORSE code, decodes it into English, and sends the message up to the captain. Now, imagine doing this when the ships are each at the other end of the ocean separated by thousands of miles and you have a fiber optic communications system installed between the two ships.
A fiber optic transmission system consists of:
transmitter - produces and encodes light signals;
fiber optic - conducts light signals (long distances);
optical regenerator - may be required for signal amplification;
optical receiver - receives and decodes light signals.
THE TRANSMITTER
The transmitter is like the sailor on the deck of the ship emitting the signals. Receives and directs the optical device to focus light into the fiber. Lasers have more power than LEDs, but vary more with changes in temperature and are more expensive. The most common wavelength of the light signal is 850nm, and 1550nm (infrared and invisible parts of the spectrum).
OPTICAL REGENERATOR
As mentioned above, a signal loss occurs when light is transmitted through the fiber, especially over long distances (more than 1km), like an underwater cable. So one or more regenerators must be placed on the cable to amplify the degraded light signal. The optical regenerator consists of optical fibers with a special coating. The coated portion is laser pumped. When the degraded signal enters the shell, the laser energy allows the molecules to become lasers themselves. The molecules then emit a new, stronger light signal with the same characteristics as the weakly received signal. The regenerator is an amplifier for the input signal.
RECEPTORUL OPTIC
The optical receiver is like the sailor on the ship receiving the signal. It receives the input light signal, decodes it and sends it as an electrical signal to the other user, computer, TV, or ceiling (the captain of the other ship).
Why are fiber optic systems the telecommunications revolution? Compared to conventional wire, optical fiber is:
cheaper - a few miles of optical cable are cheaper than the same length of copper wire;
thinner - optical fibers can be pulled in smaller diameters than copper wire;
Higher carrying capacity - because optical fibers are thinner than copper wires, more fibers can be gathered in a cable of the same diameter. This allows multiple phone lines through the same cable or multiple TV channels;
less signal degradation - the loss of signal on optical fibers is less than the loss on copper wires;
light signals - unlike copper electrical signals, fiber electrical signals do not interfere with other wires in the cable. This means a better phone conversation or a better TV reception;
low power - optical fibers degrade less, smaller transmitters can be used;
digital signals - optical fiber is ideal for transmitting digital signals (widely used for computer networks);
non-flammable - because no electric current passes through the fibers, there is no risk of fire;
light weight - an optical fiber is lighter than a copper cable, takes up less space in the ground;
flexible - optical fibers are so flexible, they can transmit and receive light, they are used in flexible digital cameras for the following purposes:
- medical image;
- mechanical image;
- installations.
Because of these advantages, you see fiber optics in many industries, especially in telecommunications and computer networks. For example, if you make a phone call from Europe to the US or vice versa, and the signal was bounced by a communications system, you often hear an echo on the line. However, with transatlantic fiber optics you have a direct connection without echoes.
Now that we know how fiber optic systems work and why they are used, how are they made? Optical fibers are made of extra pure glass. We think of a window as being transparent, but the thicker the glass, the less transparent it becomes due to impurities. However, the impurities of the glass in a fiber are much less than in a window glass. A company's description of glass clarity is: "If you were above a fiberglass ocean you could see the ocean floor."
The production of fiber optics requires the following steps:
production of a preformed cylinder;
pulling the fibers through the cylinder;
fiber testing.
Mold production.
Mold glass is made from a special process called chemical modification of condensed vapors (MCVD). In MCVD it is ballooned by silicone chloride (Si Cl4), germanium chloride (Ge Cl4) and / or other chemicals. The precise mixture that governs the principle of physical and optical properties (refractive index, coefficient of expansion, melting point, etc.). The gas vapors are conducted in a silicone or quart tube, in a special lathe. As the lathe rotates, you put the cake in the tube, the cake is moved up and down outside the tube.
The extreme heat of the cake makes two things happen:
silicone and germanium react with oxygen to form silicon dioxide and germanium dioxide;
silicon dioxide and germanium dioxide are deposited in the tube merging together to form glass.
The lathe rotates continuously to make a perfect and consistent coating. The purity of the glass is maintained by using corrosion-resistant plastic in the gas injection system and by precisely controlling the flow rate of the mixture composition. The process of producing the mold is automatic and takes several hours. After it cools, it is tested for quality.
Once the mold has been tested it is installed in a fiber pulling tower. The ecological tower in a graphite furnace (2200˚C) and the top is melted until the substance falls, cools and forms a wire. A technological process is still going on.
Materials and technologies for obtaining optical fibers
General considerations:
In the field of optical fibers, which is in full development today, the research efforts crowned, so far, with notable successes are directed in two main directions, the first being the finding of materials with superior characteristics and the second - closely related to the first. of the most efficient technologies and installations, able to ensure the desired quality, at the most affordable costs.
Regardless of the composition chosen, the dielectric material used to obtain the optical fibers must meet the following requirements:
to have the best possible transparency at the wavelength of the light signal used;
to possess the best possible chemical stability in time;
to be easily processed in all phases of the technological process;
Based on the experience of fiber optic manufacturers, the most widely used materials can be grouped into three categories:
- pure silicon dioxide and mixtures thereof with other oxides in small quantities, also called dupanti;
- multi composite bottles;
- plastic materials.
If we consider the degree of processing of the materials mentioned above, the superiority of polymers is obvious, which does not require too high working temperatures. Although the use of plastics not only for the optical coating but also for the core is an interesting topic and experienced, the optical characteristics clearly inferior to those of glass recommend them only for short distance transmissions, where attenuation of the optical signal along the fiber is of secondary importance.
So here are some considerations for which the superiority of the first two types of materials is obvious, namely silicon-based and multicomposite bottles, which -in fact- have the same basic component-silicon dioxide. The differences between the two groups of materials appear most striking when it comes to choosing the processing technology for obtaining optical fiber. Of course, the performance of the final product -fiber- depends directly on the material used, but also on the production technology, but there is also a system of restrictions by which the material conditions the technology that makes possible its processing, so that the fiber optic results with the desired parameters. .
It can be stated that, given the wide range of compositions starting from pure silicon dioxide, to multicomposite bottles, the boundary between the two groups of materials is difficult to specify, but the most used compositions are at the end of the range.
Both pure silicon dioxide and multicomposite glass have an amorphous structure, are antisotropic and are drawn into liquid wires at high temperatures. The rapid cooling of the molten material leads to the formation of a stable and homogeneous glass, despite the transition through a thermal domain in which the completely undesirable appearance of crystals is possible.
Of all the technologies that will be further analyzed, the chemical vapor deposition is the one that allows obtaining a wide range of chemical compositions, from pure silicon dioxide to multicomposite glass resulting from the addition of considerable concentrations of additives in order to sensitive change of the refractive index. Given the continuous and predictable variation of the properties depending on the chemical composition, the parameter that clearly distinguishes the glass with high SiO2 content from the multicomposite one is the melting temperature and, implicitly, the fiber drawing. While the melting temperature of multicomposite glass is in the range of 800-1200 C, silicon dioxide melts at about 2000C.
In the first case, the relatively low working temperatures allow the use of traditional ovens in the double crucible method, being possible to easily obtain fibers with large numerical aperture but with a refractive index that varies in a very narrow range of values.
On the other hand, the chemical deposition technology in the vapor phase used to obtain optical fibers from bottles with a high SiO2 content eliminates a large part of the sources of glass impurity, which, in the case of the double crucible method, are inevitably more numerous, especially during storage and handling of raw materials. Or, precisely the impurities in the material determine the undesired increase of the optical signal dispersion over the intrinsic value of the Rayleigh dispersion. Also, the technology of chemical deposition in the vapor phase allows to easily obtain the desired profile of the refractive index and an optimal core-coating interface but involves installations and equipment with a higher degree of complexity.
The disadvantages of using bottles with a high content of silicon dioxide can be summarized as follows: in the phases of material deposition and fiber extraction the speeds are low and the processes take place at high temperatures, where realized. However, these disadvantages are fully compensated by the clearly superior quality of the optical fibers obtained by any of the few variants of the chemical deposition technology in the vapor phase.
TECHNOLOGIES FOR OBTAINING OPTICAL FIBERS FROM MULTICOMPOSITE GLASS: Materials used. Multicomposite glass optical fibers can be made using a wide range of materials, provided the necessary optical properties and workability required by the manufacturing process are ensured. For example, in the case of double-sided technology it is necessary that the two materials have lowered melting points to reduce the phenomenon of corrosion and contamination on this channel of the glass and the viscosity values to be close to the firing temperature to simplify the necessary equipment and for the process to have stability.
Of all the technologies that will be further analyzed, the chemical vapor deposition is the one that allows obtaining a wide range of chemical compositions, from pure silicon dioxide to multicomposite glass resulting from the addition of considerable concentrations of additives in order to sensitive change of the refractive index. Given the continuous and predictable variation of the properties depending on the chemical composition, the parameter that clearly distinguishes the glass with high SiO2 content from the multicomposite one is the melting temperature and, implicitly, the fiber drawing. While the melting temperature of multicomposite glass is in the range of 800-1200 C, silicon dioxide melts at about 2000C.
Source: www.referatele.com
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