Fiber Optic Cables

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Fiber Optic CableS

Contents

Introduction:

Material Preprocessing & Characterization :

Outside Vapor Deposition (OVD)

Laydown

Consolidation

2 Vapor Axial Deposition process (VAD)

Modified Chemical Vapor Deposition: This

Fabrication (Manufacturing):

Fiber Cabling: (Post Process)

Comparison of Different Manufacturing Techniques

Introduction:

An optical fiber is a single, thin hair filament made by molten silica glass. These fibers are now replacing metal wires as the transmission medium has a high speed and capacity which converts information into light and then is transmitted via fiber optic cable.

The optical fiber communication system consists on cables made up of optical fibers connects data links having lasers and light detectors. For the transmission of information a datalink converts an electronic signal, telephonic conversation or a video into digital pulses of laser light. They travel over the optical fiber to other data link, where a light detector reconverts the signals into an electronic signal which can be received on receiver.

A optical fiber cable consists on many separate optical fibers which are bound together around a steel or plastic carrier for support present in the center. This is called core and is then covered with protective layers of protective materials such as aluminum, Kevlar, and polyethylene called cladding. As the core and cladding are of different materials the speed of light differs in both of the materials.

When a light wave travels in a fiber core and reaches the boundary of core and cladding, the difference in the composition between these two makes the light wave to bend back into the core. Hence when a pulse of light travels through an optical fiber, it continuously bounces away from the cladding. The moving speed of a pulse within the optical fiber is 186,290 miles per second. It loses its energy because of the impurities in the glass and absorption by irregularities of glass structure.

optical fibers have two types based on their structure. In a single-mode fiber, the size of the core is smaller around10 micrometers in diameter, and the diameter size of cladding is100 micrometers. A single-mode fiber carries only one light wave but to a very long distance. Bundles of single-mode optical fibers are used in long-distance telephone lines and undersea cables. Multimode optical fibers having a core diameter of 50 micrometers and 125 micrometers of cladding can carry hundreds of separate light wave signals but the distance travelled by them is very short. They are used in urban systems where many signals must be carried to central switching stations for distribution.

The use of fiber optics is increasing day by day as it is a better source of transferring data as compared to conventional copper wires. It expensive but the quality of signal transmission is very good as compared to other source. More over the data carrying capacity and the speed makes it un parallel.

Material Preprocessing & Characterization :

The basic material for optical fibers is silica (SiO2). The refractive index of silicon dioxide is adjusted by doping technique. The dopant most commonly used to increase the refractive index is germanium dioxide (GeO2) and Phosphorus Pent oxide (P2O5). The dopant used for lowering the refractive index is fluorine (F). For the manufacturing of low-loss optical waveguides a manufacturing process of high-purity is required particularly for the fiber core. For this purpose glass fibers are produced using chemical deposition from the vapor phase. The starting point for the process is chlorides for example SiCl4.To obtain fused silica oxidation of Sicl4 is carried out. As the reaction is reversibleso the temperature should be kept in a specific range to avoid the reaction to move in the opposite direction and complete conversion of SiO2 the temperature is kept above 1800 K. After doping process one of the Vapor Deposition technique is used for the deposition of SiO2.

Outside Vapor Deposition (OVD)

Outside Vapor Deposition consists of three steps: laydown, consolidation, and draw.

Laydown In this process a preform of soot is made from ultra-pure vapors as they travel from a burner and react with the flame to form fine soot particles of glass. The OVD process is different from other methods because of the method by which the soot is deposited. These soot particles are then moved on and are deposited on the surface of a rod which is rotating continuously. The deposition of cure material takes place before the deposition of pure silica cladding. The core and cladding both are raw materials and are deposited using vapor deposition so the preform becomes very pure as all the impurities are removed as they are converted to vapor form.

Consolidation After the completion of deposition process the bait rod on which all the material is deposited is removed from the center of the preform,at that time the pre form is in porous state and it is then placed in a furnace called consolidation furnace. During the process of consolidation the water vapor present in preform is removed from it. This is a high-temperature process which sinters the preform into a solid, dense, transparent glass. The finished glass preform is then placed on a draw tower and where it is drawn into a continuous fiber of glass. For this process first, the glass blank is lowered into the top of the draw furnace and the tip of blank is heated till a piece of molten glass is formed which starts to fall from the blank and is called gob. As the gob falls it pulls a thin strand of glass behind it. The gob is then cut off and the fine fiber strand is threaded into an automated assembly and drawn. T the diameter of the fiber is monitored carefully and to keep its diameter same throughout the speeds of the assembly increases or decreases simultaneously. The fiber then passes through a sensor that measures the diameter hundreds of times per second to ensure the specified outside diameter. After this process primary coatings are applies on the inner and outer sides and cured using ultraviolet lamps. At the end of the drawing process the fiber is wound on spools for further processing. Fiber from these spools is proof-tested and measured for performance of relevant optical and geometrical parameters. Each fiber is given a unique identification number that can be traced from all relevant manufacturing data (including raw materials and manufacturing equipment).

2 Vapor Axial Deposition process (VAD)

The properties which differentiates VAD from other vapor phase deposition methods is that it can be used as a continuous process for production of optical fiber preforms . VAD process involves several steps, dehydration and a consolidation phase follows the "porous preform" fabrication, but the growing process continues axially and the porous preform is grown gradually and extracted from the deposition chamber.it enters in a second chamber where drying gases (He-Cl2 or He-SOCl2)are flown before consolidation phase is carried out in an electrical furnace. The result achieved by this process which is the reduction of OH groups is outstanding. The most acute part of the VAD plant is the deposition chamber.in this chamber the core and cladding both are deposited all together and the position of the deposition burners should be carefully evaluated to attain the calculated refractive index of the fiber. To ensure a perfect symmetry of the deposition The preform is extracted slowly and is rotated on its axis while being pulled. A slight fluctuation in preform position can cause a change in diameter and irregularity in the refractive index profile. To prevent this irregularities’ the growth zone is kept motionless and is continuously monitored by a TV camera. Another important factor which influence the quality and the reproducibility of the preform and the regularity of the refractive index is burner shape. The burner which deposits the cladding is positioned perpendicular to the growing preform and the refractive index is produced by acting on the three-dimensional distribution of dopants in the flame. Mixtures of different dopant concentrations of the starting compounds are being flowed in different areas of the burner which allows to get complex profiles for example in "graded index" multimode fibers. A careful control of the surface temperature in the growing preform is important Ge in particular needs a temperature between 500 and 800 degree C because belowthis temperature there is not incorporation in the glass network instead it is deposited in crystalline form which can be sublimated during the consolidation process. On the other hand over 800 Degree Celsius the concentration of Ge drops because of the sublimation caused by its high vapor pressure.

The major advantage of the VAD processes includes the high rate of deposition and the ability to produce large preforms. The typical deposition rateis around 0.4 but methods have been developed for increasing the value of deposition rate by the addition of deposition burners of clad or optimizing the geometry of existing burners. ((TECH))

Modified Chemical Vapor Deposition: This is process was developed at Bell Labs in 1974. After the development of this deposition method, it is widely being used in semiconductor technology. It was initially using Hydride compounds (Silane and Germane) which were substituted successively by Chlorides to avoid the formation of OH during the deposition. The reactants are passed through the reaction tube together along with an oxidizing mixture inert in nature of carrying gases in measured quantities by using Mass Flow Controllers and is oxidized in vicinity of the heated zone.

A short area of the silica tube is heated by using an external burner which is continuously moving in the direction of the flow of reactant along the rotating tube. The rotating tube is attached on a glass working lathe. The reactants, SiCl4, and various dopants which were entered at low temperatures are heated on approaching the hot zone of the traversing torch. A circular temperature rise occurs due to the flowing gas next to the wall of the containing tube. The tubes get heated up more rapidly and reach higher temperature than the gas. A standardized gas phase reaction occurs where the gas temperature reaches about 1200 degree Celcius and solid particles nucleate from the reaction products. as the finely divided solids suspended in the gas pass through the hot zone Particle development arises as the result of thickening of striking particles. A soot particle which is placed in a field with a temperature difference move towards the cool area and as a result of the impact with particles at higher temperature. This process is called thermo phonetic effect and has been verified theoretically and experimentally to manage the deposition on the tube wall downstream the hot reaction zone. “The flux of the particles towards the wall is proportional to the temperature gradient and hence the thermo phonetic velocity Burner movement causes the hot zone to shift onto the zones where silica has been deposited as soot by thermophoresis. it is fused and vitrified by the following cooling. The translucent glassy layer has usually a thickness variable between 15 and 100 pm, subject on process flow and temperature. At the end of the cycle, the burner returns quickly to its initial position and another glass layer deposition can start. The composition of the gas and process condition can be changed in order to produce different kinds of glass layers as per the type of fiber which is to be manufactured. Numerous cladding layers are placed, followed by the core layers which can be several in the case of graded-index multimode fibres, in which reaction gas At the end of the glass deposition, a collapse phase is started: the tube's internal hole is progressively reduced by successive slower burner passes (typically 5-10) at higher temperature (1800-2000 CC), due to the action of surface tension when the high temperature lowers the viscosity of the glass until a solid cylindrical rod is produced, with a refractive index profile equal to the final optical fiber profile. During phase of high temperature, it can happen that some of the deposited oxides are favorably vaporized conditional on their vapor pressure, causing a changein the refractive index profile in the center of the preform. This is a typical case of Germanium Oxide which evaporates favorably from the silica network as GeO and yields an index concavity which can cause some problems in the communicable bandwidth to a affected level. It is possible to reimburse this defect in the drawing process. For example increasing the content of Ge in the disintegrating atmosphere during the final phaseor chemically etching the innermost glass layer with fluorinated compounds in the latter collapsing pass to eliminate the dopant depleted glass layers .The process of deposition is not very effective and the deposition rates are very low (-0.5 g/min), but the MCVD technique has been mostly used by maximum research laboratories, because of the great usefulness of this method. (Yeh, 1990.)

Fabrication (Manufacturing):

A fiber drawing system is composed of high temperature furnace, fiber diameter measurement system, coating applicator, centering device, UV curing apparatus, draw tension gauge and take up capstan in a mechanically stable tower that is normally taller than 8 meter.

The major processes in fiber drawing can be divided into three main areas including heating zone, cooling zone and coating zone. The heating zone that contains a high temperature furnace and a temperature Controller which relaxes an optical fiber preform to form a neck-down region where bare glass fiber is drawn continuously. The softening point of silica glass ranges from 1400 to 2350 degree centigrade. The viscosity of silica glass in the temperature of 1935 to 2322 degree centigrade which varies from 05.86 to 104.63 poises. To heat the silica preform to this temperature range an electrical high temperature furnace is used with an optimum heating zone settings. A broad temperature distribution along the furnace yields a slight Temperature change across the preform diameter to form a slowly varying neck-down area and offers a low fiber draw tension. The thin heat zone brings an extremely high draw tension particularly in case of drawing a fiber from a large preform, to bring strength deprivation due to the fatigue effect, producing a very weak fiber and causing the fiber to break during drawing. Temperature distribution and its significant neck-down region formation are important factors in the heating zone. Pristine plain glass fiber illustrates a very high strength which is approximately 70,000 kg/cm2 in air. But it is easily ruined due to surface damage when a uncoated fiber comes in contact with an external particle or matter. Thus, it is important to coat an optical fiber to protect their surface.

In case of high speed drawing, the temperature of the fiber entering coating material in the coating applicator plays a very critical role to define the quality of the coating. So it must be kept under a definite point to avoid the coating material from overheating. Else the meniscus made by the fiber entering the coating material can collapse in a result of improper wetting as well as burning the material or losing the integrity of material. Therefore the cooling zone right after the heating zone should be followed to control the temperature to ensure high speed coating. the coating zone contains coating die having liquid pre polymer and the polymer curing unit. Detailed viscous flow of liquid pre polymer in a coating die can expressively disturb the final coating quality and mechanical strength. In high-speed coating, the shear rate becomes so large that a fiber coated at a high speed often shows poor strength. It is supposed that high draw tension and viscous friction on the fiber surface by pre polymer solution in the coating die may add to the weak fiber strength. The common practice for reducing a high shear force in fiber manufacturing is to heat the coating applicator to reduce the polymer viscosity. The viscosity level set for coating is normally in the range of 10 poises. (Process, Introduction to fiber optic communications)

Fiber Cabling: (Post Process)

Fiber cabling should be done in order to avoid mechanical stress on the fiber and micro bending. A commonly used method for Fiber cabling is the hollow core technology in which up to 20 fibers are placed loosely in a hollow core. Generally the hollow core is filled with a gel-like filler compound after placing the fibers. Cables can be made from numbers of the hollow cores bound together. (R. Engel, 2002)

Comparison of Different Manufacturing Techniques (Process)

Sr. #

Feature

OVD

VAD

MCVD

1

Basic

approach

Deposition of core,

then cladding on a

removable mandrel

End-on deposition of

core on bait rod,

deposition of clad

from side

Deposition of core

layers inside

substrate tube, which

becomes outer

cladding

2

Reaction

mechanism

Flame hydrolysis of

chlorides using

methane fuel

Flame hydrolysis of

chlorides using

H2/O2 flame

High temperature

gas phase oxidation

of chlorides

3

Deposition

mechanism

Thermophoretic

deposition of

particles on mandrel,

and soot over it

Thermophoretic

deposition of

particles on bait rod,

and soot over it

Thermophoretic

deposition of

particles on inner

tube wall

4

Sintering

Separate viscous

sintering step for

soot body

Separate viscous

sintering step for

soot body

Viscous sintering of

particles

simultaneous with

deposition of layer

Conclusion:

The Fiber optic technology is one of the growing technology of the world and is taking over coming the conventional wiring because of its many characteristics.

One of the major properties of fiber optic is the transfer of signals with a very good quality and less deterioration and loss. Because of the modern technology its becoming comparatively cheap in a sense that it can carry very large amount of data at once as compared to conventional wiring and as the data carrying capacity is considerably high so it is becoming a preferable media for the transfer of information.

One of the major drawbacks of this technology is that it is very sensitive and a very slight change in temperature or any mechanical effect can easily damage and the damage is irreparable causing the complete change of wire which is very costly.

The scientist are working to develop new techniques for the manufacturing of fiber optics and also working on the materials to be used instead of conventional materials they are trying to discover earth materials that will be cheap and can give better performance than other sources present.

Reference:

  • R. Engel, ”Lichtwellenleiterkabel”, in E. Voges, K. Petermann, ”Optische Kommunikationstechnik”, Springer, 2002).
  • Chapter 3. Preform Fabrication and Optical Fiber Drawing Process “Introduction to fiber optic communications”
  • Chapter 6 Manufacture of optical fibers (TECH) “Introduction to fiber optic communications”
  • Yeh, Chai.Handbook of Fiber Optics.Academic Press, 1990.
  • Shuford, Richard S. "An Introduction to Fiber Optics,"Byte.December, 1984, p. 121.


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