11. Optical Fiber Technology

Optical fiber is used worldwide for transmission ofvoice, data, and content because of its ability to transmitat speeds in excess of 10 GB/second over very longdistances.

Optical fibers consist of:

1. A cor e , havi ng hi gh refractive index.

2 . Cladding. 3 . Buffer , protective polymer

layer. 4 . Jacket , protective polymer

layer.

Two methods to manufacture optical glass fiber:

Nowadays most optical fibers are made from the preform. There are three steps in this method:

1. Draw the fiber from molten glasses, which are placed in two concentric crucibles (Double Crucible method)2. Draw from a glass rod called preform.

1. Fabrication of the preform2. Drawing the fiber from the preform3. Coating and jacketing process

Preform Fabrication: Chemical Vapor Deposition (CVD)

1. Modified Chemical Vapor Deposition (MCVD)

2. Plasma Modified Chemical Vapor Deposition (PMCVD)

3. Plasma Chemical Vapor Deposition (PCVD)

4. Outside Vapor Deposition (OVD)

5. Vapor-phase Axial Deposition (AVD)

1. All these methods are based on thermal chemical vapor reaction that forms oxides2. They are deposited as layers of glass particles called soot3. Starting materials are solutions of SiCl4, GeCl4, POCl3, and gaseous BCl3.4. These liquids are evaporated within oxygen stream and form silica and other oxides.5. Chemical reactions proceed as follows:

Chemical Vapor Deposition (CVD)

SiCl4 + O2 -> SiO2 + 2 Cl2GeCl4 + O2 -> GeO2 + 2 Cl24 POCl3 + 3 O2 -> 2 P2O5 + 6 Cl24 BCl3 + 3 O2 -> 2 B2O3 + 6 Cl2

- Germanium dioxide and phosphorus pentoxide increase the refractive index of glass

- Boron oxide decreases the refractive index of glass.

- These oxides are known as dopands.

- Changing composition of the mixture during the process influences refractive index profile of the preform

Chemical Vapor Deposition is chemical reactions which transform gaseous molecules,

called precursor, into a solid material , in the form of thin film

or powder, on the surface of a substrate .

CVD

Vaporization and Transport of Precursor Molecules into Reactor

Diffusion of Precursor Molecules to the Surface

Adsorption of Precursor Molecules to Surface

Decomposition of Precursor and Incorporation into Solid Films

Integrated circuits, optoelectronic devices and sensors

micromachines, and fine metal and ceramic powders

Nanothnic protective coatings - Titanium Carbide-Coated Graphite

CVD techniques can be employed to grow a thin layer of protecting materials

Applications may be found in the areas of:

Modified Chemical Vapor Deposition (MCVD)

1. This method was developed by Bell Laboratories.2. The gaseous mixture of reactants is fed at the end of a rotating silica tube.3. This tube is heated by a traversing oxygen-hydrogen burner.4. As a result of chemical reactions glass particles, called soot, are formed.5. These particles are deposited on internal wall of the tube.6. The soot is then vitrified by the traversing burner to provide a thin glass layer.7. The process is repeated many times as the cladding layers and core layers are formed.8. When the deposition is finished, the temperature of the burner is increased to collapse the tube into a solid preform.9. The entire process is highly automated and all process parameters are precisely controlled.

MCVD Process

Plasma Modified Chemical Vapor Deposition (PMCVD)

A modification of MCVD method is a process known as PMCVD.In addition to the normal MCVD technique the radio-frequency coil aroundthe tube generates an internal high temperature plasma.

Plasma Chemical Vapor Deposition (PCVD)

The PCVD method is similar to PMCVD. The radio-frequency coil is replacedby a microwave cavity resonator. In this method reactions lead directly to of glasslayer without forming the soot.

Outside Vapor Deposition (OVD)

1. This process is also called the “soot process”.2. It was exclusively used by Corning since the 1970s, and the patent of such a technology has expired since July 2000.3. Halogens and O2 react in a hot flame to form hot glass soot, which is deposited layer by layer on an aluminium oxide or graphite mandrel.4. The central mandrel is removed after deposition.5. In the last step, called sintering, a hollow porous preform is dehydrated and collapsed in controlled atmosphere, (e. g. helium) to form desired preform.

OVD Process

Vapor-phase Axial Deposition (VAD)

1. In VAD method, the preform can be fabricated continuously.2. Starting chemicals are carried from the bottom into oxygen-hydrogen burner flame to produce glass soot which is deposited on the end of a rotating silica rod.3. A porous preform is then grown in the axial direction.4. The starting rod is pulled upward and rotated in the same way as that used to grow single crystals.5. Finally the preform is dehydrated and vitrified in ring heaters.6. This process is preferred for the mass production.

VAD Process

Fiber Drawing

1. Optical fibers are obtained by drawing from the preform at high temperature.2. The drawing process must be integrated with the coating process to avoid contamination of fiber surface.3. The tip of the perform is heated in a furnace to a molten state.4. Formed molten gob falls down under the force of gravity while shrinking in diameter into a proper diameter strand.5. It is controlled continuously during the drawing process.6. Diameter drift cannot exceed 0.1%.7. The strand is threaded through a series coating applicators immediately after drawing.8. Liquid prepolymer coatings are cured by thermal or ultraviolet apparatus.

9. Dual coating, soft inner and hard outer, is needed to avoid microbending and protect against impact and crushing forces in either manufacturing process or installation.10. Silicone coating and acrylate, Tefzel (ETFE), Teflon (PFA), nylon buffers are applied during the fiber drawing, while additional materials such as Hyrtel and PVC can be extruded after the draw process.11. The fiber with coatings is pulled down and wound on a winding drum.12. The drawing process must take place in air conditioned room, because air pollution influences fiber attenuation.