Apparatus

Fiber optic fibers are dielectric waveguides^W that take advantage of Total Internal Reflection^W to transmit light. A central core made of Silica^W (SiO2) and Germania^W, which ranges in diameter from 8um to 63 um.^[4][5] This is surrounded by a Silica sleave called the cladding.

The core and the cladding have different refractive indicies

Theory

Total Internal Reflection allows the fiber to reflect on the cladding boundary and contain the light within the core. From this, the optic fiber acts as a waveguide.

Total Internal Reflection occurs due to the differences in Refractive Indices between the core and the cladding of the fiber. The core always has a higher refractive index than the cladding because this will cause light beams to reflect inwardly and continue to travel from one end of the fiber to the other. The refractive index^W is kept low as higher indices retard the speed of light, thus slowing information transfer rates.

Chemical Reactions

The reactions that occur in the fabrication of optical fibers are found in the formation of the preform as described in the next section.

The reactions that occur are as follows:

SiCl4 + O2 -> SiO2 + 2Cl2

GeCl4 + O2 -> GeO2 + 2Cl2

Preform Manufacturing

A Preform is a cylindrical piece of material commonly measuring 1 meter long and 10 centimeters in diameter.

Modified Chemical Vapor Depos (MCVD)

MCVD is a process that uses a hollow Silica (SiO2) Tube as a blank and fills the interior with a solid Silica and Germania (Ge02) mixture.

The hollow tube is constantly rotated at 100 RPM about its center axis. Silicon Tetraoxide, Germanium Tetraoxide and Oxygen (02) gasses are pumped into the blank. An external heat source, which varies with available technology, is applied to the tube and transverses from one end to the other. The system reaches temperatures of 1200oC and creates a reaction in the gasses. Si02 and Ge02 form as particles and fuse to the outer diameter of the tube while residual gasses pass through the opposite end of the tube. With the transversing heat source, the entirety of the tube is filled with solid particles until only a thin passage remains in the center of the Preform. The heat is increased by reducing the speed at which the heat source transverses the tube and the Preform is collapsed eliminating the passage-way.

The end product is the Preform that is used in the next step of the process.^[6]

Overcladding

Overcladding is performed for one of two reasons.

1. Overcladding will increase the diameter of the preform so that it reaches the specifications required for Fiber Drawing in the next step.
2. Overcladding can create multiple cladding layers of varying refractive indices. This can be used to create a more beneficial refractive index profile to reduce attenuation in the Optical Fiber.

This process requires at least two hollow tubes made of cladding materials, one overclad tube must be bigger than the pre-existing preform core, the second overclad larger than the first overclad tube. The first overclad tube is positioned around the preform core rod and the second is positioned around the first overclad tube. A pressure gradient is produced where the pressure outside the second overclad tube is higher than the area inside the second overclad tube. This will cause the materials to fuse inwardly. A heat source provides heat to all three components causing the first overclad to collapse onto the preform core rod and the second overcald to collapse on the first overclad. This process produces a multiple overclad preform.^[7]

Fiber Drawing

Furnace

The furnace used in the drawing process has a longitudinal cavity in which the preform can be inserted vertically. The furnace is constructed of high temperature ceramics to allow high temperatures to be reached without causing changes in the furnace materials. An high frequency coil surrounds the vertical cavity and produces an electromagnetic field in order to heat the lower end portion of the preform and cause reflow in the material.^[8]

Preform Heating

The preform is heated in the furnace to the point where it begins to flow. A portion of the preform is pulled from the bottom of the furnace and inserted into the roller mechanisms. The roller will pull the preform from the furnace at regulated speeds to produce fibers of adequate diameter.^[9][10][11]

Response System

During the drawing process, dynamic measuring devices observe the diameter of the fiber being drawn at different areas of the process. These diameters are fed into a response system, which alters the speed of the rollers which pull on the fiber being formed from the heated preform material.

Laser measurement systems are commonly used because of their high accuracy and rapid response relay time.^[12]

Germania Content

The Germania content in a preform determines its refractive index. Using this, processors can alter the refractive properties of the core material and the cladding material.

Refractive Index Profile

The profile of the refractive index in optical fibers determine the quality of the optic fiber. Multimode optic are more affected by creating gradient indices, where as single mode^W optic fibers are commonly left as step-index fibers since their diameter is minuscule. A gradient refractive index causes light to travel through the fiber as a sinusoidal wave and never reach a critical angle of reflection with the cladding. This increases the fibers resistance to attenuation with length of the fiber.

Apparatus

• Two Randcastle extruders^W with 12.7 mm diameters
• Single screw
• coextrusion die block
• diffusion section
• Doped material set in feeder 1 for core fabrication
• Pure material set in feeder 2 for cladding fabrication^[13]

Process

The materials are heated to their subjective melting points and fed into the co-extrusion die block. The two materials form a core-cladding structure during extrusion. The material reaches the end of the diffusion sections and is drawn the same as conventional fiber drawing methods.^[14]

Variances in Process

Viscosity Parameter

The refractive index profile in the co-extrusion process is controlled by the viscosity of the materials under high-temperature plasticizing.^[15]

Diffusion

In the diffusion chamber of the process, a material is formed that has the doped material occupying the inner diameter and the outer radial layer being comprised of the pure undoped material. Under the heated conditions, diffusion occurs with the doped particles between the two concentrations of materials. Dopant diffuses outwards to the surface of the material. From simple diffusion laws, this action creates a parabolic refractive index profile in the radial direction of the material.^[16]