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How Are Optical Fibers Made?

1. Materials for optical fibers

There are two major types of optical fibers: plastic optical fibers (POF) and glass optical fibers

Plastic optical fibers are usually made for lighting or decoration such as fiber optic christmas trees. They are also used on short range communication applications such as on vehicles and ships. Because of plastic optical fiber's high attenuation, they have very limited information carrying bandwidth.

When we talk about fiber optic networks and fiber optic telecommunications, we actually mean glass optical fibers. Glass optical fibers are mostly made from fused silica (90% at least). Other glass materials such as fluorozirconate and fluoroaluminate are also used in some specialty fibers.

2. Glass optical fiber manufacturing process

Before we start talking how to manufacture glass optical fibers, let's first take a look at its cross section structure.

Optical fiber cross section is a circular structure composed of three layers inside out.

A. The inner layer is called the core. This layer guides the light and prevent light from escaping out by a phenomenon called total internal reflection. The core's diameter is 9um for single mode fibers and 50um or 62.5um for multimode fibers.

B. The middle layer is called the cladding. It has 1% lower refractive index than the core material. This difference plays a vital part in total internal reflection phenomenon. The cladding's diameter is usually 125um.

C. The outer layer is called the coating. It is actually epoxy cured by ultraviolet light. This layer provides mechanical protection for the fiber and makes the fiber flexible for handling. Without this coating layer, the fiber will be very fragile and easy to break.

Because of optical fiber's extreme tiny size, it is not practical to produce it in a single step. Three steps are required as we explain below.

1). Preparing the fiber preform

Standard optical fibers are made by first constructing a large-diameter preform, with a carefully controlled refractive index profile. Only several countries including US have the ability to make large volume, high quality fiber preforms.

The process to make glass preform is called MOCVD (modified chemical vapor deposition).

In MCVD, a 40cm long hollow quartz tube is fixed horizontally and rotated slowly on a special lathe. Oxygen is bubbled through solutions of silicon chloride (SiCl4), germanium chloride (GeCl4) and/or other chemicals. This precisely mixed gas is then injected into the hollow tube.

As the lathe turns, a hydrogen burner torch is moved up and down the outside of the tube. The gases are heated up by the torch up to 1900 kelvins. This extreme heat causes two chemical reactions to happen.

A. The silicon and germanium react with oxygen, forming silicon dioxide (SiO2) and germanium dioxide (GeO2).
B. The silicon dioxide and germanium dioxide deposit on the inside of the tube and fuse together to form glass.

The hydrogen burner is then traversed up and down the length of the tube to deposit the material evenly. After the torch has reached the end of the tube, it is then brought back to the beginning of the tube and the deposited particles are then melted to form a solid layer. This process is repeated until a sufficient amount of material has been deposited.

2). Drawing fibers on a drawing tower

The preform is then mounted to the top of a vertical fiber drawing tower.

The preforms is first lowered into a 2000 degrees Celsius furnace. Its tip gets melted until a molten glob falls down by gravity. The glob cools and forms a thread as it drops down.

This starting strand is then pulled through a series of buffer coating cups and UV light curing ovens, finally onto a motor controlled cylindrical fiber spool.

The motor slowly draws the fiber from the heated preform. The formed fiber diameter is precisely controlled by a laser micrometer. The running speed of the fiber drawing motor is about 15 meters/second. Up to 20km of continuous fibers can be wound onto a single spool.

3). Testing finished optical fibers

Telecommunication applications require very high quality glass optical fibers. The fiber's mechanical and optical properties are then checked.

Mechanical Properties:

A. Tensile strength: Fiber must withstand 100,000 (lb/square inch) tension
B. Fiber geometry: Checks fiber's core, cladding and coating sizes

Optical Properties:

A. Refractive index profile: The most critical optical spec for fiber's information carrying bandwidth
B. Attenuation: Very critical for long distance fiber optic links
C. Chromatic dispersion: Becomes more and more critical in high speed fiber optic telecommunication applications

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