Making Optical Fibers

BY: Christina Hansen


The impact of the technology of optical fiber in our communication system is astounding. Many have wondered how these optical fibers are made. There are several steps involved in making an optical fiber, which include making a preform glass cylinder and drawing fibers from the preform.

Optical Glass

Optic fiberglass, a replacement for copper wires, is an ultra-high-purity silica glass that can be stretched into long, hair-thin fibers and used to transmit information over long distances. Fiber optic strands consist of an inner core of high purity glass with a high refractive index that transmits light, and an outer core of low refractive glass that keeps the light signal from seeping out the sides. The basic unit from which fibers are drawn is called a "preform."

Preform Glass Cylinder

A preform is a glass cylinder that might be several inches long and several inches thick with a different refractive index to provide the core and cladding of the fiber. The fiber's capability to reflect light is decided by the creation of the cladding glass relative to the core glass. The reflection usually occurs by creating a higher refractive index in the core of the glass than in the surrounding cladding glass.

Modified chemical vapor deposition, which is a chemical process used for producing high-performance solid materials, is used for making the glass for the preform. Vapor deposition, outside vapor deposition, and vapor axial deposition are the three methods usually used in this process.

The most common process used is the outside vapor deposition because it yields a low-loss fiber that is very well suited for long-distance cables. This process is highly automated and usually takes several hours for completion. Once the preform blank is cool, it is tested for quality control and then placed into a fiber drawing tower.

Drawing Fibers from Preform

The drawing tower has a temperature of 3,452 to 3.992 degrees Fahrenheit or 1,900 to 2,200 degrees Celsius. This tower consists of a furnace that heats the tip of the blank until a piece of molten glass falls from the blank, pulling a thin strand of glass, which is the beginning of an optical fiber. Then the fiber goes through a monitor to ensure a specified outside diameter. Next, ultraviolet lamps are used for applying and curing coatings. The fiber is wound on spools at the bottom of the draw and each fiber is assigned a unique identification number. The optic fiber cables are formed by being coated, colored and bundled in protective jackets.

Once the optical fibers are finished they are tested for bandwidth, tensile strength, fiber geometry, operating temperature and range of humidity, refractive index profile, attenuation, and temperature dependence of attenuation. Cables that are going to be used undersea must have the ability to conduct light under the water.

From Optical Fiber Production to Many Uses

The fibers, after they have passed quality control, are sold to many companies to improve capacity and speed, and to replace their old copper wire systems. Some types of companies that use fiber optics include network providers, telephone, power, and cable companies, industrial plants, and computer technicians.