How Are Optical Fibers Made?
Now that we know how fiber-optic systems work and why they are useful, how do they make them? Optical fibers are made of extremely pure optical glass. We think of a glass window as transparent, but the thicker the glass gets, the less transparent it becomes due to impurities in the glass. However, the glass in an optical fiber has far fewer impurities than windowpane glass.
Making optical fibers requires the following steps:
- making a preform glass cylinder
- drawing the fibers from the preform
- testing the fibers
Making the Preform Blank
In MCVD, oxygen is bubbled through solutions of silicon tetrachloride (SiCl4), germanium tetrachloride (GeCl4), phosphorus trichloride (PoCl3) and/or other chemicals. The precise mixture governs the various physical and optical properties (index of refraction, coefficient of expansion, melting point, etc.). The gas vapors are then conducted to the inside of a synthetic silica or quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up and down the outside of the tube. The extreme heat from the torch causes two things to happen:
- The silicon and germanium react with oxygen, forming silicon dioxide (SiO2) and germanium dioxide (GeO2).
- The silicon dioxide and germanium dioxide deposit on the inside of the tube and fuse together to form glass.
The lathe turns continuously to make an even coating and consistent blank. The purity of the glass is maintained by using corrosion-resistant plastic in the gas delivery system (valve blocks, pipes, seals) and by precisely controlling the flow and composition of the mixture. The process of making the preform blank is highly automated and takes several hours. After the preform blank cools, it is tested for quality control (index of refraction).
Drawing Fibers from the Preform Blank
Once the preform blank has been tested, it gets loaded into a fiber drawing tower.
The blank gets lowered into a graphite furnace (3,452 to 3,992 degrees Fahrenheit or 1,900 to 2,200 degrees Celsius) and the tip gets melted until a molten glob falls down by gravity. As it drops, it cools and forms a thread.
The operator threads the strand through a series of coating cups (buffer coatings) and ultraviolet light curing ovens onto a tractor-controlled spool. The tractor mechanism slowly pulls the fiber from the heated preform blank and is precisely controlled by using a laser micrometer to measure the diameter of the fiber and feed the information back to the tractor mechanism.
Fibers are pulled from the blank at a rate of up to 66 ft/s (20 m/s) and the finished product is wound onto the spool. It is not uncommon for individual spools to contain miles or kilometers of optical fiber.
Testing the Finished Optical Fiber
The finished optical fiber is tested for the following:
- Tensile strength. Must withstand 100,000 lb/in2 or more
- Refractive index profile. Determine numerical aperture as well as screen for optical defects
- Fiber geometry. Core diameter, cladding dimensions and coating diameter are uniform
- Attenuation. Determine the extent that light signals of various wavelengths degrade over distance
- Information carrying capacity (bandwidth). Number of signals that can be carried at one time (multi-mode fibers)
- Chromatic dispersion. Spread of various wavelengths of light through the core (important for bandwidth)
- Operating temperature/humidity range
- Temperature dependence of attenuation
- Ability to conduct light underwater. Important for undersea cables
Once the fibers have passed the quality control, they are sold to telephone companies, cable companies and network providers. Most companies have replaced their old copper-wire-based systems with new fiber-optic-based systems to improve speed, capacity and clarity.