The two main major types of optical fibers: plastic optical fibers (POF) and glass optical fibers – so how are optical fibers made?
1. Materials for optical fibers
Plastic optical fibers are often made for lighting or decoration like FTTH Cable Production Line. They are also applied to short range communication applications including on vehicles and ships. Because of plastic optical fiber’s high attenuation, they may have very limited information carrying bandwidth.
Whenever we speak about fiber optic networks and fiber optic telecommunications, we actually mean glass optical fibers. Glass optical fibers are mainly made from fused silica (90% at the very least). Other glass materials including fluorozirconate and fluoroaluminate will also be found in some specialty fibers.
2. Glass optical fiber manufacturing process
Before we start talking the best way to manufacture glass optical fibers, let’s first check out its cross section structure. Optical fiber cross section is a circular structure made up of three layers inside out.
A. The interior layer is referred to as the core. This layer guides the light and stop light from escaping out with 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 center layer is known as the cladding. It offers 1% lower refractive index compared to the core material. This difference plays an essential part overall internal reflection phenomenon. The cladding’s diameter is generally 125um.
C. The outer layer is known as the coating. It is actually epoxy cured by ultraviolet light. This layer provides mechanical protection for your fiber and makes the fiber flexible for handling. Without it coating layer, the fiber will be really fragile and simple to break.
Because of optical fiber’s extreme tiny size, it is not practical to create it in a single step. Three steps are required while we explain below.
1. Preparing the fiber preform
Standard optical fibers are created 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, good quality Fiber Drawing Machine preforms.
This process to create glass preform is known as 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 will be injected in to the hollow tube.
Because 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 occur.
A. The silicon and germanium interact with oxygen, forming silicon dioxide (SiO2) and germanium dioxide (GeO2).
B. The silicon dioxide and germanium dioxide deposit on the within 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. Right after the torch has reached the final in the tube, it is then brought back to the beginning of the tube and the deposited particles are then melted to create a solid layer. This procedure is repeated until a sufficient quantity of material continues to be deposited.
2. Drawing fibers on the drawing tower.
The preform will be 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 because it drops down.
This starting strand will then be 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 ltxsmu fiber diameter is precisely controlled by a laser micrometer. The running speed in the fiber drawing motor is all 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.
A. Tensile strength: Fiber must withstand 100,000 (lb/square inch) tension
B. Fiber geometry: Checks Sheathing Line core, cladding and coating sizes
A. Refractive index profile: The most critical optical spec for fiber’s information carrying bandwidth
B. Attenuation: Very crucial for long distance fiber optic links
C. Chromatic dispersion: Becomes more and more critical in high speed fiber optic telecommunication applications.