There are two major varieties 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 generally made for lighting or decoration such as Fiber Coloring Machine. They are also used on short range communication applications like on vehicles and ships. Because of plastic optical fiber’s high attenuation, they may 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 mainly produced 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 how you can manufacture glass optical fibers, let’s first have a look at its cross section structure. Optical fiber cross section is a circular structure composed of three layers inside out.
A. The interior layer is called the core. This layer guides the light preventing light from escaping out by way of 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 known as the cladding. It has 1% lower refractive index compared to the core material. This difference plays a crucial part overall internal reflection phenomenon. The cladding’s diameter is normally 125um.
C. The outer layer is called the coating. It is in reality epoxy cured by ultraviolet light. This layer provides mechanical protection for the fiber and helps make the fiber flexible for handling. Without it coating layer, the fiber will be very fragile and easy to break.
Due to optical fiber’s extreme tiny size, it is not practical to create it in a single step. Three steps are needed since we explain below.
1. Preparing the fiber preform
Standard optical fibers are made by first constructing a large-diameter preform, having a carefully controlled refractive index profile. Only several countries including US have the ability to make large volume, top quality SZ Stranding Line preforms.
The process to create glass preform is referred to 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) or other chemicals. This precisely mixed gas will be injected into the hollow tube.
Since the lathe turns, a hydrogen burner torch is moved up and down the outside 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 inside the tube and fuse together to create glass.
The hydrogen burner will be traversed up and down the size of the tube to deposit the material evenly. After the torch has reached the end of the tube, this will make it brought back to the start of the tube as well as the deposited particles are then melted to form a solid layer. This process is repeated until a sufficient level of material has been deposited.
2. Drawing fibers over a drawing tower.
The preform will be mounted for 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 since it drops down.
This starting strand is then pulled through a number of buffer coating cups and UV light curing ovens, finally onto a motor controlled cylindrical fiber spool. The motor slowly draws the fiber from your heated preform. The ltxsmu fiber diameter is precisely controlled by a laser micrometer. The running speed of the fiber drawing motor is approximately 15 meters/second. As much as 20km of continuous fibers can be wound onto just one 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 Fiber Drawing Machine core, cladding and coating sizes
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 a lot more critical in high speed fiber optic telecommunication applications.