There are 2 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 generally designed for lighting or decoration like Secondary Coating Line. They are also applied to short range communication applications including on vehicles and ships. As a result of plastic optical fiber’s high attenuation, they have restricted information carrying bandwidth.
Once we speak about fiber optic networks and fiber optic telecommunications, we actually mean glass optical fibers. Glass optical fibers are generally created from fused silica (90% a minimum of). Other glass materials including fluorozirconate and fluoroaluminate are also utilized in some specialty fibers.
2. Glass optical fiber manufacturing process
Before we start talking how to manufacture glass optical fibers, let’s first check out its cross section structure. Optical fiber cross section is actually a circular structure composed of three layers inside out.
A. The interior layer is called the core. This layer guides the light and prevent 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 called the cladding. It has 1% lower refractive index compared to core material. This difference plays a crucial part in total internal reflection phenomenon. The cladding’s diameter is usually 125um.
C. The outer layer is called the coating. It is in reality epoxy cured by ultraviolet light. This layer provides mechanical protection for your fiber and helps make the fiber flexible for handling. Without it coating layer, the fiber can be really fragile as well as simple to break.
Due to optical fiber’s extreme tiny size, it is not practical to generate 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 sizable-diameter preform, with a carefully controlled refractive index profile. Only several countries including US have the capacity to make large volume, good quality Secondary Coating Line preforms.
This process to make 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 the 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 from the torch up to 1900 kelvins. This extreme heat causes two chemical reactions to happen.
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 of the tube and fuse together to form glass.
The hydrogen burner is then traversed up and down the size of the tube to deposit the content evenly. After the torch has reached the end from the tube, it is then brought back to the start 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 has been deposited.
2. Drawing fibers over a drawing tower.
The preform will be mounted for the top of the 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 through the heated preform. The ltxsmu fiber diameter is precisely controlled by a laser micrometer. The running speed in the fiber drawing motor is about 15 meters/second. Approximately 20km of continuous fibers can be wound onto one particular 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 Secondary Coating Line core, cladding and coating sizes
A. Refractive index profile: By far 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 increasingly more critical in high-speed fiber optic telecommunication applications.