How to make optical fibers?
This article explains in detail the process of fiber production.
Rod-in-Tube Glass Fibers
The simplest way to make a glass-clad fiber is by inserting a rod of high-index glass into a tube with lower refractive index. The two are heated so the tube melts onto the rod, forming a thicker solid rod. Then this rod (called a preform) is heated at one end and a thin fiber is drawn from the soft tip. The process is used for image-transmission and illuminating fibers but not for communication fibers.
For the fiber to transmit light well, the core-cladding interface must be very clean and smooth. This requires that the rod inserted into the tube must have its surface fire-polished, not mechanically polished. Although mechanical polishing gives a surface that looks very smooth to the eye, tiny cracks and debris remain, and if that surface becomes the core cladding boundary, they can scatter light, degrading transmission.
Another way to draw glass fibers is to pull them from the bottom of a pair of nested crucibles with small holes at their bottoms. Raw glass is fed into the tops of the crucibles, with core glass going into the inner one and cladding going into the outer one. The fiber is pulled continuously from the bottom, with the cladding glass covering the core glass from the inner crucible. The double-crucible process is very rare today, but it has been used in the past and may be used with some special materials.
Limitations of Standard Glasses
Fibers made from conventional optical glasses typically have attenuation of about 1 dB/m, or 1000 dB/km. This is adequate for an image-transmitting bundle to look into a patient’s stomach but not for communications from town to town.
The main cause of this high loss is absorption by impurities in the glass. Traces of metals such as iron and copper inevitably contaminate the raw materials used in glass manufacture, and those metals absorb visible light. To make extremely clear glass, you need to start with extremely pure silica, which has virtually no absorption at wavelengths from the visible to about 1.6 pm in the near infrared. The concentrations of critical impurities that absorb light at 0.6 to 1.6 pm— including iron, copper, cobalt, nickel, manganese, and chromium— must be reduced to a part per billion (1 atom in 109). That level is impractical with standard glass-processing techniques.
The starting point for modern communication fibers is fused silica, an extremely pure form of S i0 2. It is made synthetically by burning silicon tetrachloride (SiCkj) in an oxyhydrogen flame, yielding chloride vapors and S i0 2, which settles out as a white, fluffy soot. The process generates extremely pure material, because SiCl4 is a liquid at room temperature and boils at 58°C (136°F). Chlorides of troublesome impurities, such as iron and copper, evaporate at much higher temperatures than SiCl4, so they remain behind in the liquid when SiCl4 evaporates and reacts with oxygen. The result is much better purification than you can get with wet chemistry, reducing impurities to the part-per-billion level required for extremely transparent glass fibers.
Dopants, Cores, and Claddings
You cannot make optical fibers from pure silica alone. Optical fibers require a high-index core and a low-index cladding, but all pure silica has a uniform refractive index, which declines from 1.46 at 0.550 pun to 1.444 at 1.81 pun. You need to add dopants to change the refractive index of the silica, but they must be chosen carefully to avoid materials that absorb light or have other harmful effects on the fiber quality and transparency.
Most glasses have higher refractive index than fused silica, and most potential dopants tend to increase silicas refractive index. This allows them to be used for the high-index core of the fiber, with a pure silica cladding having a lower refractive index. The most common core dopant is germanium, which is chemically similar to silicon. Germanium has very low absorption, and germania, like silica, forms a glass.
Only a few materials reduce the refractive index of silica. The most widely used is fluorine, which can reduce the refractive index of the cladding, allowing use of pure silica cores.
Boron also reduces refractive index, but not as much as fluorine. In practice, single-mode and multimode step-index silica fibers fall into the three broad categories shown in Figure 6.1.
The fiber core may be doped to raise its refractive index above that of pure silica, which is used for the entire cladding. Alternatively, a smaller level of dopant may raise the core index less, but the surrounding inner part of the cladding may be doped— generally with fluorine—to reduce its refractive index. This design is called a depressed-clad fiber; normally the fluorine-doped zone is surrounded by a pure silica outer cladding. (Doping at the proper levels complicates processing, so manufacturers prefer to make as much as possible of the fiber from pure silica.) Both designs are used for single-mode fiber. An alternative used for multimode step-index fiber is a pure silica core clad with a lower index plastic.
Silica Fiber Manufacture
The trickiest stage in the manufacture of fused-silica optical fibers is making the preform from which the fibers are drawn. Several processes have been developed; they share some common features but have important differences.
The crucial common feature is the formation of fluffy fused-silica soot by reacting SiCl4 (and GeCl4, when it is used as a dopant) with oxygen to generate S i0 2 (and G e 0 2 if the silica is doped). The crucial variations are in how the soot is deposited and melted into the final preform.
One approach is to deposit the soot on the inside wall of a fused-silica tube, as shown in Figure 6.2. Typically, the tube serves as the outer cladding, onto which an inner cladding layer and the core material are deposited. Variations on the approach are called inside vapor deposition, modified chemical vapor deposition, plasma chemical vapor deposition, and plasma-enhanced chemical vapor deposition. The major differences center on how the reaction zone is heated.
The chemicals react to deposit a fine glass soot, and the waste gas is pumped out to an exhaust. To spread soot along the length of the tube, the reaction zone is moved along the tube. Heating melts the soot, and it condenses into a glass.
The process can be repeated over and over to deposit many fine layers of slightly different composition, which are needed to grade the refractive index from core to cladding in graded-index fibers. The doping of input gases is changed slightly for each deposition step,
producing a series of layers with small steps in the refractive index. Step-index profiles are easier to fabricate, because the whole core has the same doping. A final heating step collapses the tube into a preform.
Another important approach is the outside vapor-deposition process, which deposits soot on the outside of a rotating ceramic rod, as shown in Figure 6.3. The ceramic rod does not become part of the fiber; it is merely a substrate. The glass soot that will become the fiber core is deposited first, then the cladding layers are deposited on top of it. The ceramic core has a different thermal expansion coefficient than the glass layers deposited on top of it, so it slips out easily when the finished assembly is cooled before the glass is sintered to form a preform. The central hole is closed either in making the preform or drawing the fiber.
The third main approach is vapor axial deposition, shown in Figure 6.4. In this case, a rod of pure silica serves as a “seed” for deposition of glass soot on its end rather than on its surface. The initial soot deposited becomes the core. Then more soot is deposited radially outward to become the cladding, and new core material is grown on the end of the preform. Vapor axial deposition does not involve a central hole.
All three processes yield long, glass cylinders or rods called preforms. They are essentially fat versions of fibers, composed of a high-index fiber covered with a lower-index cladding.
They have the same refractive-index profile as the final fiber.
Optical fibers are drawn from preforms by heating the glass until it softens, then pulling Fibers are drawn hot g[ass away from the preform. This is done in a machine called a drawing tower.
Drawing towers typically are a couple of stories high and loom above everything else on the floor of a fiber factory. The preform is mounted vertically at the top, with its bottom end in a furnace that heats the glass to its softening point. Initially a blob of hot glass is pulled from the bottom, stretching out to become the start of the fiber. (This starting segment of the fiber normally is discarded.)
The hot glass thread emerging from the furnace solidifies almost instantaneously as it cools in open air. As shown in Figure below, the bare glass fiber passes through a device that monitors its diameter, then is covered with a protective plastic coating. The end is attached to a rotating drum or spool, which turns steadily, pulling hot glass fiber from the bottom of the preform and winding plastic-coated fiber onto the drum or spool. The actual length of the draw zone is longer than shown in the figure, to allow the fiber to cool and the plastic coating to cure properly.
Typically the fiber is drawn at speeds well over a meter per second. A single, large preform can yield over 20 kilometers of fiber; smaller preforms yield a few kilometers. After the fiber is drawn, it is proof tested and wound onto final reels for shipping.