Fiber optic cables are available in numerous types. When choosing cable, two primary considerations are the ease of installation and service life.
Cables are designed for different environments. Some are intended for indoor installation. Outside cables may require extensive protection from the elements, chemicals, temperature extremes, and gnawing rodents. Some cables have extra flexibility to accommodate small radius bends, and some must withstand high tension.
Most cables are based on 250 micron fiber optics with one or more buffer coatings that protect the fibers. A gel prevents water entry. Strength members are added, usually Kevlar, to absorb any tension. Some cables use a fiberglass rod to prevent kinks. The outermost jacket is black polyethylene for outside cables. It can be color coded, and may even be double-jacketed and armored if rodents are a problem.
Single Mode and Multimode Fibers.
The typical outside diameter of all optical fibers is 125 microns, not much larger than a human hair. This includes both the fiber and cladding, so fibers are identified by their core and cladding dimensions. A 50/125 multimode fiber, for instance, would have a 50 micron core with a 125 micron cladding. Single mode fibers are smaller than multimodes, with 9 micron cores.
Multimode fibers are almost always 50 or 62.5 microns. They work well with either LEDs or lasers depending on the desired bandwidth and length of the cable run. The larger fiber diameter allows multiple paths for light to travel. Some goes through the center, but some is reflected along the sides. At the receiver, the light pulses arrive at slightly different times, causing some distortion. The effect is magnified as cable length increases.
Multimode fibers are designated as OM1, OM2, OM3, or OM4. OM1 has the lowest bandwidth at 160-200 MHz while OM4 has far more at 3600 MHz.
Single mode fibers have a 9 micron core and are intended to be used with lasers. The small diameter eliminates internal reflections that can cause a laser pulse to arrive slightly distorted at the receiver. This permits higher data transmission rates than multimode fiber, with the disadvantage of increased cost.
Single mode fiber is designated as OS1, B1.1, or G.652 for telecommunications, cable television or high speed LANS. Its bandwidth is greater than a terrahertz. Other single mode fibers, OS2, B2, B1.2, G.653 and G.654 have similarly high bandwidths, and each is designed for a particular application. OS2 and B1.2 have low water dispersion, meaning that light isn’t absorbed by water molecules, allowing for wider spectrum use. G.653 is designed to reduce light dispersion at 1310 nm, helping to reduce signal deformation. G.654 is known as cutoff-shifted fiber or low-attenuation fiber and is most often used for transoceanic cables.
Cables intended for indoor use are tight buffered types, with their fibers encased in a 900 micron buffer. They include simplex, zipcord, and distribution cables. Simplex is a single fiber strand surrounded by its buffer and casing, while a zipcord consists of two simplex cables side-by-side. It looks remarkably similar to an indoor power cord for a lamp.
Distribution cables consist of multiple fibers, often with Kevlar to assist with pulling. Some incorporate fiberglass rods to prevent kinks. The fibers are not individually supported, so they must be terminated in a breakout box or similar panel.
Outdoor cables are the loose tube type. They withstand higher pulling tension than indoor cables and are designed for much longer runs of up to several miles. Their fibers are encased in a watertight sheath with gel or other waterproof materials. Like indoor cables, these have Kevlar to prevent putting tension on the fibers during pulling operations. They may have double jackets and armor to resist weather and rodents.
Loose tube cables using single mode fibers are easily damaged and are generally terminated with pigtails for protection. Multimode fibers are more robust and can be terminated in a breakout box, with each fiber inside a protective sleeve.
Ribbon cable is popular because it offers a high fiber count inside a small diameter package. A 6mm cable typically has 144 fibers, and offers fast termination with a fusion splicer or pigtails.
How are Fiber Optics Made?
Every time you talk on the phone or go in the internet, what you see or type travels to its destination through fiber optics: The process of transmitting voice or data via pulses of light through hair thin glass fibers.
Those fibers start out as large glass tubes. First, workers unwrap the tubes then they submerge them into a corrosive bath of hydrofluoric acid. That removes any oil residues. Then they set the tube into each end of a lathe. As the tubes spin they are heated with a hydrogen oxygen flame. When the glass turns white it’s getting close to heating peak temperature. At 2000 degrees Celsius the 2 tubes fuse together. They put this new longer tube onto another lathe. As the tube spins they inject a mixture of chemical gases inside while a traversing burner heats everything up. The gas mixture contains liquid forms of silicon, an abundant chemical element found in nature and germanium, a chemical element similar to tin that is use a semiconductor in transistors and other electronic devices.
As the gases heat they undergo a chemical reaction that leaves a white silt on the inside of the glass tube. The heat fuses the silt forming what will eventually become the core of the optical fiber. The glass tube itself will form the fiber’s covering. When there is enough fused silt, they turn up the heat until the silt itself turns into glass. Then they heat the glass tube enough to soften it as well as the new glass inside. The intense heat eventually makes the tube collapse on itself to form a solid rod. The internal structure of the optical fiber has been achieved. But it’s in the form of a big bulky rod called a preform so the next step is to thin it out. First they excise the preform from the uncollapsed section of the glass tube. Then they install it vertically into the drawing tower which will draw out the final shape. The drawing tower’s oven heats one end of the preform to 2000 degrees Celsius. The glass softens. Gravity helps pull it down like honey dripping from a spoon. Then using a glob of glass as a weight, they stretch the soft glass and keeps stretching it until they form a thin glass fiber.
A series of pulleys measures the tension on the fiber as it’s being drawn. A special monitor makes sure the fiber is precisely the right diameter, 125 micrometers. That’s about 1/8 of millimeter thick. Then the fiber passes through UV lamps, and acrylic coating to protect against dust and other contaminants. Finally the fiber is rolled on to a drum. From here it’s either shipped out as is or put into a cable. Fiber optic cables are expensive to produce but they are smaller and lighter than traditional copper cables. They carry more information and need fewer repeaters to keep the signal from deteriorating. And unlike copper cables they are immune to electromagnetic interference. They are also hard to tap without being detected and all this is made possible by a complicated process based on a very simple principle, light travelling through glass.