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The ubiquity of today's fiber optic cable types is rooted in research from the 1950s. During the 1950s, research and development into the transmission of visible images through optical fibers led to some success in the medical world, where it was being used in remote illumination and viewing instruments. In 1966, Charles Kao and George Hockham proposed the transmission of information over glass fiber and realized, to make it a practical proposition, much lower losses in the cables were essential.
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This was the driving force behind the developments to reduce the optical losses in fiber manufacturing. Today, those losses are significantly lower than the original targets set by Kao and Hockham.
The advantages of using fiber optics
Because of the low-loss, high-bandwidth properties of fiber optic cabling, it can be used over greater distances than copper cables. In data networks, this can be as much as 2 kilometers without the use of repeaters. Being lightweight and small-size also makes them ideal for applications in which running copper cables would be impractical; by using multiplexers, one fiber can replace hundreds of copper cables. This is pretty impressive for a tiny glass filament, but the real benefit in the data industry is its immunity to electromagnetic interference -- and the fact that glass is not an electrical conductor.
Because fiber is nonconductive, all fiber optic cable types can be used where electrical isolation is needed -- for instance, between buildings where copper cables could experience ground potential differences. Fibers also eliminate threats in dangerous environments -- such as chemical plants, where a spark could trigger an explosion. Last, but not least, is the security aspect: It is difficult to tap into a fiber cable to read data signals.
There are many different fiber optic cable types, but for the purposes of this explanation, we will deal with one of the most common types: 62.5/125-micron loose tube. The numbers represent the diameters of the fiber core and cladding. These are measured in microns, which are millionths of a meter.
Loose tube fiber cable can be indoor, outdoor or both. Outdoor cables usually have the tube filled with gel to act as a moisture barrier to the ingress of water. The number of cores in one cable can be anywhere from four to 144.
Over the years, a variety of core sizes have been produced. Today, there are three main sizes that are used in data communications: 50/125, 62.5/125 and 8.3/125. The 50/125- and 62.5/125-micron multimode cables are the most widely used in data networks; although, recently, the 62.5 has become the more popular choice. This is rather unfortunate, because the 50/125 has been found to be the better option for Gigabit Ethernet applications.
The 8.3/125-micron loose cable is a single-mode cable that, until now, hasn't been widely used in data networking because of the high cost of single-mode hardware. Things are beginning to change, because the length limits for Gigabit Ethernet over 62.5/125 fiber have been reduced to around 220 meters. Thus, using 8.3/125 may be the only choice for some campus networks.
Single-mode vs. multimode fiber optic cable types
With copper cables, larger size means less resistance and, therefore, more capacity. But with fiber, the opposite is true. To explain this, we first need to understand how light propagates within the fiber core.
Light travels along a fiber cable by a process called total internal reflection; this is made possible by using two types of glass that have different refractive indexes. The inner core has a high refractive index, and the outer cladding has a low index. This is the same principle as the reflection you see when you look into a pond. The water in the pond has a higher refractive index than the air, and if you look at it from a shallow angle, you will see a reflection of the surrounding area; however, if you look straight down at the water, you can see the bottom of the pond.
At some specific angle between these two viewpoints, the light stops reflecting off the surface of the water and passes through the air-water interface, allowing you to see the bottom of the pond. In multimode fibers, as the name suggests, there are multiple modes of propagation for the rays of light. These range from low-order modes, which take the most direct route straight down the middle, to high-order modes, which take the longest route, as they bounce from one side to the other all the way down the fiber.
This has the effect of scattering the signal because the rays from one pulse of light arrive at the far end at different times; this is known as intermodal dispersion -- sometimes referred to as differential mode delay, or DMD. To ease the problem, graded index fibers were developed. Unlike fiber optic cable types that have a barrier between core and cladding, these have a high refractive index at the center that gradually reduces to a low refractive index at the circumference. This slows down the lower-order modes, allowing the rays to arrive at the far end closer together, thereby reducing intermodal dispersion and improving the shape of the signal.
So, what about the single-mode fiber?
What's the best way to get rid of intermodal dispersion? Easy: Only allow one mode of propagation. So, a smaller core size means higher bandwidth and greater distances. It's as simple as that.
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