The different fiber optic cable types: Tips for network professionals

We explain the different types of fiber optic cables, how the 62.5 / 125 micron compact loader is structured and what role size plays.

That the various fiber optic cable types are to be found almost everywhere today has its origins in the research of the 1950s. At that time, scientists experimented with fiber-optic cables to transmit visible images, leading to some success in the medical field. There the technology was used in the remote lighting and observation instruments. In 1966, Charles Kao and George Hockham proposed the transmission of information about optical fibers. They realized that much lower losses in the cables were a prerequisite for practical application.

This has been the driving force behind efforts to reduce optical losses in fiber production. Today’s losses are significantly lower than the original targets set by Kao and Hockham.

The advantages of using fiber optic cables

Because fiber optic cables are characterized by low losses and high bandwidth, they can be used over longer distances than copper cables. In data networks, two kilometers are possible without repeaters being necessary. As a slim solution, they are also ideal for applications where laying copper wires would be impractical. When using multiplexers, a single fiber can replace hundreds of copper cables. This is quite impressive for a tiny fiber, but the real advantage in the data field is the insensitivity to electromagnetic interference – and the fact that glass is not an electrical conductor.

Because fiber optics is nonconductive, all types of fiber optic cable can be find at Sopto and used where electrical insulation is needed – for example, between buildings where copper ground could have different potentials. Glass fibers also eliminate hazards in critical environments – such as chemical plants, where a spark can cause an explosion. Last but not least, there is also a safety aspect: it is difficult to tap into a fiber-optic cable in order to read the data traffic.

Construction of fiber optic cables

There are many different fiber optic cable types, but in this article we limit ourselves to one of the most common types: the 62.5 / 125 micron compact loader. The numbers indicate the diameter of the fiber core and the shell, respectively. As a unit micrometers are used, that is millionths of a meter.

Fiber optic cable designed as a compact charger can be used both inside and outside. Outdoor cables are filled with a water-repellent gel that acts as a barrier to moisture penetration. The number of fiber cores in a cable can be between four and 144.

Over the years, a variety of different core sizes have been produced. Today there are three main sizes used for data communication: 50/125, 62.5 / 125 and 8.3 / 125. The multimode 50/125 and 62.5 / 125 micron fiber optic cables are the most common types in data networks. More recently, however, the latter is in the lead in popularity an unfortunate development because the 50/125 micron cable has proven to be a better option for Gigabit Ethernet applications.

The 8.3 / 125-micron compact loader is a singlemode cable that has not yet been used extensively due to the high cost of single-mode hardware in data networks. This is starting to change as the Gigabit Ethernet length limits have been reduced to approximately 220 meters over 62.5 / 125-micron fiber-optic cable. As a result, using the 8.3 / 125 cable may be the only option for some campus networks.

Fiber optic cable types: singlemode cable versus multimode cable

The thicker a copper cable is, the lower the resistance and the more capacity it offers. For fiber optic cables, the opposite is true. First, we need to understand how the light propagates inside the fiber core.

Light propagation

The light travels in a fiber optic cable using a process called total reflection. This is made possible by using two types of glass, each having a different refractive index. The inner core has a high refractive index, while the outer shell has a low refractive index. It is the same principle as the reflection that 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 environment. However, if you look directly down at the water, you can see the bottom of the pond.

At a certain angle between these two perspectives, the light is no longer reflected off the water surface and breaks through the air-water interface, so you are able to see the bottom of the pond. In multimode glass fibers, as the name implies, there are several so-called modes for the light rays to spread out. The spectrum ranges from low-order modes, which take the direct route through the middle, to the high-order modes, which take the longest, route because they keep on their route through the fiber bounce off one side.

This leads to a scattering of the signal, because the rays of one light pulse reach the other end at different times. This phenomenon is called intermodal dispersion, sometimes called Differential Mode Delay, DMD for short. To get a handle on the problem, gradient index fibers were developed. Unlike fiber optic cable types, which have a barrier between the core and the cladding, they have a high refractive index in the center, which gradually changes to a low refractive index towards the periphery. This slows down the low-order modes, causing the beams to reach the other end more closely together. In this way the intermodal dispersion is reduced and the signal shape improved.

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