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Here at DesignSpark, we are immersed in the world of electronics, but there is a subject about which we don’t talk very often. This is surprising because it is a technology that is increasingly important in global communications and is constantly finding applications in the world of industry. That subject is fibre optics.
Fibre optics are finding applications in all industries
There are a few articles on DesignSpark that do talk about fibre, but I have noticed that many of them assume that the reader is familiar with the technology. Many of us shy away from the subject of fibre because it is different. The intention of this article is to give you some information to prove that fibre optics need not be scary.
Just as with conventional electronic communications, fibre optics have their advantages and disadvantages. Optical transmission using fibre has the capacity to carry huge amounts of information over distances that would prove inefficient for conventional electrical signals. They are secure and immune to the disruption caused by electromagnetic interference (EMI), and they are remarkably small and light considering the amount of information they can carry.
Communicating with Light
The purpose of an optical fibre is to transfer information. In conventional copper cables, this communication is carried out with electricity, which is really just negatively charged electronics in a conductor that allows them to move freely. Optical fibres use the same principle, substituting electrons for light, and using a medium that allows it to propagate.
Optical fibre uses glass of extremely high purity to minimise the loss of signals over long distances, drawn-out into tiny filaments. A phrase often used (even overused) is that fibre is thinner than human hair, but it is useful to give a clear impression of quite how fine optical fibre is. In fact, an optical fibre is actually made of two layers of glass, each with a different refractive index, with the cladding wrapped around the core.
The light travels along the core of the fibre using a principle called total internal reflection. It is possible to see this effect in action when you look at a large body of water. When looking straight into a large body of water, the light can travel freely between the interface between the water and the air. When viewed at a shallow angle, the light from the sky is reflected back to the viewer, and the water appears opaque.
This same effect is at work in an optical fibre. The interface between the core and the cladding – both of which exhibit a different refractive index – keeps the light bouncing along inside the core like a rubber ball bouncing along a steel pipe. In fact, the core of a modern fibre is often constructed with a graded refractive index, which curves light gently back to the centre rather than relying on internal reflection.
The size of the core governs the volume of data that can be transmitted along a fibre. In contrast to electrical systems, in which a larger conductor provides a lower resistance, the smallest fibre cores offer the largest capacity. The reason for this is down to the mode. In fibre optics, mode is the name for the path which light takes through the fibre.
Making Sense of Modes
A multimode fibre has a larger diameter core, typically 50 or 62.5 µm. This allows more than one path or mode that light can follow through core of the fibre. This means that a signal can take a number of different paths to reach its destination. Therefore, it is necessary to delay transmission of the following signal so that there is no risk of it interfering with the previous signal that has taken the slowest path.
In contrast, a single-mode is far narrower, typically with a diameter of just 9 µm. This construction allows the light to travel along just a single path. Therefore, it is possible to send more data through the fibre, safe in the knowledge that one signal with not interfere with another. All of the undersea cables that form the backbone of global communications use single-mode fibres.
The tiny diameter of the fibre optic core is one of the reasons that employing fibre optics can be challenging. This becomes especially important when the time comes to connect two lengths of fibre together. Unlike an electrical system that simply requires conductors to touch in order to pass current, fibre connections work differently. In order to join two fibres, it is necessary to align the core of each fibre and bring them into direct physical contact. If the cores are misaligned, there is a risk of losing some of the signals at the join. With the cores of fibre so fine, this means that precision is vitally important.
To create the join, optical fibres are fitted into ceramic ferrules that are manufactured to a very fine tolerance in order to provide the precision needed for a good connection. This is one of the key reasons why multimode is used, despite its poorer performance compared to single-mode fibre. The precision required to bring two 9 micron cores into perfect alignment translates into a complex and costly process, and so if extreme performance is not necessary, multimode fibre is often the more user-friendly choice.
When installing an optical cable, it is also important to remember that fibre needs careful handling. Conventional electrical cable, especially if it has a stranded construction, can withstand a certain amount of physical abuse. In contrast, optical fibres can be damaged if crushed or are bent at a sharp angle. In fact, cables are constructed to protect the vulnerable fibres housed within, often using yarns and even armour to provide the needed strength.
Fibre optics in telecommunications applications often do not have to contend with harsh environments, and conventional fibre connectors are often lightweight and compact in construction. The popular LCand FC connector standards are perfect examples of commercial fibre connectors, while for applications that need multiple fibres the MT ferrule offers an ideal solution.
Harsh environment fibre connectors from Souriau
However, the advantages that fibre can provide – lightweight, high data rates and resistance to EMI – means that it has found applications in a wide variety of industries. From outside broadcast to military communication, there is a considerable appetite for fibre connectors that can withstand harsh conditions. Solutions from Bulgin, Souriau and HARTING .
In part 2 of this article, we’ll look in greater detail about how fibres are terminated into connectors.