An Introduction to Fiber Optics and How They Work

The backbone of the internet is provided by cables transmitting information with light, arrayed in a huge network around the world. Fiber optics has proven ideal for this network and has many attributes that provide benefits for applications in other industries.

Instead of conventional electrical signals, optical fibers transmit information using pulses of light. This provides optical systems with the capacity to carry huge amounts of data over distances that are inefficient for normal electrical signals. Unlike electrical cables, fiber conduits are secure and immune to electromagnetic interference (EMI). In addition, fiber cables are small and lightweight when compared to their electrical counterparts carrying the same volume of data.

Communicating with Light

In a conventional copper cable, the conductive core carries electricity in the form of negatively charged electrons. Optical fibers substitute electrons for light. Instead of conductive metal, the signals are carried through glass.

The glass of an optical fiber consists of tiny filaments of extreme purity. This purity can be compared to copper’s low electrical resistance, minimizing the signal loss over long distances. The construction of an optical fiber comprises two layers of glass: the core and cladding each of which has a different refractive index.

Light is introduced into the fiber and travels along the core using total internal reflection. This effect can be seen in real life by looking at a large body of water. When looking directly down into the water, light can travel freely through the interface between the water and the air. However, if viewed at a shallow angle, light from the surroundings is reflected by the surface, making the water appear opaque.

This same principle is at work inside the optical fiber. The core and the cladding each exhibit a different refractive index, and the interface between the two acts like the surface of the water in the example above, causing the light to bounce along inside the core like a rubber ball moving along a pipe.

The size of the core determines the volume of data that can be carried along an optical fiber. Unlike electrical systems, where larger conductors provide a higher capacity, smaller fibers deliver greater performance. To understand how this functions, it is necessary to introduce the concept of modes.

What is The Difference Between Single Mode and Multimode Fiber?

Multimode Fiber

A multimode fiber is one with a larger core, most frequently of 50 or 62.5 microns in diameter. Inside this fiber, light can follow more than one path, known as a mode, through its core. A signal can take any of several different routes to reach its destination. To prevent a signal from interfering with the one that went before, it is necessary to delay transmission of any following signal. This allows the previous signal sufficient time to be received, even if it has taken the slowest path.

Notably, multimode fibers are typically used with shorter wavelengths of light and for shorter distance transmissions. This is because the multiple modes of light travel can lead to greater signal dispersion over long distances, making multimode fibers more suitable for shorter-range applications such as local area networks (LANs) or data centers.

Single-mode Fiber

A single-mode fiber is much smaller, often with a diameter of just 9 microns, which limits the light to just a single path. The construction of modern fiber uses a core with a graded refractive index. This gently curves light back to the middle of the fiber instead of relying on internal reflection. The practical result is that it is possible to send far more data at greater distances through a single-mode fiber, because any one signal will not interfere with another. To achieve the bandwidth demanded by today’s internet users, all undersea cables use single-mode fibers.

Single-mode fibers are typically used with longer wavelengths of light, which allows for transmission over much greater distances. This combination of a single light path and longer wavelengths results in minimal signal degradation, making single-mode fibers ideal for long-distance communication applications such as intercontinental data transmission.

Fiber Connections

The small diameter of the core of optical fibers creates a number of engineering challenges, especially when connecting two cables together. An electrical system does not require conductors to touch in a particular orientation in order to pass current. In contrast, it is necessary to carefully align each fiber’s core for optical connections to work. If the cores are not aligned correctly, some of the signal will be lost at the join. As the core of each fiber is so fine, precision is vitally important.

To create an optical connector, each fiber is fitted into a ferrule that is manufactured to extremely precise tolerance. This ensures the precision needed for a good connection by keeping the cores centered and aligned. This is one of the reasons why multimode is popular in some applications, despite its lower performance when compared to single-mode fiber. The precision needed to align two 9-micron cores translates into tighter tolerances, higher project complexity and cost. If the high performance of single-mode fiber is not required, multimode offers as a more expedient and less costly option. Ferrules can be ceramic and plastic (MTP/MPO)

Once cables are created, it is also important to remember that fiber requires careful handling. Copper electrical cable is robust and can withstand a reasonable amount of rough handling. However, optical fibers can be damaged if they are crushed or bent at a sharp angle. To prevent this, optical cables are often constructed specifically to protect vulnerable fibers using a combination of high-tensile strength members and armor to deliver strength.

The connector to which the cable is terminated is designed to protect the delicate fibers. Optical fibers require direct physical contact between the mating surface for effective communication, and contamination can prevent accurate alignment. The body of the connector protects the fibers, provides an anchor for the cable, and ensures alignment for reliable communications.

Regular cleaning and maintenance of fiber optic connections is an important aspect of operations, especially for tactical and deployable systems. Connectors should be cleaned before each use with specialized tools like lint-free wipes or cleaning pens and inspected with a fiber scope. In harsh environments, maintenance frequency is increased and ruggedized components are often required for optimal performance and longevity of the fiber optic network.

Contact your Major Custom Cable Account Executive for additional information.