WHAT IS AN OPTICAL FIBER, EXACTLY?

Are you new to the field of optical fibers, or just curious about what they are? This article is an entry point to the basic working principles of optical fibers and a sneak peek of the wide world of optical fibers applications.

The Basic Principles of an Optical Fiber

Since the dawn of civilization, humans have been using light as a signal for information or as a communication method. Think of the light of a fire seen from afar, a light house to guide ships, or even different colored smoke signals for visual communication over a long distance.

With light travelling at the fastest possible speed in the universe, it’s no wonder scientists and engineers have always tried to use it to transmit information and messages.

The first published works on the underlying physical principles that led to optical fibers date back at least 1840, where the refraction of light was starting to be understood. This is the phenomenon that makes light move at different speeds in different materials. Modern uses of optical fibers began much later, in the 1960’s, one limiting factor being the complex process needed to manufacture optical fibers.

Guiding light over long distances is complex, since you can lose some of it if your material is not completely transparent, and it can reflect or dim if hitting an interface. In order to overcome this, we make use of a phenomenon known as total internal reflection that enables us to “capture” light.

Here’s how it works: If light travelling in one transparent material arrives at the interface of another transparent material, a portion of it will be reflected and a portion will pass through. Some small quantity of light will also be absorbed in the material, while a portion will be scattered more randomly. The behavior of the light is determined by the refractive index and chemical proprieties of both materials.

If the two materials are carefully chosen for their refractive index, we ensure that, for a certain incoming angle, the light is reflected back in the first material. Using this feature, we can guide our light “indefinitely” over a long distance, giving us a simplified version of an optical fiber.

Optical fibers are very thin glass tubes that use the total internal reflection phenomenon to guide light. The main material used is glass since it’s transparent over wide wavelength ranges. Of course, nothing is perfect and there are some intrinsic losses of light over the length of the fiber, but the light can remain almost identical over many kilometers. Light will be guided in the fiber’s glass core. It will have a difference of refractive index with the glass cladding, designed for the light to remain captured in the core by total internal reflection. One or multiple layers of protective coating (usually polymers) are also always applied to the fiber because of the fragility of the glass core and cladding.

Single Clad Fiber

Starting from this fundamental representation of an optical fiber, many parameters can vary which can lead to a large amount of different optical fibers:

The addition of different chemical elements to the core gives additional physical proprieties and, depending on application, the core can vary from pure silica glass to specialty glass doped with various chemical elements (germanium, rare earths, etc.);
The core and cladding diameters can vary to enable more power or different guiding modes;
Another total internal reflection feature can be added between the glass cladding and another material over it, enabling light to be guided both in the core and in the cladding. This defines what are called double-clad fibers. An interesting characteristic of this type of fiber is that the first cladding can have a different shape than round. In some cases an octagonal cladding is used to change the behavior of the guided light;
Different types of glass can be used to guide different wavelengths of light.

Double Clad Fiber

Typical Uses and Applications of Optical Fibers Today

Optical fibers are used in many fields and in industry. The most common and widely adopted application is in the telecom network! Other uses of optical fibers include high power fiber lasers, used for metal sheet cutting, welding, marking and more. They can also be used as the light source of LiDAR systems and many sensing applications. This is just an overview of how optical fibers have a key role in today’s world.

Conclusion

At Coractive, we manufacture specialty optical fibers for many applications.

Given how diverse optical fibers can be, this was just a small preview of all the possibilities. Keep following Coractive for new insights into emerging applications and additional glimpses into the world of optical fibers.

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