You may have noticed cables installed in the offices or homes that are providing internet access. These cables also do have a familiar name, optical fibers. In this post, we will gain knowledge on what are these optical fibers? What is their design? And for which purposes are they used?

What are optical fibers?

An optical fiber or fiber optic is a fiber made from transparent materials such as glass (SiO2) or plastic in order to transmit a photon-based (light) signal. These fibers are extremely thin, usually no wider than a strand of human hair. Now, there might be a question, since the thickness of these fibers are extremely thin, why do the cables installed in our homes and offices seem like just some regular cables? This is because these fiber optic cables observed in our daily lives might have hundreds of these optical fibers inside them. An exact number depends on the application for which the cables are used.

Coming back to our story, these fibers are used to transmit light over long distances because without any light guiding medium, the light diffracts or bends from the particles present in the air, and thus, it loses energy. For communication or power-related applications, the phenomenon of diffraction is extremely detrimental as the light loses valuable information or energy. In order to stop the diffraction, these optical fibers are employed so that the signal or light remains confined in particular strands of fiber. The optical fibers are usually designed to carry visible light, which means their wavelength of operation is from 400-800 nm, or their frequency of operation is 375-750 THz.

There are many names who helped in the development of fiber optics, and the development has been ongoing since the start of the 19th century. However, the first working fiber optic data transmission system was demonstrated by Manfred Börner, who also patented the device. After that, Charles Kuen Kao (also known as the father of fiber optic communication) worked extensively on fiber optics for its development.

Working and design of optical fibers

The fiber optic cables work on the principle of total internal reflection, where a light wave, upon entering a medium of lower refractive index from a medium of higher refractive index, undergoes reflection. The wave is never able to come out of this medium of higher refractive index. The only requirement for this condition is that a wave must make an angle greater than the critical angle required for total internal reflection. The critical angle (θc) for any interface of two mediums can be calculated as follows.


where nh and nl represent mediums of higher refractive index and lower refractive index. The design of an optical fiber is shown in Figure 1.

Optical fiber cable design (fiber optic cable design)
Figure 1: Optical fiber cable design (fiber optic cable design).

The optical fiber cable usually consists of five layers which are the core, cladding, coating, strength member, and cable jacket. The transmission of light is primarily done through the core, where cladding is used as a medium with a lower refractive index than the core in order to satisfy the condition for total internal reflection. The other layers used in fiber optic cables, such as coating, strength member, and cable jacket, are added to provide strength and durability to the cable. Moreover, these layers are used to provide environmental sustainability to the cable.

Applications of optical fibers

Fiber optic cables are used for the following applications.

  • Communications networks: To transmit light from one to another, where the light carries some information.
  • Power transmission: Visible light carries high frequency, which means it carries more power. Therefore, fiber optic cables are also used to transfer power.
  • Sensing: Optical fibers are also used for sensing applications where light from optical fibers can be used for sensing when it has interacted with an object.
  • There are many other applications of optical fibers, such as the testing of materials and microscopy.

Further reading

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