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Fiber Optics: Not Something To Make Light Of – Part 1

- December 15, 2017 - 4 Comments

Written by Priya Maratukulam, Product Manager, Transceiver Modules Group, Cisco

This is the first part in a series on the basic principles of fiber optic communication.  Have you ever wondered how the data we transmit through the internet is able to travel thousands of miles around the globe?  Well, for the majority of the distance, the data travels as light signals in fiber optic cable.  Signal propagation through optical fiber is based on the phenomena known as total internal reflection (TIR).  When light travels through a material and encounters a new material which has a different refractive index, that light changes direction, or bends, when it enters the new material.  The amount the light bends is determined by the indices of refraction of the two materials and the angle of incidence.  A special case is when light travels from a medium with higher refractive index to one with lower refractive index. If the incident angle exceeds a critical angle, the incident light will be totally internally reflected and remain in the first material.

A great example of this is in water, whose index of refraction is greater than that of air.   Have you ever been swimming and looked up at the sky from under the water and noticed that the surface of the water is mirror like? That effect is due to total internal reflection.

Turtle Swimming In Sea

Fiber optic cable takes advantage of this property of light with its internal structure.  The glass fiber has a cylindrical core that is surrounded by a cladding. See the figure below for a cross-sectional diagram.

The core has a higher index of refraction than the cladding so that light traveling down the core is totally internally reflected and remains in the fiber as it travels. The difference in index of refraction between core and cladding is small, but it’s enough to trap the light traveling at glancing angles.  That is how light signals can travel long distances, in some cases over 100km, in fiber optic cable. Imagine looking out a window that is 100km thick, while still seeing the other side!

Internet content rides on top of data communication between switches, routers, and servers, and fiber optic links play a key role at all distances, even less than 1 meter. These links help support traffic at the fastest rates possible so that you can stream your movies and music videos reliably.

In the upcoming second part of this series we will discuss two different types of optical fiber: single-mode and multi-mode.



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    If we have Total Infernal Reflection on place, like in fiber, do we have different angles theta1 and theta2? Should it be the same angle like in the mirror?

    • Hi Grzegorz, Yes you are correct! Theta1 equals theta2 because that light is being reflected.

    The same (see critical angle picture) is when You are underwater and trying to see the light. Even in a sunny day You can see a circle of light (deeper You are, smaller it is). And this circle is surrounded by dark space - strange isn't it? But all You have to do is to remember that if the light comes to You with the angle higher than critical, those light is mirrored, and from this direction You have no light - "no signal" for Your eyes, which is dark space.

    • I think what you are referring to is “Snell’s window”. Here is a link to a wikipedia article on this effect. Snell’s window occurs when looking up from underwater and light coming from outside the water is refracted when entering the water. The total internal reflection described in the blog post is concerning light that originates from underwater and is reflected at the water air boundary.