A light ray that is being turned on and off to send data (1s and 0s) into an optical fiber must stay inside the fiber until it reaches the far end. The ray must not refract into the material wrapped around the outside of the fiber. The refraction would cause the loss of part of the light energy of the ray. A design must be achieved for the fiber that will make the outside surface of the fiber act like a mirror to the light ray moving through the fiber. If any light ray that tries to move out through the side of the fiber were reflected back into the fiber at an angle that sends it towards the far end of the fiber, this would be a good “pipe” or “wave guide” for the light waves.

The laws of reflection and refraction illustrate how to design a fiber that guides the light waves through the fiber with a minimum energy loss. The following two conditions must be met for the light rays in a fiber to be reflected back into the fiber without any loss due to refraction:

• The core of the optical fiber has to have a larger index of refraction (n) than the material that surrounds it. The material that surrounds the core of the fiber is called the cladding.
• The angle of incidence of the light ray is greater than the critical angle for the core and its cladding.

When both of these conditions are met, the entire incident light in the fiber is reflected back inside the fiber. This is called total internal reflection, which is the foundation upon which optical fiber is constructed. Total internal reflection causes the light rays in the fiber to bounce off the core-cladding boundary and continue its journey towards the far end of the fiber. The light will follow a zigzag path through the core of the fiber.

A fiber that meets the first condition can be easily created. In addition, the angle of incidence of the light rays that enter the core can be controlled. Restricting the following two factors controls the angle of incidence:

• The numerical aperture of the fiber – The numerical aperture of a core is the range of angles of incident light rays entering the fiber that will be completely reflected.
• Modes – The paths which a light ray can follow when traveling down a fiber.

By controlling both conditions, the fiber run will have total internal reflection. This gives a light wave guide that can be used for data communications.

Web Links Total Internal Reflection http://www.glenbrook.k12.il.us/gbssci/ phys/Class/refrn/u14l3b.html