Total Internal Reflection

Beyond a certain angle, the refraction will cause light to be reflected from the surface. Optical fiber uses this reflection to “trap” fiber in the core of the fiber by choosing core and cladding materials with the proper index of refraction that will cause all the light to be reflected if the angle of the light is below a certain angle. We call that “total internal reflection.”

There is an angle that for any given fiber defines total internal reflection. At higher angles a ray of light will still be refracted but not enough to be reflected back into the core, so it is lost in the cladding of the fiber. Below that angle, it will be reflected back into the core of the fiber and transmitted to the end of the fiber. The angle of total internal reflection defines the “numerical aperture” (NA) of the fiber, a standard fiber specification.

In the drawing above, the left shows a section of an optical fiber with a ray of light. On the right is a diagram of the index of refraction of the glass in the fiber. The core glass is n1 and the cladding glass is n2, and the core glass is of slightly higher refractive index, ~1.46 to the cladding at ~1.45.

Snell’s Law Remember basic trigonometry? The sine is the ratio of the side of a right triangle to the hypotenuse - the long side opposite the right angle. The refractive index of the glass (n) is defined as the ratio of the speed of light in a vacuum (c) to the actual speed of light in the material (v) like this: