Fiber Optics

Fiber optics work on the principle of total internal reflection. Light reaching the boundary between two materials is reflected such that it never leaves the first material. In the case of fiber optics, light is reflected from the optical fiber core-cladding interface in such a way that it propagates down the core of the fiber. This can be explained by a brief discussion of Snell's law of refraction and law of reflection, and a physical quantity known as index of bottom material. According to Snell's law, the light will be bent from its original path to a larger angle in the second material. As the incoming, or incident angle increases, so does the refracted angle. For the properly chosen materials, the incident angle can be increased to the point that the ray is refracted at 90 degrees and never escapes the first medium. The equation can be solved to give the incoming, or incident, angle which will result in a refracted angle of 90 degrees.

This is known as the critical angle (see Figure 2).

Figure 1. A cable cross section. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Light hitting the boundary or interface at angles greater than or equal to this value would never pass into the second material, but would rather undergo total internal reflection.

Now change the model slightly so that the higher index material is sandwiched between two lower index layers (see Figure 3).

Light enters the higher index material, hits the upper interface and is reflected downward, then hits the second interface and is reflected back upward, and so on. Like a marble bouncing off rails, light will make its way down the waveguide. This picture essentially corresponds to an optical fiber in cross-section. Light introduced to the fiber at the critical angle will reflect off the interface, and propagate down the fiber.

The second law of thermodynamics cannot be disregarded, however. Light will not travel down the fiber indefinitely. The strength of the signal will be reduced, or attenuated. Some light will be absorbed by impurities in the fiber, or scattered out of the core. Modern fibers are made of very pure material so that these effects are minimized, but they cannot be entirely eliminated. Some light will be diverted by microbends and other imperfections in the glass. Recall the law of reflection. If a microbend is encountered by light traveling through the fiber, the light may hit the interface at an angle smaller than the critical angle. If this happens, the light will be reflected out of the core and not continue propagating (see Figure 4).