How Optical Fibers Work

Fiber optics is one of the newer words these days. Optical fiber has a number of advantages over the copper wire used to make connections electrically. For example, optical fiber, being made of glass (or sometimes plastic), is immune to electromagnetic interference, such as is caused by thunderstorms. Also, because light has a much higher frequency than any radio signal we can generate, fiber has a wider bandwidth and can therefore carry more information at one time.

But just how does it work? We're talking about a thin, flexible "string" of glass. Looking sideways at it, we can see right through it. How can we keep light that's inside the fiber from getting out all along the length of the fiber?

Consider an ordinary glass of water. We know that if we look through the water at an angle, images will appear distorted. This happens because light actually slows down a little bit when it enters the water, and speeds up again when it moves back into the air again.

Since the light has a slight but measurable width if it hits the water at an angle, the part of the light that hits the water first will slow down first. The result is that the direction the light is traveling changes, and the path of the light actually bends at the surface of the water.

No matter what angle the light is traveling as it approaches the water, it will take a steeper angle once it actually enters the water. You can see this at any time by looking at a picture or newspaper through a glass of water, and by looking at different angles. Even a straw in a glass of water looks bent, although it really isn't. This phenomenon is called refraction.

Any substance that light can travel through will exhibit this phenomenon to some extent. Glass happens to be a very practical choice for optical fiber because it is reasonably strong, flexible, and has good light transmission characteristics.

Now, consider looking into a glass of water from below the surface of the water. If you look up through the bottom of the glass, you will see a somewhat distorted view of the ceiling or whatever is above the glass. However, if you look in from the side of the glass and observe the underside of the top surface, you will begin to note an interesting and useful effect: the light you see is reflected from the surface, rather than being refracted through it. This effect persists for all angles shallower than the critical angle at which the phenomenon first appears. As you might expect, glass or any other material through which light might pass exhibits the same phenomenon.

Consider a single glass fiber. The actual fiber is so thin that light entering one end will experience the "mirror effect" every time it touches the wall of the fiber. As a result, the light will travel from one end of the fiber to the other, bouncing back and forth between the walls of the fiber.

This is the basic concept of optical fibers, and it correctly describes the fundamental operation of all such fibers. Unfortunately, it is not possible to use fibers of this basic construction for any practical application. The reason for this has to do with the physical realities of the phenomenon of reflection within the fiber, and how the parameters involved will change under different conditions.

The basic fact governing the reflection of light within the fiber has to do with the speed of light inside the fiber and the speed of light in the medium just outside the fiber. Every possible material through which light can pass has a characteristic called the refractive index, which is a measure of the speed of light through that material as compared to the speed of light in open space.

One of the requirements of an optical fiber is that its diameter remains constant throughout its length. Any change in the thickness of the fiber will affect the way light reflects from the inner walls of the fiber. In some cases, this could even mean that the reflected light could exceed the critical angle required for total reflection, and so be lost through the walls of the fiber.

Unfortunately, the same effect will be noticed if the characteristics of the medium outside the fiber should change. For example, if the fiber gets wet (as it would in rain, fog, or some underground situations), the characteristics of the boundary between the inside and the outside of the fiber will change, and hence the effective shape of the fiber will change and will keep changing as drops of water move along the surface of the fiber.

The easiest way to ensure that the boundary between the inside of the fiber and the outside of the fiber remains constant and unchanging no matter what is to create a permanent boundary of known characteristics. The practical approach is to surround the glass fiber with another layer of glass while making sure that the speed of light in the outer layer remains faster than the speed of light in the inner fiber.

The original fiber is now the core of a two-layer construction. The diameter of the core is kept constant at approximately 50 to 60 µm (micrometers, at one time designated "microns") and its surface is kept as perfectly smooth as possible. The outer layer, known as cladding, is bonded at all points to the surface of the core.

To the outside world, this construction is effectively one solid piece of glass, even though it is constructed of two different types of glass. Thus, it is impervious to water, dirt and other materials. If the outer surface gets wet, that makes no difference because it still doesn't affect the boundary between the core and the cladding. The whole composite fiber may be covered with rubber or plastic for easier handling and visibility.