How Do Fiber Optics Work and What are the Advantages vs. Copper

Fiber Optics

In recent years it has become apparent that fiber optics are steadily replacing copper wire as an appropriate means of communication signal transmission. Fiber optic systems are currently used most extensively as the transmission link between terrestrial hardwired systems. They span the long distances between local phone systems as well as other system users which include cable internet and television services, university campuses, office buildings, industrial plants, and electric utility companies.

Fiber Optic Technology

A fiber-optic system can generally be seen as a system with three main components: a transmitter, a transmission medium and a receiver. As a model, it is similar to the copper wire network that the fiber optic network is replacing. The difference is that fiber optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down copper lines. Looking at the three main components in the fiber optic infrastructure will give a better understanding of how the system works versus copper based systems.

At the head end of the chain is a transmitter. This is the place of origin for information coming on to fiber optic lines. The transmitter accepts coded electronic pulse information coming from copper wire. It then processes and translates that information into equivalently coded light pulses. A light-emitting diode (LED) or an injection-laser diode (ILD) can be used for generating the light pulses. Using a lens, the light pulses are tunneled into the fiber-optic medium where they transmit themselves down the line.

Light pulses move easily down the fiber-optic line because of a principle known as total internal reflection. This principle of total internal reflection states that when the angle of incidence exceeds a critical value, light cannot get out of the glass; instead, the light bounces back in. When this principle is applied to the construction of the fiber-optic strand, it is possible to transmit information down fiber lines in the form of light pulses.

There are generally five elements that make up the construction of a fiber-optic cable: the optic core, optic cladding, a buffer material, a strength material and the outer jacket. The optic core is the light carrying element at the center of the optical fiber. It is commonly made from a combination of silica and germanium. Surrounding the core is the optic cladding made of pure silica. It is this combination that makes the principle of total internal reflection possible. The difference in materials used in the making of the core and the cladding creates an extremely reflective surface at the point in which they interface. Light pulses entering the fiber core reflect off the core/cladding interface and thus remain within the core as they move down the line.

Surrounding the cladding is a PVC (polyvinyl chloride) buffer material used to help shield the core and cladding from damage. A strength material surrounds the buffer, preventing stretch problems when the fiber cable is being pulled. The outer jacket is added to protect against abrasion, solvents, and other contaminants.

Once the light pulses reach their destination they are channeled into the optical receiver. The basic purpose of an optical receiver is to detect the received light incident on it and to convert it to an electrical signal containing the information impressed on the light at the transmitting end. In other words, the coded light pulse information is translated back into its original state as coded electronic information. The electronic information is then ready for input into electronic based communication devices such as a computer, telephone or TV.


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 greater bandwidth and can therefore carry more information at one time. Fiber based networks also allow transmission over greater distance because they experience much lower attenuation or loss of signal.

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 a vacuum.

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 in multimode fiber is kept constant at approximately 50 to 62.5 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 jacket for easier handling and visibility.