How do optical fibres work?

Everybody's heard of them. They are used to connect your computer to the internet, your sound boxes to your surround system at home, in the cable TV system. To understand why these optical fibres are used, we first have to know how optical fibres work. Unlike other cable-wiring, optical fibres use light instead of electrical signals to transport information. How is it possible that light can transport information? How is it even possible that light can travel through an optical fibre? The answer to these questions is total internal reflection. When, for example, a beam of light (traveling in a glass plate) strikes the glass-air interface, a part of the light is reflected (reflected beam) and a part of the light is transfered through (refracted beam) and escapes the glass plate. This phenomenon is depicted below for a water-air interface.



As the reflected beam stays inside the glass plate, we can see that the problem lies in the refracted beam: all light that escapes the optical fibre causes the light signal to be less strong. When the incident light beam, however, makes a large enough angle with the normal to the surface, all the light is being reflected.



There still is one detail missing in this explanation to be true, and that is that the light must be incident to a material with a lower index of refraction (this is indeed the case in the example stated). The index of refraction is a property of the material, determined by the speed of light in this material. In an optic fibre, the core of glass is surrounded by the cladding (a coating with a lower index of refraction), which makes sure total internal refraction occurs. The cladding itself is coated with a material that prevents physical damage and protects it from moisture, the buffer. This buffer may be further surrounded with a jacket that improves the strength properties of the fibre.



The situation we just described doesn't really exist. It's not possible to produce a core without any impurities, these impurities cause loss of information. This loss of signal, however, is less than that in copper wiring. Another advantage is that optical fibres are very thin: 10-70 microns. Because optical fibers are thinner than copper wires, more fibers can be bundled into a given-diameter cable than copper wires and more information can be transported. Since optical fibres use light signals, no interference occurs between adjacent fibres and the signal stays clean and clear. All this makes optical fibres the wiring of the future!