Tuesday, April 27, 2010

Quickfire Question: How do Fiber Optics work?

When you stick something in water - something like a pencil or a ruler is best since they are straight, you can see the object appear to bend at the surface of water.  This is due to the differences in refractive index between the water and the air.

All materials have a refractive index, because of the way that they interact with light. The vacuum, free space, has a refractive index "n" of 1, and all normal materials (negative refractive index is something I can cover another time!) have a refractive index higher than one. To give a couple of examples, for air, n is 1.0008, for water n is 1.330, for most ordinary glass, n is 1.51 and for diamond, n is 2.417. 

In a previous post, I mentioned Snell's Law,  this simple law relates the angles of incidence and refraction, and the refractive indices of the materials.

For something passing from a low refractive index to a high one at any angle, we can see that the light gets through, but what about the other way? If we try to calculate snell's law for certain angles, we see that the formula can't produce a result. At a very particular angle known as the critical angle, light can no longer escape from a high index material to a low one, and the light reflects from the surface.

This reflection is known as Total Internal Reflection. You can see total internal reflection when swimming underwater in a pool - look at the water's surface at a shallow angle, and it looks like a mirror.

This principle of total internal reflection is used in fiber optics to keep the light inside the fiber. A simple fiber optic is made of two materials - a core, with a high refractive index and a cladding with a low refractive index. Because of the TIR effect, light continuously reflects from the boundary, and is carried along the fiber.

There are a number of different sorts of optical fibre. Multimode fibers are generally wide compared to the wavelength of light, and as a result light can bounce at different angles (modes). some light may pass straight along the core, and some may bounce a lot from the edges. This causes the light to spread out. When the core is much narrower, then we may have a Monomode fibre, where the light can only pass in a straight line through the core (the mathematics of this are more complicated). As a result, the light does not spread out (due to reflections anyway!). The kind of fiber described above is known as a step index fibre, because the core immediately jumps from high to low index in the cladding. However one may also have a graded index fibre where the refractive index drops slowly towards the edge.

FIber optics are used in a broad range of applications, from telecommunications, to lighting applications, sensor applications and are commonly used for imaging in surgery. There are many other issues, complexities and types of fibres which build on the basic background introduced here.

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