Part 1: Why Is Optical Fiber Flexible?

In the Innovations gallery we get many questions about all sorts of topics involving glass. One of the most frequently asked questions, in reference to optical fiber, is “How can glass be so flexible?” To answer this we will be doing a two part blog; the first to figure out why things bend at all, and the second to find out why things break, or don’t break, when bending.

Asking why fiber bends so much is a really good question because most of us have never seen glass bend. We know glass to be rigid and brittle, to break with very little impact. We think of bottles and windows and light bulbs, none of which bend in any noticeable way without breaking.

Believe it or not though, all types of glass can bend. If you go to the Flameworking Demo or the Hot Glass Demo, the glassworkers can pull a piece of glass to less than a millimeter thick and it will bend. Not a whole lot, grant you, but it will bend. But how can glass be flexible at all, and more specifically how can optical fiber be so flexible that you can nearly tie it in a knot?

The short answer to why glass bends is that it does so only when the glass is very thin, and in the case of optical fiber, it bends so much better because it is very pure glass, manufactured with very few defects and with a pristine surface.

But why do these things matter?

Let’s look first at why any material bends at all. If we look at the mechanics of how material changes as it is bent, you will see that on the inside of the curve the material will compress and the outside of the curve the material will stretch. If you do this with a piece of leather, you will notice the inside of the curve wrinkles and the outside of the curve starts developing little cracks. The thicker the material, the further the outside of the curve will have to stretch to match the inside curve. This is true for all materials.

In metal, the molecules are bonded together loosely with metallic bonds and are in an orderly arrangement. These allows the atoms to slide over one another as the material flexes. This makes the material able to stretch and bend as sheets of molecules slide over one another, pulling apart or pressing together without the material breaking. Materials like metal stretch fairly well because of the flexibility of the material at an atomic level.

Glass is an interesting material on an atomic level. The atoms are not very well ordered so that they cannot slip past one another, which makes it structurally very stiff. A good way to think about this is looking at a simple bookcase made of wood. If all you have are the shelves and the sides, the bookcase can easily be forced to lean to one side or another since all the connections are lined up. But if you add braces at an angle, then the connections do not line up and the bookcase is much more rigid. It will not bend, but it will break under enough force. Metal has only the shelves and sides, glass has angled braces.

Thus, glass does not stretch very well at all when cool. It is indeed a rigid and brittle material because the molecules are locked into place with little wiggle room. So bending even a relatively thin rod of glass will cause the outer curve to have to stretch far more than the glass is capable of doing. This is why glass must be extremely thin to allow bending at all. Remember the thinner the material the less stretching that outer curve must do.

So we know how things bend, and that sort of helps us, but we really need to know is why do they break? Next time we will look at how these materials behave while bending and what that means in terms of their strength and brittleness.