Many polymers can form neat fibers, as long as certain intermolecular forces can occur between chains, holding them together in a crystal-like fashion. Polymer fibers are strong materials with excellent tensile strength, which makes them very useful as textiles.

1. Garth Naar - What are Optical Fibers Made Of ?
Surely the word fiber is not new for you, is it? Here is a fiber page that you could visit right
now, just in case you don't know what fibers are.
Many polymers can form neat fibers, as long as certain intermolecular forces can occur
between chains, holding them together in a crystal-like fashion. Polymer fibers are strong
materials with excellent tensile strength, which makes them very useful as textiles.
But have optical fibers something to do with those fibers we are talking about? Well, yes
and no. In fact, optical fibers have polymers in their composition. However, the term optical
fiber doesn't tell us anything about the polymers, but about two transparent, dielectric
tubes or cylinders, one surrounding the other, like this:
The First Steps
Back in the 1870s, British scientist John Tyndall performed a demonstration in which he
showed light guiding by means of a curved stream of water flowing from an illuminated
tank.
Between 1900 and 1930 numerous experiments followed Tyndall's demonstration. It was
discovered that bent thin glass rods not only could transmit light, but using a bundle of glass
rods (or optical fibers, as they were called later) complete images could be carried as well.
What type of phenomenon was involved in these sensational findings? Well, it was just
reflection, total internal reflection, as the walls of the thin fiber acted like mirrors in which
the incident light bounced back and forth.
You have many examples of total internal reflection in your everyday life. Just look for down
asphalt road on a hot, sunny day, and you should see reflection from the road surface, as if
the road had become a mirror. Well, it has due to the same property that causes internal
reflection: if light hits at a low enough angle, it can't penetrate the surface, and just bounces
off. Weird, hey?
This reflection, governed by Snell's law, is made possible due to the differences between the
refractive index of the glass and the air, the latter having the lowest value.
However, the usefulness of light guidance would not be completely appreciated until 1950,
when many scientists began to think in its potential applications. They were smart enough
to understand that such applications ranged from medicine, enabling the visualization of
inaccessible regions of the human body, to communication networks, in place of metal
wires.

2. But there were a number of drawbacks that arose in those first applications: light couldn't
be carried too far, and the transmitted images came pretty poor and distorted.
That was because the fibers had no coating at all surrounding them to increase the
difference in refractive index. For that reason, if such fibers got wet, for example, the glass-
air interface didn't remain fixed and the way the light reflected inside was changed.
Only in the mid 50s, Dutch Abraham van Heel designed a covering layer surrounding the
fiber with another cylinder of glass, of lower refractive index than that in the middle. This
way, the total reflection was not affected by water, dirt or other contamination. Finally, a
third layer was added, in order to protect the glass fiber from damage, and to make it easier
to handle. Guess what type of material was used in this third layer?
So what are optical fibers made of then?
Turns out they can be made of just glass, glass plus polymers, or just polymers ("plastic
optical fibers" -POF-). The most basic optical fiber consist of:
An inner cylinder with high refractive index, called the core.
A middle cylinder with a lower refractive index, called the cladding.
An outer protective polymer layer (usually polyurethane or PVC) called the jacket.
For glass optical fibers, the diameter of the core ranges between 10-600 microns, the
cladding thickness is between 125-630 microns, and that of the jacket varies between 250-

3. 1040 microns. For POF all diameters range between 750-2000 microns. As can be seen, one
of the main differences between glass and plastic optical fibers is their diameter. This makes
POF easier to handle.
The material used for currently commercialized fibers (core and cladding) include pure glass
(SiO2), plastic, or a combination of both. The use of one or the other material will be
determined by such factors as quality and economics.
Plastic optical fibers (POF) have the advantage of being made of cheaper materials than
glass and to operate in the visible range of the spectrum. However, they show a high loss,
and for that reason their applications are confined to short distance transmission. In spite of
this, POF is widely used for medical and industrial instruments, and currently research is
carried out about using POF as a replacement of copper wiring for data transmission in
automobiles.
If you use silica glass for the core, it must be high purity in order to allow the light to be
transmitted along the core with minimal loss.
Many Optical Fibers In The Market
There are several types of fibers based both on the transmission modes and the refractive
index profile. We won't discuss them in detail, but it is important to know that the design of
an optical fiber depends on the needs it is supposed to meet. Key parameters like
attenuation, bandwidth, dispersion, and tensile strength are the most considered.
Also the protection of the fiber from external factors like humidity, heat, cold, and water is
contemplated. That's why plastic sheaths where the fiber is enclosed in, are used. The
whole material comprising the single fiber or bundle of fibers, the sheaths, and the jacket, is
usually referred as a fiber cable.
Obviously these cables also must satisfy requirements such as high flexibility, resistance to
kinks and crushing, and light weight.
Designing Your Own Fiber Cables
In the first example on the right we have a multi-layered system in which the fiber is firstly
surrounded by a buffer tube. This buffer tube is usually a layer of silicone or epoxy resin,
softer than the external jacket, and has no optical function. It keeps the fiber from
"microbends" due to physical contact with the other components of the cable. As relatively
fragile materials, fibers need some mechanical reinforcement. Many materials in form of
strands or filaments can play this role. One of them is fiberglass, and in the example, we can
see a bunch of fiberglass strands surrounding the buffer tube.

4. Of course, since fiberglass is a stiff material, another layer of polyurethane is added, to
provide cushioning. If your fiber cable needs a good tensile strength as well as electrical
insulation, which is highly desirable, you can place a layer of Kevlar®. Have you ever heard of
Kevlar® and its wonderful properties? If not, click here to learn something about it. Usually
Kevlar® is arranged into the cable in the form of filaments. Finally, you must think that your
fiber cable may be placed in a number of different environments: in the air, into the water,
or under ground. Therefore an external protective jacket becomes essential. PVC and
polyurethane are the most used materials for that purpose. But be careful with the choice:
PVC is a better material than polyurethane when considering its resistance to: water, flame,
acids, alkalis, hydrocarbons, and alcohol. Conversely, polyurethane has certain advantages
over PVC when dealing with its resistance to: abrasion, nuclear radiation, and low
temperatures.
In the second example on the left (redrawn from "Fibre Optics" / S. Ungar, John Wiley &
Sons, 1990) we have an exterior fiber cable with a buffer jacket which can be constituted by
two sheaths: one of silicone, and the other of Hytrel®, extruded on silicone. Hytrel® is a neat
polyether-ester block copolymer, a thermoplastic elastomer with optimum water resistance,
sold by DuPont. Between two layers of Kevlar® filaments, a moisture barrier is inserted. This
moisture barrier can be made of plastic (often polyethylene), metal (particularly aluminum),
or both. Lastly, a PVC coating ensures the utility of this fiber cable to be installed in free air.

5. The third example on the right (redrawn from "Fibre Optics" / S. Ungar, John Wiley & Sons,
1990) is a slight modification of the second. Only this time an underground fiber cable is
shown. Since it requires a further protection against humidity, two sheaths have been
inserted between a Kevlar® layer: a moisture barrier and a metallic shield.
Last But Not Least: Why Optical Fibers Are So Useful?
Don't get confused:
Fiber optics (FO) is a science, by which electrical energy is converted into light (or optical
energy); that light is transported through optical fibers to some other place, and finally is
converted again in electrical energy.
Sounds amazing that the waves along our telephone line or our cable television travel in
form of light, which many times falls in the visible frequency spectrum.
The enormous advantages of the optical fiber telecommunications compared to metal wire
or coaxial transmission may be summarized as follows:
High bandwidth, completely independent of the cable size.
Low attenuation, i.e., optical loss is reduced to a minimum.
Extremely low electromagnetic induction, induced noise, and crosstalk.
Availability of long distance light-weight cables.
Lower cost in installation and maintenance.