Good day, reader! Surely everyone has heard of fiber optics, many have dealt with it in telecommunications, someone even has a fiber-optic cable installed in the house. In general, this term is well known; and the essence of the question - the transmission of light over a distance along a thin fiber - is generally clear. But I suggest going a little deeper into this technology, which has changed the world in many ways. In this post I will try to explain in a simple and understandable language the essence of fiber optics, how it works - at the level of simple physical understanding, with simplifications and examples, without scary formulas. In other words, “on the fingers”. If interested, then welcome to cat. Caution : a lot of text, there are pictures.
By the will of fate, it so happened that my education, and then my professional activity, is closely connected with optical fiber and lasers. After working in telecom for some time, and then moving into the field of scientific development and measurements, I had the opportunity to notice that not everyone is familiar with optical fiber at the level of deep understanding, even among laser technicians and telecom operators. Telecoms tend to perceive fiber at the patch cord or cable level. For them, it is a patch cord or an abstract communication line. Yes, with attenuation, dispersion, welds and reflectograms, but only with a superficial understanding of the physical principle of operation. Of course, this is not a bad thing at all, just such are the features of their work. Anyway, the desire to write a popular science article on the very essence of fiber optics has arisen more than once, especially since education and experience allow us to do this: everything that is written in this article is not only "material from textbooks", but also my personal experience. On the one hand, I would like to dwell on many points in detail, but on the other, the article will be too large. It was decided to do this: this article is an introductory overview. If the public is interested, a series of posts will follow, devoted to the most interesting questions in the vast fiber-optic topic. I hope it will be interesting. It was decided to do this: this article is an introductory overview. If the public is interested, a series of posts will follow, devoted to the most interesting questions in the vast fiber-optic topic. I hope it will be interesting. It was decided to do this: this article is an introductory overview. If the public is interested, a series of posts will follow, devoted to the most interesting questions in the vast fiber-optic topic. I hope it will be interesting splicing fiber optic cable.
How it works?
The first thought that arose in my, then a child's brain, when I first saw such a light-guide lamp , was "How does it work !?" At school we were taught that light in a homogeneous environment propagates in a straight line. How to bend the light? I learned the answer a little later. We have all heard about the effect of total internal reflection, the so-called. Air defense. If light leaves a denser optical medium (glass) into a less dense optical medium (air), n glass> nair, then at a certain angle of incidence the light may not come out, but will be reflected back. This is all well known to us since school. This phenomenon hides rather thick equations of wave propagation of light and volume theory. But we do not need it now, it is enough to know what it is. Almost everyone watched air defense in everyday life. At least the one who dived with a mask under the water. From the water we can see everything that is directly above us, but at some periphery we see the mirror surface of the water and do not see what is above it - this is the air defense.
Now let's imagine we have a sheet of glass in the air. If you shine a laser pointer at the end of it at a small angle to the plane of the surface, then its light, repeatedly reflected, will come out from the other side of this glass - this is the phenomenon of air defense. Now let's take a glass rod - the effect will be the same. In this case, the light is limited not in one, but already in two planes, of course, if the angle of incidence of the light does not exceed the TIR angle.
But if we replace the glass rod with a transparent fishing line, then the light will spread in it, but it can already be "bent". Of course, as long as the bending radius is large enough. When the bend radius becomes small, light will come out of the line at this point, since the angle of incidence of the light on the surface of the line will be greater than the TOR angle. Notice that the line doesn't have a mirror finish, the light is held in it by itself. This is roughly how optical fibers work. The light in them spreads until the air defense law is violated and the light leaves the light-guiding vein. Optical fiber is essentially the same line in a decorative lamp, but with a more complex structure.
In general, there are a huge number of types of optical fibers, differing in shape, size, material, coating, properties, fields of application, etc. Reviewing and comparing different types of fibers is a topic for another huge article. However, all these fibers are structurally united by one thing: they have a light-carrying core (core) with a higher refractive index and a cladding (cladding) with a lower refractive index. Due to this, the air defense effect is achieved. As for the dimensions of the fibers, then, depending on the design and field of application, they can be from 50 microns to 1 mm or more in diameter (meaning the fiber itself without protective sheaths). Applying various protective sheaths increases the fiber diameter several times. In this article, I will consider only the simplest and most common types of fibers used in telecommunications.
How is it done?
Telecommunication fibers, and many others, in 99.9% of cases are made of pure quartz glass. Chemical formula SiO2. Window glasses are made from it, but with impurities that block UV radiation: Na2C03, K2CO3, CaCO3. There is enough information on Wikipedia about silicon dioxide. Yes, flexible optical fibers are actually made of glass - some people didn't believe me. The stereotype that glass does not bend, but pricks and beats is firmly embedded in human heads.
It is well known that glass has an amorphous structure, which means that it does not have a fixed melting point like crystalline substances. When heated, the glass softens and becomes viscous and you can easily pull the "thread" out of it. However, such a fiber, although it bends, is very fragile, since microcracks quickly form on its surface, which destroy the fiber when the stress on their surface increases during bending. The freshly drawn fiber is immediately covered with a polymer film that protects against microcracks. But first things first.
I will describe the "classical" scheme for the manufacture of single-mode telecommunication fibers with vapor deposition. First, a glass tube is taken about a meter long or slightly more and a few centimeters thick. It is hollow inside. Its inner diameter determines the thickness of the light-carrying core. The main difference between such glass is a very high degree of purification from impurities and OH-groups. This is necessary in order for the fiber to have maximum transparency. The pipe is laid on the machine and begins to rotate around its axis, gradually warming up with a burner with a temperature of 1200-1500 ° C. A mixture of gases O2, SiCl4, GeCl4, etc., provided by the technology, is blown into the pipe from the end under pressure.
Epitaxy of germanium dioxide and SiO2 occurs on the surface of the hot glass billet pipe. Germanium dioxide increases the refractive index of pure quartz and has practically no effect on transparency. The desired profile of the refractive index of the core is grown from the gas phase by adjusting the ratio of gases supplied to the preform.
After building up a layer of the required thickness, the burner temperature increases. The glass softens more and the core cavity through which the gas was blown gradually collapses under the action of the surface tension force. It looks something like this:
And this is how the blanks ready for drawing look like:
It turns out a one-piece glass rod with a high refractive index inside - this is the future light-guiding core. Then the rod is installed vertically and the burner heats up its lower end, softening it more. A dummy bar is brought to the workpiece, after which the stretching process begins. Who ever glued with glue a la "Moment" perfectly knows what it looks like.
