Now, perhaps the most interesting part. We talked about the manufacturing process and a little about the physical principles of work. Well, now let's figure out why, in fact, fiber optics is now the basis of modern telecommunications.
We know how to generate traffic. The development of semiconductor technology has led to a tremendous increase in the computing power of computers. This could not but lead to an increase in the volume of generated information. But there is little use from huge deposits of information if it cannot be quickly conveyed. How to transmit a large amount of information over a distance quickly? That's right, you need to take a high-speed communication channel. And here mankind, at some point in its technological development, buried itself in a problem. By a certain time, the realization came that there were no sufficiently high-speed communication channels. And if theoretically they are, then they are prohibitively expensive and complex. Of course, all this was long ago, when computers were big. But even then, the issue of expanding the channels arose more and more clearly. By the 60-70s, he already demanded a solution, despite the fact that the amount of information generated by computers was negligible compared to the traffic generated by telephone networks, television and radio. It's all different now.
We know that information can be transmitted by electromagnetic waves. They can spread both in air (vacuum) and through wires - copper coaxial or twisted. It is quite obvious that whatever information is - analog or digital - the speed of its transmission depends on the frequency of the electromagnetic wave carrying this information. The higher the carrier frequency, the higher the information transfer rate can be. Thus, it is quite obvious that in order to increase the transmission rate in any medium, it is necessary to fundamentally increase the carrier frequency. It is important.
Now let's remember a physics course from school - a lesson about electromagnetic waves. Imagine the frequency scale of electromagnetic waves:
Have you presented? Radio waves and microwaves - infrared light - visible light - ultraviolet - X-ray - gamma radiation. Picture from a physics textbook. Far infrared light conditionally borders on the radio range, and near IR is already the optical range, which includes visible light. What frequencies does the radio work on? Hundreds of megahertz. What about WiFi, Bluetooth, etc.? Several gigahertz. It is possible to further increase the frequency of the radio signal. But a high frequency generator, especially at high powers, required for transmission over long distances, is a non-trivial and very complicated thing. Semiconductor electronics have a "ceiling" of operating frequencies. This is already a fundamental limitation - a pn junction simply cannot run faster. Fastest semiconductor transistoroperates at a frequency of about 1 THz at a temperature of 4.7K. And in the 60s such a frequency was never dreamed of. Therefore, in the radio range, the frequency can no longer be increased. A new source of high-frequency electromagnetic oscillations with a much higher frequency is needed.
What available sources of sufficiently high-frequency electromagnetic waves can be offered? If you look at the picture above, you can see that infrared and visible light go further along the frequency scale from radio waves. We can generate light, we can somehow control it too. Already good. In 1960, the world's first laser appears . A laser is a generator of high-frequency electromagnetic waves with a specific frequency, light waves. Unlike a light bulb, a laser generates a very narrow spectrum, almost one wavelength. And his radiationcoherent . Therefore, the laser is suitable for the role of a carrier frequency generator for high-speed data transmission. Near-IR frequencies - hundreds of THz - the frequency is higher than traditional radio waves by 4-5 orders of magnitude. The source of the carrier high-frequency electromagnetic wave appeared, and the prospect for the development of high-speed data transmission arose.
The first gas laser showed the theoretical possibility of creating a coherent source of electromagnetic waves of light frequency. The emergence and development of other types of lasers is a matter of time, because the principles of their operation have become widely known. But the issue of transmission of such high-frequency electromagnetic radiation over long distances has become extremely urgent. Light lives according to the laws of optics, unlike radio waves, which means that it was necessary to find an analogue to coaxial cables, only for light.
Already in 1966, researchers Kao and Hookam from STC Laboratory presented the first optical fibers in the form of ordinary glass filaments. The attenuation of light in them was about 1000 dB / km, which made it impossible to transmit any signal over long distances. Such losses were due to the presence of a large amount of impurities in the glass.
In the 1970s, Corning optical fibers appeared with an attenuation of about 20 dB / km. Now such values seem incompatible with data transmission, but then they seemed acceptable for organizing fiber communication. Around the same time, fairly compact gallium arsenide semiconductor lasers were invented... From 1975 to 1980, the first commercial communication line with a speed of 45 Mbit / s was implemented, and already in 1988 the first transatlantic fiber-optic cable was laid.
Fiber types
Any fiber, incl. telecommunications are divided into two types: single-mode and multi-mode. Despite the huge variety of species, each of them belongs to either one or another type. How they differ - let's figure it out. Historically, it so happened that the first commercial fibers, due to imperfect manufacturing technology, had a fairly thick light-carrying core. Several light modes could propagate in it, so they are called multimode. Let's understand on our fingers what a light mode is.
Light is an electromagnetic wave. The light from the laser is a coherent wave, which means it can interfere. It can also interfere in the fiber, i.e. fiber. Contrary to popular belief, laser light is introduced into the fiber not in the form of an absolutely ideal parallel narrow beam, but with a certain angular divergence. And it is not that small. And it is impossible to form an ideal parallel beam - there is always some divergence. Imagine that such a beam was introduced into the fiber with some divergence. The beam, propagating in the core, at some point will begin to reflect from the upper and lower boundaries of the core and the substrate. The reflected portions of the beam will form interference as they are coherent. Interference, as you know, is an alternation of light and dark stripes, a discrete spatial structure of the redistribution of light intensity.
Complicated? Not really. The light mode is just a standing wave of light that has arisen in the cross section of the fiber. I think there is no need to explain what a standing wave is. If the standing wave has one antinode in the fiber section, then this is the first mode, if 2 is the second, 3 is the third, etc. Modes are stable discrete spatial-energy structures of the distribution of the electromagnetic field of a light wave, caused by the appearance of interference on light reflections from the walls of the fiber. A mode in a fiber only occurs if light has been introduced into the fiber at a certain angle. The angle at which light enters the fiber at which a particular light mode is formed is called the mode angle. Light introduced at a non-mode angle will pump its energy into the nearest modes or radiate outward. In other words, light in the fiber can only propagate at certain angles - mode. At these angles, standing waves appear in the fiber cross section.
By reducing the size of the light-carrying core, a single-mode operation of the fiber can be achieved. Moreover, the standing wave in it has only one antinode.
In thick fibers, the size of the core of which is much larger than the wavelength of light, the number of modes is very large. Such fibers are called ordinary light guides, and the laws of ray optics can be applied to them with some reservation. Light guides with a relatively small number of modes, as well as single-mode ones, are usually called waveguides, and when calculating them, it is necessary to take into account the wave properties of light. Optical waveguides are analogous to coaxial cables for light.
As mentioned, historically, multimode fibers were the first. They have a significant disadvantage that limits the transmission rate and range: intermode dispersion.
The first multimode waveguides with a stepped refractive index profile (Fig. A) also had a significant temporal broadening of the light pulse and distorted its shape. The figure clearly shows the mechanism of this process. The light pulse introduced into the fiber decayed into discrete modes, however, due to different angles, each mode had a different optical path, and hence a different propagation time. In practice, this led to the fact that the light pulse was stretched in time and could overlap with the next one following it. This meant a lot of mistakes and loss of information.
Subsequently, the technology made it possible to manufacture multimode waveguides with a gradient profile of the refractive index (Fig. B). This led to a decrease in the intermode dispersion and an increase in the transmission rate, but did not fundamentally solve the problem.Single-mode fiber made it possible to significantly increase the speed and transmission distance. If there are no extraneous modes, then there is no intermode dispersion, and the light pulse is not broadened.
Nowadays both types of fiber are in use, however, single-mode is much more common. For the price, it no longer exceeds the multimode ones. The standard single mode is ubiquitous. The modern telecom operator rarely thinks about the type of fiber. The default is single-mode everywhere. Of course, there are also specific types of fibers used in telecommunications: non-zero dispersion, non-zero dispersion shifted, negative dispersion, active fibers with dopants, etc., but it is not possible to consider them within the framework of this article.
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