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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Fiber-optic communication line. History of invention and production
Directory / The history of technology, technology, objects around us Fiber-optic communication line (transmission) - a fiber-optic system consisting of passive and active elements, designed to transmit information in the optical (usually near infrared) range.
In the XNUMXth century, mankind has witnessed a huge leap in the development of various types of communication, especially telephony, radio and television. Thanks to them, as well as thanks to the emergence of a satellite space communication system, modern man has received an opportunity inaccessible to previous generations to communicate with the most distant and remote corners of the planet, to see, hear and know about everything that happens in the world. However, with all the advantages of traditional types of communication, each of them has a number of disadvantages, which become more and more sensitive as the volume of transmitted information increases. Despite the latest technology, which can significantly condense the information transmitted over the cable, the main telephone lines are still often overloaded. Approximately the same can be said about radio and television, in which information signals are carried using electromagnetic waves: an ever-increasing number of TV channels and radio stations, broadcasting and service, has led to the emergence of mutual interference, to a situation called "crowded air". This was one of the impetuses for the development of more and more shortwave radio wave bands. It is known: the shorter the waves used for broadcasting, the more radio stations without mutual interference can fit in a given range (this is easy to see by rotating the radio setting: if we can catch only a few radio stations on long waves, then there are already dozens of such radio stations on short and ultrashort waves and hundreds, they literally "sit on every millimeter"). Another disadvantage of traditional types of communication is that it is generally unprofitable to use waves radiated into free space to transmit information. After all, the energy per a certain area of the front of such a wave decreases as the wave front increases. For a spherical wave (that is, one that propagates uniformly in all directions from the source), the attenuation is inversely proportional to the square of the distance from the wave source to the receiver. As a result, modern radio engineering spends a lot of money on isolating and amplifying a useful signal. A completely different picture would be if the information was sent by a narrow directed beam or beam. Losses in this case would be much less. These shortcomings suggest that humanity is on the verge of an important revolution in the communication system, which will lead to the fact that in the XNUMXst century, optoelectronics will become its main type, which does not have all these shortcomings. It is expected that already in the first decades of the coming century, all new telephone, television and computer systems will be connected by fiber optic cables using laser radiation as an information carrier. The era of modern optical communication began in 1960 with the creation of the first laser. The invention of lasers in general gave rise to the hope for a quick and easy overcoming of the problems of "aether crowding". Indeed, the use of micron-waves of visible light for communication needs instead of centimeter and millimeter radio waves made it possible to expand the volume of transmitted information almost indefinitely. For example, a helium-neon laser communication system has a bandwidth that can simultaneously accommodate about a million television channels. However, the first experiments dispelled the rosy illusions. It turned out that the earth's atmosphere very actively absorbs and scatters optical radiation and that lasers (in the event that the beam propagates directly through the air) can only be used for communication needs at a very short distance (on average, no more than 1 km) All attempts were unable to overcome this difficulty. This was the case when, in 1966, two Japanese scientists, Kao and Hokema, proposed the use of long glass fibers to transmit the light signal, similar to those already used in endoscopy and other fields. Their article laid the foundation for fiber optic communications. What is the basis of the action of light guides? It is well known from optics that if a light beam is directed from a denser medium to a less dense one (for example, from water or glass into air), then a significant part of it is reflected back from the boundary of the two media. In this case, the smaller the angle of incidence of the beam, the greater part of the light flux will be reflected. Through experiment, one can choose such a gentle angle at which all the light is reflected and only an insignificant part of it gets from a denser medium to a less dense one. In this case, the light turns out to be like a prisoner in a dense medium and propagates in it, repeating all its bends. This effect of "retaining light" can be seen in the example of the propagation of light inside a jet of water, which it cannot leave, constantly reflecting from the boundary of water and air. In the same way, a light signal is transmitted through an optical glass fiber. Entering inside it, the light beam propagates in different directions. Rays traveling at a small angle to the boundary of two media are completely reflected from it. Thus, the shell firmly holds them, providing an opaque channel for signal transmission at almost the speed of light.
In ideal light guides made of absolutely transparent and homogeneous material, light waves should propagate unabated, but almost all real light guides more or less strongly absorb and scatter electromagnetic waves due to their opacity and inhomogeneity. (Absorption appears externally as heating of the fiber; scattering is when some of the radiation leaves the fibre.) Glass that appears so transparent in windows, storefronts, and binoculars is actually far from uniform. This is easy to see by looking through the end face of sheet glass. At the same time, its faint bluish-green color becomes immediately visible. Studies show that this coloration is caused by small amounts of iron and copper found in the glass. Even the purest glasses made for astronomical and photographic lenses contain large amounts of colored impurities. In the first light guides made of such glass, the energy losses were very high (more than 1% of the light introduced into it was lost per 50 m of the light guide). However, even with this quality, it was possible to create devices that made it possible to transmit light through curved channels, observe the internal surfaces of metal cavities, study the state of the internal organs of the human body, etc. But for the creation of trunk communication lines, such light guides were of little use. It took about a decade to create laboratory samples of optical fibers capable of transmitting 1% of the light power introduced into them per 1 km. The next task was to make a light-guide cable suitable for practical use from such a fiber, to develop sources and receivers of radiation. The simplest optical fiber is a thin filament of a transparent dielectric. The transmitted light waves travel at small angles to the axis of the fiber and experience total internal reflection from its surface. But such a light guide can only be used in a laboratory, since under normal conditions the unprotected glass surface is gradually covered with dust particles, many defects develop on it: microcracks, irregularities that violate the conditions for total internal reflection of light inside the fiber, very strongly absorb and scatter rays. Significant additional losses occur at the points of contact between the optical fiber and supports supporting the unprotected cable.
A radical change in the situation was associated with the creation of two-layer light guides. Such light guides consisted of a light guide strand enclosed in a transparent sheath, the refractive index of which was lower than that of the strand. If the thickness of the transparent shell exceeds several wavelengths of the transmitted light signal, then neither dust nor the properties of the medium outside this shell have a significant effect on the process of light wave propagation in a two-layer light guide. These light guides can be coated with a polymer sheath and turned into a light guide cable suitable for practical applications. But for this it is necessary to create a high perfection of the boundary between the vein and the transparent shell. The simplest fiber manufacturing technology is that the glass core is inserted into a tightly fitted glass tube with a lower refractive index. Then this structure is heated. In 1970, Corning Glass pioneered the development of glass light guides suitable for transmitting light signals over long distances. And by the mid-70s, light guides made of ultra-pure quartz glass were created, the light intensity in which was halved only at a distance of 6 km. (How transparent such glass is can be seen from the following example: if you imagine that ultra-clear optical glass 10 km thick is inserted into the window, then it will transmit light as well as ordinary window glass of a centimeter thickness!)
In addition to the light guide, the fiber-optic communication system includes an optical transmitter unit (in which the electrical signals entering the system input are converted into optical pulses) and an optical receiver unit (receiving optical signals and converting them into electrical pulses). If the line is long, repeaters also operate on it - they receive and amplify the transmitted signals. In devices for inputting radiation into optical fibers, lenses are widely used, which have a very small diameter and a focal length of the order of hundreds and tens of microns. Radiation sources can be of two types: lasers and light emitting diodes, which work as carrier wave generators. The transmitted signal (it can be a television broadcast, telephone conversation, etc.) is modulated and superimposed on the carrier wave in the same way as it is in radio engineering. However, it is much more efficient to transmit information in digital form. In this case, again, it does not matter at all what information is transmitted in this way: a telephone conversation, printed text, music, a television program or an image of a picture. The first step in converting a signal to digital form is to determine its values at certain time intervals - this process is called signal sampling over time. It has been proven (both mathematically and practically) that if the interval T is at least 2 times less than the highest frequency contained in the spectrum of the transmitted signal, then this signal can be further restored from a discrete form without any distortion. That is, instead of a continuous signal, without prejudice to the transmitted information, you can apply a set of very short pulses that differ from each other only in their amplitude. But there is no need to transmit these impulses in this form. Since they all have the same shape and are shifted relative to each other by the same time interval T, it is possible to transmit not the entire signal, but only the value of its amplitude. In our example, the breakdown by amplitude goes into eight levels. This means that the value of each pulse can be interpreted as a binary number. The value of this number is transmitted over the communication line. Since only two digits, 0 and 1, are needed to transmit each binary number, it is greatly simplified: 0 corresponds to the absence of a signal, and 1 to its presence. In our example, it takes 1/3 T to transmit each digit. The transmitted signal is restored in the reverse order. Applying a signal in digital form is very convenient, as it virtually eliminates any distortion and interference.
The optical communication system is still relatively expensive, which hinders its widespread adoption, but there is no doubt that this is only a temporary obstacle. Its merits and advantages are so obvious that it must certainly receive widespread use in the future. First of all, fiber optic cables are very resistant to interference and are light in weight. When mastering the technology of their mass production, they can be much cheaper than the electric cables currently used, since the raw materials for them are already much cheaper. But their most important advantage lies in the fact that they have a huge bandwidth - in a unit of time, such huge amounts of information can be passed through them, which cannot be transmitted by any of the currently known communication methods. All these qualities should provide fiber-optic communication lines with multifaceted applications, primarily in computer units (a lot of experience has already been accumulated in creating microcircuits that use microscopic light guides; the speed of such microcircuits is about 1000 times greater than that of conventional ones), in cable television; then there will be a replacement of telephone cables on trunk lines and the creation of television cables; in the future, it is expected to combine all these networks into a single information network. In many developed countries (and primarily in the USA), many telephone communication lines have already been replaced by light guides. The creation of urban fiber-optic networks is being practiced. So, in 1976, the urban digital fiber-optic telephone communication system was installed in the large American city of Atlanta. Author: Ryzhov K.V.
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