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Fiber-optic communication lines


Fiber-optic communication lines
In the XX century, mankind has witnessed a huge leap in the development of various forms of communication, especially telephony, radio and television. Thanks to them and thanks to the emergence of satellite space communication systems modern man was inaccessible to previous generations of the opportunity to communicate with the most distant and remote corners of the planet, to see, hear and know everything that happens in the world. However, with all the advantages of traditional forms of communication inherent in each of them and a number of disadvantages that are becoming more sensitive with the growth of the volume of information transmitted. Although the latest technology to significantly condense transmitted by cable information, main telephone lines still often overloaded. Much the same can be said about the radio and television, in which information signals are transferred via electromagnetic waves: an increasing number of TV channels and radio stations, broadcasters and service has led to the emergence of interference, a situation known as "crowding in the air." This was one of the shocks to the development of more and more short-wave radio wave bands. It is well known: the shorter used for broadcasting wave, the more frequencies without interference can be accommodated in this range (it is easy to see, turning the radio settings: if at long wavelengths, we can catch a few radio stations, the short and ultra-short waves of radio dozens and hundreds, they are literally "sitting on every millimeter"). Another drawback of conventional types of communication lies in the fact that for the transmission of information in general is unprofitable to use waves emitted into free space. After all, the energy per a specific area of ​​the front of this wave decreases with increasing wave front. For a spherical wave (ie one that applies uniformly in all directions from the source), the weakening of the inverse square of the distance from the wave source to the receiver. As a result, in modern radio spent huge sums on the selection and amplification of the desired signal. A completely different picture would be in if the information was sent to a narrow directional beam or ray. Losses at this would be much less. These shortcomings suggest that mankind is on the threshold of an important revolution in the communication system, which will lead to the fact that in the XXI century will be its main views optoelectronics, not having all these drawbacks. It is expected that in the first decades of the coming century, new telephone, television and computer systems will be connected by fiber-optic cables, using as a vehicle the laser information.

The era of modern optical communication began in 1960 after the creation of the first laser. The invention of lasers in general has given rise to hope for a quick and easy to overcome the problems of the "ethereal tightness." In fact, the use of micron waves of visible light for the needs of communication, instead of centimeter and millimeter radio waves created almost limitless opportunity to expand the volume of the transferred information. For example, the communication system for a helium-neon laser has a bandwidth, which can simultaneously accommodate one million television channels. However, the first experiments have dispelled optimistic illusions. It was found that the earth's atmosphere is actively absorbs and diffuses the optical radiation and lasers (in the case when the beam propagates directly from the air) can be used for communication needs only a very short distance (on average - not more than 1 km) All attempts to overcome this difficulty were not successful. This was the case when in 1966 two Japanese scientists Kao and Hokema proposed to use for the transmission of the light signal long glass fibers, such as those already used in endoscopy and other areas. Their article has laid the foundations of fiber-optic communication.

On what action is based fibers? Well known from optics, if direct light beam from a denser medium to a less dense (for example, from water glass or the air), a large part of it is reflected back from the interface of two media. The smaller the angle of incidence, the greater part of the light will be reflected. By experiment, we can choose a gentle angle at which reflected the whole world and only a tiny part of it becomes more of a dense medium to less dense. The light in this case is like a prisoner in a dense medium and apply it, repeating all her curves. This effect "light hold" can be seen in the light propagation inside the jet of water, which he can not escape, constantly bouncing off the water and air borders. Similarly, there is a transfer of a light signal on the optical glass fibers. Once inside it, the light beam propagates in different directions. Rays coming at a small angle to the boundary between two media, is fully reflected. Thus, the shell keeps them firmly, providing a light-tight channel for transmitting a signal with almost the speed of light.

In an ideal optical fibers made of completely transparent and homogeneous material, the light waves have spread unabated, but almost all of the real fibers more or less strongly absorb and scatter electromagnetic waves because of its opacity and heterogeneity. (Absorption outwardly manifested as heating the fiber; scattering - is when part of the radiation leaving the fiber.) Glass, which seems so clear in the windows, the windows and binoculars, in reality is far from homogeneous. It is easy to see, by looking through a sheet of glass butt. This will immediately become visible to his weak bluish-green color. Studies show that this color is caused by small amounts of iron and copper contained in the glass. Even in the cleanest glass produced for astronomical and photographic lenses, there are a large number of colored impurities. The first optical fibers made of such glass, the energy losses were very high (1 m fiber lost more than 50% of light input thereto). However, even with such as managed to create a device that allows light to pass through the curved channels, watch the inner surface of the metal cavities, examine the condition of the internal organs of the human body, etc. But to create the main communication lines such fibers have been of little use.

It took about a decade to establish laboratory samples fibers that can pass to 1 km 1% of the power light introduced therein. The next task was to make of such a fiber light guide cable is suitable for practical use, develop sources and radiation detectors. The simplest optical fiber is a thin thread of a transparent dielectric. The transmitted light waves are at small angles to the fiber axis and undergo total internal reflection from the surface. But to use such an optical fiber can be only in the laboratory, as unprotected glass surface under normal conditions gradually covered with dust particles, it develops on a lot of defects: cracks, irregularities that violate the conditions of total internal reflection of light within the fiber, very strongly absorb and scatter radiation. Substantial additional losses occur in places of contact with the optical fiber supports, supporting unprotected cable.

A radical change in the situation has been associated with the creation of two-layer fibers. Such fibers consist of light guiding core enclosed in a transparent envelope, whose refractive index was smaller than the refractive index of the core. If the thickness of the transparent shell exceeds a few wavelengths of the transmitted signal light, no dust, no properties of the medium out of the shell does not have a significant impact on the process of the light wave propagation in a two-layer optical fiber. These fibers can be coated with a polymeric shell and turn them into the light-guiding cable is suitable for practical use. But for this it is necessary to create a high degree of perfection the boundaries between the living and transparent shell. The simplest fiber manufacturing techniques is that the glass rod is inserted in the core-tight glass tube fitted with a lower refractive index. Then this design is heated.

In 1970, the company "Corning Glass" was first developed glass fibers suitable for the transmission of the light signal over long distances. But by the mid 70-ies were created fibers of ultra-pure silica glass, the intensity of light that is halved only at a distance of 6 km. (How is the transparent glass can be seen from the following example: if we imagine that the inserted ultrapure optical glass thickness of 10 km in the window, it will transmit light as well as an ordinary window glass centimeter thick!)

Also the fiber-fiber optical communication system includes an optical transmitter (in which the electrical signals from the input system are converted into optical pulses) and the optical receiver unit (receiving optical signals and converting them to electrical pulses). If the line has a large length, acting on it as repeaters - they accept and amplify the transmitted signals. The devices for the input radiation in optical fibers are widely used lenses, which have a very small diameter and focal length of the order of hundreds or tens of microns. The radiation sources can be of two types: lasers, light emitting diodes, which act as the carrier wave generator. The transmitted signal (which may be a television program, a telephone conversation, etc.) And superimposed on a modulated carrier wave in the same way as occurs in the radio.

However, much more efficient to transmit information in digital form. In this case, again, it does not matter what kind of information is transmitted as follows: telephone conversation, typed text, music, TV program or a picture image. The first step to convert the signal to digital form is to determine its value at certain time intervals - this process is called discretization signal time. Proved (mathematically and practically) that if interval T, at least 2 times less than the highest frequency contained in the spectrum of the transmitted signal, the signal can be further recovered from discrete shapes without distortion. That is, instead of a continuous signal without affecting the transmitted information can be fed a set of very short pulses, differing 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 form and are offset from each other on the same time interval T, it is possible not to transmit the entire signal, but only the value of its amplitude. In this example, the amplitude is split into eight levels. This means that the value of each pulse can be interpreted as a binary number. The value of this number and is transmitted through the communication line. As for the transmission of each of the binary number are required only two digits - 0 and 1, it is very simplified: 0 corresponds to the absence of a signal, and one - his presence. On the transfer of each digit in this example is the time in 1/3 T. recover the transmitted signal occurs in reverse order. Sending a signal in digital form is very convenient, since virtually eliminates any distortion and noise.

The optical communication system is still relatively expensive, which hinders its widespread, but there is no doubt that this is only a temporary obstacle. The advantages and benefits it is so obvious that it should certainly receive in the future widespread use. Firstly, fiber optic cables are very resistant to noise and are light weight. With the development of technology of mass production, they can be much cheaper than electric cables currently used as raw material for them is already much cheaper. But most importantly, their advantage is that they have a great capacity - in a time unit can pass therethrough such huge amounts of information which can not pass by any of the currently known methods of communication. All these features should provide a fiber-optic communication lines multifaceted use primarily in the computer units (already accumulated extensive experience in creating chips that use microscopic fibers, the speed of chips is about 1000 times higher than normal), in cable television; then will replace telephone cables on the main lines and the creation of television cables; in the long term it is expected to unite all of these networks into a single information network.

In many developed countries (primarily the US) now many telephone lines are replaced by optical fibers. The practice of creating urban fiber-optic networks. So, in 1976 the urban fiber-optic digital telephone system was installed in a large US city of Atlanta.

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