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Fiber optic network


Characteristics of optical systems

Fiber-optic lines - this type of communication, where information is transmitted by dielectric optical waveguides, known as "optical fiber". Optical fiber is now considered the most perfect physical environment for the transmission of information, as well as the most promising medium for the transmission of large flows of information at longer distances. Reasons to consider both derive from a number of features of an optical waveguide.

Physical Features

Broadband optical signals due to the extremely high frequency carrier (Fo = 10 ** 14 Hz). This means that for the optical lines can transmit information at speeds of about 10 ** 12 bps or Terabit / sec. In other words, one fiber can transmit 10 million simultaneous phone calls and a million video. Data transfer rate can be increased through the transfer of information at once in two directions, as well as light waves can propagate in a fiber independently of each other. In addition, the optical fiber can propagate light signals of different polarizations, thus doubling the capacity of optical communication channel. To date, the limit on the density of information transmitted by optical fiber is not reached.

very small (compared with other media) attenuation of the light in the fiber. The best Russian fibers have attenuation 0.22 dB / km at a wavelength of 1.55 microns, which allows you to build a link length of up to 100 kilometers without regeneration of signals. For comparison, the best Sumitomo fiber at a wavelength of 1.55 microns is attenuation 0.154 dB / km. In the U.S. optical laboratories have developed a more "transparent", so-called ftortsirkonatnye fibers with a theoretical limit of approximately 0.02 dB / km at a wavelength of 2.5 microns. Laboratory studies have shown that on the basis of such fibers can be created through the connection with the regeneration sites across 4600 km at a speed of transmission of the order of 1 Gbit / sec.

Technical features

Fiber made of quartz, based on the silicon dioxide are widely distributed, but because of cheap materials, unlike copper. Optical fibers have a diameter of about 100 microns., That is very compact and lightweight, which makes them promising for use in aviation, instrumentation, in cable technology.

Glass fiber - not metal, during the construction of communications systems is automatically achieved Galvanic isolation segments. Applying very durable plastic, the cable plant manufactures self-hanging cables, which do not contain metal and therefore safer in electrical terms. Such cables can be mounted on the mast of existing power lines, both individually and embedded in the phase wire, saving considerable funds for laying cable through rivers and other obstacles.

systems based on optical fibers are resistant to electromagnetic interference, and the transmitted light on the information is protected from unauthorized access. Fiber-optic lines can not be podslushat non-destructive way. Any effect on the fiber could be recorded by monitoring (continuous monitoring), the integrity of the line. Theoretically, there are ways to circumvent the protection by monitoring, but the cost of implementing these options would be so great that exceed the value of intercepted information.

Is there a way to covert transmission of information on optical lines. When the hidden transfer of the signal from the source of radiation is not modulated in amplitude, as in conventional systems, and in phase. The signal is mixed with itself, detained for some time, more than the coherence time of a radiation source. In this manner the transfer of information can not be intercepted by the receiver of the radiation amplitude, as it will record only the signal of constant intensity.

For the intercepts the signal will need a special restructuring Michelson interferometer design. And vidnost interference pattern can be reduced as a 1:2 N, where N - the number of signals simultaneously transmitted over the optical communications system. You can distribute the information transmitted by the set of signals or transmit some noise signal, without hurting those conditions information. It takes considerable power from the fiber to take the optical signal is hijacked, and this intervention is easy to register the monitoring systems.

An important characteristic of optical fibers - long life. The lifetime of fiber, that is, the preservation of their properties within certain limits, more than 25 years, thus paving the optical-fiber cable once, and, where necessary, to increase channel capacity by replacing the receiver and transmitter to a fast.

is in fiber technology and its shortcomings:


When you create a connection requires highly active elements that convert electrical signals into light and light into electrical signals. Also needed optical connectors (connectors) with low optical loss and greater resources to the connection-disconnection. Precision manufacturing of such elements should be consistent with the line wavelength, ie, the error should be the order of the proportion of a micron. Therefore, the production of such components of optical communication lines are very expensive.

Another drawback is that for the installation of optical fibers requires precision, and therefore expensive process equipment.
As a result, when the accident (breakage) of optical cable cost recovery is higher than with copper wires.
Advantages of the use of fiber-optic lines (fiber-optic) so great that in spite of the listed disadvantages of optical fiber, these lines are increasingly being used to transmit information.

Fiber


Industry in many countries production of a wide range of products and components FOCL. It should be noted that the production of fiber-optic components, especially optical fiber with a high degree of concentration. Most enterprises are concentrated in the United States. With the main patent, U.S. company (in the first place it belongs to us "CORNING") have an impact on production and market fiber-optic components in the world, through licensing agreements with other firms and joint ventures.

A key component of FOCL - optical fiber. To send a signal are two kinds of fiber: single and multiple. The name of the mode fibers have been the spread of radiation in them. Fiber consists of core and shell with different indices of refraction n1 and n2.

In singlemode fiber diameter svetovodnoy veins around 8-10 microns, which is comparable with the length of light waves. With this geometry in the fiber can be extended only one beam (one mode). In multimode fiber size svetovodnoy veins around 50-60 microns, which makes it possible to disseminate a large number of rays (a lot of fashion). Both types of fiber are two key parameters: attenuation and dispersion.
Attenuation is usually measured in dB / km and is determined by losses in the absorption and scattering of radiation in the optical fiber. Loss absorption depends on the purity of the material loss at the scattering depends on the inhomogeneities of the refractive index material. Attenuation depends on wavelength, introduced in the fiber. Currently, transmission of signals on the fiber is carried out in three bands: 0.85 micron, 1.3 micron, 1.55 micron, as it is in these ranges of quartz, a higher transparency.

Other important parameters of optical fibers - dispersion. Dispersion - this time in the scattering spectrum and mode of the optical signal. There are three types of dispersion: mode, material and waveguide. Dispersion inherent in multi-mode fiber and is due to the large number of modes, the time distribution of which varies. Material dispersion is due to the dependence of the refractive index of the wavelength. Waveguide dispersion is due to processes within the fashion and is characterized by dependence of the velocity of propagation modes of the wavelength.

Since the LED or laser emits a certain range of wavelengths, the dispersion leads to pulse broadening in the distribution of fibers and thus leads to distortion of signals. In evaluating the use the term "bandwidth" - this is a value inverse to the amount of pulse broadening as they pass them on optical fiber distance of 1 km. Bandwidth is measured in MHz * km. The definition of bandwidth can be seen that the dispersion imposes a limitation on the range of transmission and the upper frequency of transmitted signals.

If the spread of light on the multi-fiber usually dominates dispersion mode, the single mode fiber has only two last type of dispersion. At a wavelength of 1.3 micron material and waveguide dispersion in singlemode fiber compensate each other, that provides the highest bandwidth.

Attenuation and dispersion of different types of optical fibers are different. Single fiber has the best characteristics of attenuation and bandwidth, because they are subject to only one beam. However, the single source of radiation is several times more expensive than multimode. In single fiber more difficult to enter the radiation from the small size of svetovodnoy vein, for the same reason, single mode fiber is difficult to splice with small losses. Okontsevanie singlemode optical cable connector is also more expensive.

Multimode fibers are more convenient for mounting, as well as their size svetovodnoy vein is several times higher than in singlemode fiber. Multimode cable is easier okontsevat optic connectors with small losses (up to 0.3 dB) at the junction. At the multi-fiber emitters calculated at wavelength 0.85 microns - the most accessible and inexpensive emitters produced in a very wide range. But the attenuation at that wavelength from the multimode fiber is within 3-4 dB / km, and can not be significantly improved. Bandwidth of the multimode fiber is 800 MHz * km, which is acceptable for local networks, but not enough for the trunk.

Electronic components of optical communication

Now Concerning the problem of transmitting and receiving optical signals. The first generation of transmitter signals on optical fiber was introduced in 1975. The basis of the transmitter of light-emitting diode operating at a wavelength of 0.85 microns in the multimode regime. Over the next three years, a second-generation - single transmitter operating at a wavelength of 1.3 microns. In 1982, the third generation born transmitters - diode lasers operating at a wavelength of 1.55 microns.

Research continued and now a fourth generation of optical transmitters, which gave rise to the coherent communications systems - that is, systems in which information is modulated frequency or phase of radiation. Such systems provide a much greater range of propagation of signals on optical fiber. Our specialists have built bezregeneratornuyu NTT coherent FOCL STM-16 at the rate of transmission 2.48832 Gbit / s length of 300 km, and in the laboratories of NTT in early 1990, scientists created the first system to use optical amplifiers at the speed of 2.5 Gb / s at a distance of 2223 km.

advent of optical amplifiers based on optical fiber doped erbiem able to increase the passing of light signals at 30 dB, has started a fifth-generation optical communication systems. Currently rapidly evolving system of long-distance optical communication over distances of thousands of kilometers. Successfully operated transatlantic link US-Europe and the TAT-8 TAT-9, Pacific Line USA Hawaii-Japan-TRS-3. Work is underway to complete the construction of a global optical rings, Japan-Singapore-India-Saudi Arabia-Egypt-Italy.

In recent years, along with a coherent system, developing an alternative line: solitonovye systems. Soliton - this light pulse with unusual properties: it retains its shape and can theoretically apply to "ideal" light infinitely far away. Solitons are ideal light signals for communication. The duration of the soliton is approximately 10 trillionth of shares seconds (10 ps). Solitonovye system in which individual bits of information encoded the presence or absence of the soliton can have a capacity of not less than 5 Gbit / s at a distance of 10 000 km.



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