Fiber-optic communication lines (fiber) - we are building an enterprise network. Fiber Optic Signaling: Principles

Methods for transmitting signals of various types, data and control commands over fiber optic communication lines began to be actively introduced in the last decade of the last century. However, for a long time they could not seriously compete (at least in the TSB segment) with coaxial cable and twisted pair. Despite such shortcomings as high resistance and capacitance, which significantly limits the signal transmission range, coaxial cable and twisted pair prevailed in security systems. Today the situation is beginning to change, and I would venture to say that these changes are cardinal. No, in small systems where video and control signals need to be transmitted over short distances, coaxial cable and twisted pair are still indispensable. In large and especially distributed systems, there is practically no alternative to fiber.
The fact is that fiber optic equipment today has become much more affordable and the trend towards its further reduction in price is quite stable.
So fiber optics now makes it possible to offer the customer of security systems not only a reliable, but also a cost-effective solution. The use of a light beam for signal transmission, a wide bandwidth allow you to transmit a high quality signal over long distances without the use of amplifiers and repeaters.
The main advantages of using fiber optics are known to be:
– wider bandwidth (up to several gigahertz) than copper cable (up to 20 MHz);
- immunity to electrical interference, the absence of "earth loops";
– low signal transmission losses, signal attenuation is about 0.2–2.5 dB/km (for RG59 coaxial cable – 30 dB/km for 10 MHz signal);
– does not cause interference in adjacent “copper” or other fiber optic cables;
– long transmission range;
– increased security of data transmission;
– good quality of the transmitted signal;
– fiber optic cable is miniature and lightweight.

Principle of operation fiber optic line
Fiber optics is a technology in which light is used as a carrier of information, and it does not matter what type of information it is: analog or digital. Infrared light is usually used, and fiberglass is the transmission medium.
Fiber optic equipment can be used to transmit analog or digital signal various types.
In the simplest embodiment, a fiber optic communication line consists of three components:
– a fiber-optic transmitter for converting the input electrical signal from a source (for example, a video camera) into a modulated light signal;
– fiber optic line, through which the light signal is transmitted to the receiver;
- a fiber-optic receiver that converts the signal into an electrical one, almost identical to the source signal.
The light emitting diode (LED) (or semiconductor laser - LD) is the source of light propagated through optical cables. At the other end of the cable, the receiving detector converts the light signals into electrical signals. Fiber optics relies on a special effect - refraction at the maximum angle of incidence, when there is total reflection. This phenomenon occurs when a beam of light leaves a dense medium and enters a less dense medium at a certain angle. The inner strand (thread) of a fiber optic cable has a higher refractive index than the sheath. Therefore, a light beam passing through the inner core cannot go beyond its limits due to the effect of total reflection (Fig. 1). Thus, the transported signal goes inside a closed medium, making its way from the signal source to its receiver.
The remaining elements of the cable only protect the fragile fiber from damage by the external environment of various aggressiveness.

Introduction

Currently, the telecommunications industry is undergoing unprecedented changes associated with the transition from voice-based systems to data transmission systems, which is a consequence of the rapid development of Internet technologies and various network applications. Therefore, one of the main requirements for transport networks for data transmission is the ability to quickly increase their throughput in accordance with the growth of traffic volumes.

Digital communication via optical cables, which is becoming increasingly important, is one of the main directions of scientific and technological progress.

The advantages of digital streams are in their relatively easy processing with the help of a computer, the possibility of increasing the signal-to-noise ratio and increasing the density of the information flow.

The advantages of optical transmission systems over transmission systems operating on a metal cable are:

Opportunities to obtain light guides with low attenuation and dispersion, which means an increase in the communication range;

Wide bandwidth, i.e. large information capacity;

The optical cable does not have electrical conductivity and inductance, that is, the cables are not subjected to electromagnetic influence;

Negligible crosstalk;

Low cost of optical cable material, its small diameter and weight;

High secrecy of communication;

Possibilities for system enhancements while maintaining full compatibility with other transmission systems.

Linear paths of fiber-optic transmission systems are built as two-fiber single-band single-cable, single-fiber single-band single-cable, single-fiber multi-band single-cable (with wavelength division multiplexing).

Taking into account that the share of costs for cable equipment makes up a significant part of the cost of communication, and the prices for optical cable currently remain quite high, the task arises to increase the efficiency of using the bandwidth of optical fiber by simultaneously transmitting more information over it.

The purpose of the work is to consider various ways to increase the throughput of an optical fiber.

Principles of signal transmission over optical fiber and the main parameters of optical fibers

Principles of signal transmission over optical fiber

The use of optical fiber networks is based on the principle of propagation of light waves through optical fibers over long distances. In this case, electrical signals carrying information are converted into light pulses, which are transmitted with minimal distortion over fiber-optic communication lines (FOCL). Such systems have become widespread due to a number of advantages that FOCL has in comparison with transmission systems using copper cables or radio lines as a transmission medium. The advantages of FOCL include a wide bandwidth due to the high carrier frequency - up to 10 14 Hz. Such a band makes it possible to transmit information streams at a speed of several terabits per second. An important advantage of FOCL is also such factors as low signal attenuation, which allows, using modern technologies, to build sections of optical systems of a hundred or more kilometers without repeaters, high noise immunity associated with the low susceptibility of optical fiber to electromagnetic interference, and much more.

Optical fibers are one of the main components of FOCL. They are a combination of materials with different optical and mechanical properties.

The outer part of the fiber is usually made of plastics or epoxy compositions, which combine high mechanical strength and a high light refractive index. This layer provides mechanical protection of the fiber and its resistance to impact. external sources optical radiation.

The bulk of the fiber consists of a core and a sheath. The core material is ultra-pure quartz glass, which is the main medium for transmitting optical signals. The light pulse is retained due to the fact that the refractive index of the core material is greater than that of the cladding. Thus, with an optimally selected ratio of the refractive indices of the materials, the light beam is completely reflected inside the core.

For transmission, light is introduced at a slight angle into the end face of the optical fiber. The maximum angle of penetration of a light pulse into the fiber core b 0 is called the angular aperture of the optical fiber. The sine of the angular aperture is called the numerical aperture NA and is calculated using the formula:

It follows from the above formula that the numerical aperture of the optical fiber NA depends only on the refractive indices of the core and cladding - n 1 and n 2 . In this case, the condition is always fulfilled: n 1 >n 2 (Figure 1).

Figure 1 - Propagation of light in an optical fiber. Numerical aperture of the light guide.

If the angle of incidence of light b is greater than b 0, then the light beam is completely refracted and does not enter the core of the optical fiber (Fig. 2a). If the angle b is less than b 0, then there is a reflection from the boundary of the core materials on the shell, and the light beam propagates inside the core (Fig. 2b).

Figure 2 - Conditions for the propagation of light in an optical fiber

The speed of light propagation in an optical fiber depends on the refractive index of the fiber core and is defined as:

where C is the speed of light in vacuum, n is the refractive index of the core.

Typical refractive indices of the core material are in the range of 1.45 - 1.55.

In order to transmit light through optical waveguides, a source of strictly coherent light is needed. To increase the transmission range, the transmitter spectrum width should be as small as possible. Lasers are particularly suitable for this purpose, which, thanks to the induced emission of light, make it possible to maintain a constant phase difference at the same wavelength. Due to the fact that the diameter of the fiber core is comparable to the wavelength of optical radiation, interference occurs in the fiber. This can be proved by the fact that light propagates in the core glass only at certain angles, namely, in the directions in which the introduced light waves are amplified when they are superimposed. There is a so-called constructive interference. The allowed light waves that can propagate in an optical fiber are called modes (or eigenwaves). In accordance with the types of propagation of light rays, optical fibers are divided into multimode, that is, using a number of light waves, and single-mode, in which only one light beam propagates. To describe the processes of light propagation in optical fibers, several basic parameters are used.

Russian State Pedagogical

University.

Essay

on computer architecture

on the topic of:

“Fiber Optic Networks”

Performed: YunchenkoT.

student II course

f-ta IOT, group 2.2

Checked:

St. Petersburg 2004

1. Optical cable device

2. Classification of optical fibers

3. Information transmission over fiber optics

4. DWDM and traffic

5. DWDM tomorrow

6. Literature

Fiber optic networks and technologyDWDM

Optical cable device

The main element of an optical cable (OC) is an optical waveguide - a round rod made of an optically transparent dielectric. Optical waveguides due to the small cross-sectional dimensions are usually called optical fibers (FO) or optical fibers (OF).

The dual nature of light is known: wave and corpuscular. Based on the study of these properties, quantum (corpuscular) and wave (electromagnetic) theories of light have been developed. These theories cannot be opposed. Only in their totality they allow us to explain the known optical phenomena.

An optical fiber consists of a core through which light waves propagate and a cladding. The core serves to transmit light waves. The purpose of the shell is to create the best conditions for reflection at the “core-shell” interface and protection from energy radiation into the surrounding space.

In the general case, three types of waves can propagate in the OF: directed, leaky, and radiated. The action and predominance of any type of waves are associated primarily with the angle of incidence of the wave on the interface “core – shell” OF. At certain angles of incidence of rays on the end of the OF, the phenomenon of total internal reflection takes place at the “core-shell” interface of the OF. Optical radiation, as it were, is locked in the core and propagates only in it.

Classification of optical fibers

There are single-mode and multi-mode modes of radiation transmission through OF. In the multimode mode of radiation propagation along the OF, the condition of total internal reflection is satisfied for an infinite set of rays. This is only possible for OFs whose cores are much larger than the wavelengths of the propagated waves. Such optical fibers are called multimode.

In single-mode optical fibers, unlike multi-mode optical fibers, only one beam propagates, and, consequently, signal distortion caused by different times propagation of various rays are absent.

All optical fibers are divided into groups according to the type of propagating radiation, into subgroups according to type - according to the type of refractive index profile, and into types - according to the material of the core and shell.

The following groups of agents are distinguished:

Multimode (M)

Single-mode without conservation of radiation polarization (E)

Single-mode with conservation of radiation polarization (P)

The group of multimode optical fibers is divided into two subgroups:

Stepped index (C)

Gradient refractive index (G)

In addition, OVs are divided into the following types:

Core and shell quartz

The core is quartz, and the shell is polymer

Multi-component glass core and cladding

Polymeric core and sheath

By purpose, optical communication cables are divided into:

Urban

Zonal

Trunk

Depending on the laying conditions, fixed and linear optical cables are distinguished. The latter, in turn, are divided into cables intended for laying in sewers and sewers, in the ground, for suspension on supports and racks, for underwater laying.

Information transmission over fiber optics

If compared with other methods of information transfer, then the order of magnitude of TB / s is simply unattainable. Another plus of such technologies is the reliability of transmission. Fiber optic transmission does not have the disadvantages of electrical or radio signal transmission. There is no interference that can damage the signal, and there is no need to license the use of the radio frequency. However, not many people understand how information is transmitted over fiber in general, and even more so are not familiar with specific implementations of technologies. We will consider one of them - DWDM technology (dense wavelength-division multiplexing).

First, let's look at how information is generally transmitted over an optical fiber. An optical fiber is a waveguide through which electromagnetic waves propagate with a wavelength of the order of a thousand nanometers (10-9 m). This is a region of infrared radiation not visible to the human eye. And the main idea is that with a certain selection of the fiber material and its diameter, a situation arises when for some wavelengths this medium becomes almost transparent, and even when it hits the boundary between the fiber and the environment, most of the energy is reflected back into the fiber. This ensures the passage of radiation through the fiber without much loss, and the main task is to receive this radiation at the other end of the fiber. Of course, such a brief description hides the huge and difficult work of many people. One should not think that such material is easy to create or that this effect is obvious. On the contrary, this should be treated as a great discovery, since now it provides The best way transfer of information. You need to understand that the waveguide material is a unique development and the quality of data transmission and the level of interference depend on its properties; The waveguide insulation is designed to minimize the amount of energy escaping to the outside.

As for specifically the technology called "multiplexing", it means that you transmit several wavelengths at the same time. They do not interact with each other, and when receiving or transmitting information, interference effects (the superposition of one wave on another) are insignificant, since they are most pronounced at multiple wavelengths. Here we are talking about the use of close frequencies (the frequency is inversely proportional to the wavelength, so it doesn’t matter what to talk about). A device called a "multiplexer" is an apparatus for encoding or decoding information into a waveform and vice versa. After this short introduction, let's move on to a specific description of DWDM technology.

The main characteristics of DWDM multiplexers that distinguish them from just WDM multiplexers are:
the use of only one transparency window of 1550 nm, within the amplification region EDFA nm (EDFA - optical amplification system; EDFA - optical repeater, it allows you to restore the optical power of the signal lost when passing along a long line, without converting to an electrical signal and vice versa. Optical fiber , doped with the rare earth element erbium, has the ability to absorb light of one wavelength and emit light at another wavelength.An external semiconductor laser sends infrared light at a wavelength of 980 or 1480 millimicrons into the fiber, exciting erbium atoms.When an optical signal with a length of waves from 1530 to 1620 millimicrons, excited erbium atoms emit light with the same wavelength as the input signal. The exclusion of the conversion of light signals into electrical signals and vice versa simplifies and reduces the cost of amplifying equipment and makes it possible not to introduce additional distortions during conversion. EDFA amplifiers are used on " long-range "lines, where it is difficult to install complex intermediate amplifying equipment (for example, a submarine cable). For reference, let's say that the wavelength of visible light is 400-800 nm.

In addition, since the name itself speaks of dense (dense) transmission of channels, the number of channels is greater than in conventional WDM schemes, and reaches several tens. Because of this, there is a need to create devices that are able to add a channel or remove it, in contrast to conventional schemes, when all channels are encoded or decoded at once. With such devices operating on one channel out of many, the concept of passive wavelength routing is associated. It is also clear that working with a large number of channels requires greater accuracy of signal encoding and decoding devices and places higher demands on the quality of the line. Hence the obvious increase in the cost of devices - while reducing the price for the transfer of a unit of information due to the fact that now it can be transferred in a larger volume.
This is how the demultiplexer works with a mirror (diagram in Fig. 1a). The incoming multiplex signal enters the input port. Then this signal passes through the waveguide-plate and is distributed over a plurality of waveguides, which are an AWG (arrayed waveguide grating) diffraction structure. As before, the signal in each of the waveguides remains multiplexed, and each channel is represented in all waveguides, that is, so far only parallelization has occurred. Next, the signals are reflected from the mirror surface, and as a result light streams are reassembled in the waveguide-plate, where they are focused and interfered. This leads to the formation of an interference pattern with spatially separated maxima, and the geometry of the plate and mirror is usually calculated so that these maxima coincide with the output poles. Multiplexing occurs in the opposite way.

Rice. 1. Schemes of DWDM multiplexers: a) with a reflective element; b) with two waveguides-plates

Another way to build a multiplexer is based not on one, but on a pair of waveguides-plates (Fig. 1b). The principle of operation of such a device is similar to the previous case, except that here an additional plate is used for focusing and interference.
DWDM multiplexers, being purely passive devices, introduce a lot of attenuation into the signal. For example, the loss for a device (see Fig. 1a) operating in the demultiplexing mode is 10-12 dB, with far crosstalk less than -20 dB and a half-width of the signal spectrum of 1 nm (according to Oki Electric Industry). Due to high losses, it is often necessary to install an optical amplifier before and/or after the DWDM multiplexer.
The most important parameter in DWM technology is undoubtedly the distance between adjacent channels. Standardization of the spatial arrangement of channels is needed, if only because on its basis it will be possible to start conducting tests for the mutual compatibility of equipment different manufacturers. The telecommunications standardization sector of the International Telecommunication Union ITU-T has approved a DWDM frequency plan with a distance between adjacent channels of 100 GHz, which corresponds to a wavelength difference of 0.8 nm. The issue of transmitting information with a difference in wavelengths of 0.4 nm is also being discussed. It would seem that the difference can be made even smaller, thereby achieving a greater throughput, but in this case, purely technological difficulties arise associated with the manufacture of lasers that generate a strictly monochromatic signal (of a constant frequency without interference) and diffraction gratings that separate the maxima in space corresponding to different wavelengths. When using a 100 GHz split, all channels fill the used band evenly, which is convenient when setting up equipment and reconfiguring it. The choice of separation interval is determined by the required bandwidth, the type of laser and the degree of interference on the line. However, it should be taken into account that when operating even in such a narrow range (nm), the influence of non-linear noise at the boundaries of this region is very significant. This explains the fact that with an increase in the number of channels it is necessary to increase the laser power, but this, in turn, leads to a decrease in the signal-to-noise ratio. As a result, the use of a stiffer seal has not yet been standardized and is under development. Another obvious disadvantage of increasing the density is the reduction in the distance over which the signal can be transmitted without amplification or regeneration (a little more about this will be discussed below).
Note that the nonlinearity problem mentioned above is inherent in silicon-based amplification systems. Now more reliable fluorine-zirconate systems are being developed that provide greater linearity (over the entire nm range) of the gain. With an increase in the working area of ​​the EDFA, it becomes possible to multiplex 40 STM-64 channels with an interval of 100 GHz with a total capacity of 400 GHz per fiber (Fig. 2).

Rice. 2. Spectral arrangement of channels in a fiber

The table shows the technical characteristics of one of the powerful multiplex systems using the 100/50 GHz frequency plan, manufactured by Ciena Corp.

Let us dwell in more detail on the optical amplification system. What is the problem? Initially, the signal is generated by the laser and sent to the fiber. It propagates along the fiber, undergoing changes. The main change to deal with is signal scattering (dispersion). It is associated with nonlinear effects that arise during the passage of a wave packet in a medium and is obviously explained by the resistance of the medium. This raises the problem of transmission over long distances. Large - in the sense of hundreds or even thousands of kilometers. This is 12 orders of magnitude greater than the wavelength, so it is not surprising that even if the nonlinear effects are small, then in total at such a distance they must be taken into account. Plus, the nonlinearity can be in the laser itself. There are two ways to achieve reliable signal transmission. The first is the installation of regenerators that will receive a signal, decode it, generate a new signal that is completely identical to the one that arrived, and send it further. This method is effective, but such devices are quite expensive, and increasing their bandwidth or adding new channels that they must handle is associated with difficulties in reconfiguring the system. The second method is simply optical amplification of the signal, completely analogous to amplifying the sound in a music center. This amplification is based on EDFA technology. The signal is not decoded, but only its amplitude is increased. This allows you to get rid of speed losses in the amplification nodes, and also removes the problem of adding new channels, since the amplifier amplifies everything in a given range.

Based on EDFA, line power losses are overcome by optical amplification (Fig. 3). Unlike regenerators, such a "transparent" amplification is not tied to the signal bit rate, which allows you to transmit information to more high speeds and increase throughput until other limiting factors such as chromatic dispersion and PMD come into play. EDFAs are also capable of amplifying a multi-channel WDM signal, adding another dimension to the bandwidth.

Rice. 3. Optical communication systems based on: a) a cascade of regeneration repeaters; b) cascade of optical amplifiers EDFA

Although the optical signal generated by the original laser transmitter has a well-defined polarization, all other nodes along the path of the optical signal, including the optical receiver, should show a weak dependence of their parameters on the polarization direction. In this sense, EDFA optical amplifiers, characterized by a weak polarization dependence of the gain, have a tangible advantage over semiconductor amplifiers. On fig. Figure 3 shows how both methods work.
Unlike regenerators, optical amplifiers introduce additional noise that must be taken into account. Therefore, along with the gain, one of the important parameters of the EDFA is the noise figure. EDFA technology is cheaper, for this reason it is more often used in real practice.

Since EDFA, at least in terms of price, looks more attractive, let's break down the main characteristics of this system. This is the saturation power, which characterizes output power amplifier (it can reach and even exceed 4 W); gain, defined as the ratio of the power of the input and output signals; the power of amplified spontaneous emission determines the level of noise generated by the amplifier itself. Here it is appropriate to give an example of a music center, where you can trace analogies in all these parameters. The third one (noise level) is especially important, and it is desirable that it be as low as possible. Using an analogy, you can try to turn on the music center without playing any disc, but at the same time turn the volume knob to the maximum. In most cases, you will hear some noise. This noise is created by amplification systems simply because they are powered. Similarly, spontaneous emission occurs in our case, but since the amplifier is designed to emit waves in a certain range, then photons of this particular range will be more likely to be emitted into the line. This will create (in our case) light noise. This imposes a limitation on the maximum line length and the number of optical amplifiers in it. The gain factor is usually chosen to restore the original signal level. On fig. Figure 4 shows the comparative spectra of the output signal in the presence and absence of a signal at the input.

Rice. 4. EDFA output spectrum taken with a spectrum analyzer (ASE - Noise Spectral Density)

Another parameter that is convenient to use when characterizing an amplifier is the noise factor - this is the ratio of the signal-to-noise parameters at the input and output of the amplifier. In an ideal amplifier, this parameter should be equal to one.
There are three applications for EDFA amplifiers: preamplifiers, line amplifiers, and power amplifiers. The first are installed directly in front of the receiver. This is done to increase the signal-to-noise ratio, which allows the use of simpler receivers and can reduce the cost of the equipment. Linear amplifiers are intended to simply amplify the signal in long lines or in the case of a branching of such lines. Power amplifiers are used to amplify the output directly after the laser. This is due to the fact that the power of the laser is also limited and sometimes it is easier to simply install an optical amplifier than to install a more powerful laser. On fig. 5 schematically shows all three EDFA applications.

Rice. 5. Application different types optical amplifiers

In addition to the direct optical amplification described above, an amplifying device using the Raman amplification effect for this purpose and developed at Bell Labs is currently preparing to enter the market. The essence of the effect is that a laser beam of a certain wavelength is sent from the receiving point towards the signal, which shakes the crystal lattice of the waveguide in such a way that it begins to emit photons in a wide frequency spectrum. Thus, the overall level of the useful signal rises, which allows you to slightly increase the maximum distance. Today this distance is 160-180 km, compared to 70-80 km without Raman enhancement. These Lucent Technologies devices will hit the market in early 2001.

What was said above is technology. Now a few words about implementations that already exist and are actively used in practice. First, we note that the use of fiber optic networks is not only the Internet and, perhaps, not so much the Internet. Fiber optic networks can transmit voice and TV channels. Secondly, let's say that there are several different types of networks. We are interested in long-distance backbone networks, as well as localized networks, for example, within one city (the so-called metro solutions). At the same time, for trunk communication channels, where the rule “the thicker the pipe, the better” works perfectly, DWDM technology is an optimal and reasonable solution. Another situation develops in urban networks, in which the requests for traffic transmission are not as large as those of backbone channels. Here, operators use the good old SDH/SONET-based transport operating in the 1310 nm wavelength range. In this case, to solve the problem of insufficient bandwidth, which, by the way, is not very acute for urban networks yet, you can use the new SWDM technology, which is a kind of compromise between SDH / SONET and DWDM (read more about SWDM technology on our CD-ROM ). With this technology, the same fiber ring nodes support both single-channel data transmission at 1310 nm and WDM at 1550 nm. Savings are achieved by "inclusion" of an additional wavelength, which requires the module to be added to the appropriate device.

DWDM and traffic

One of important points when using DWDM technology is transmitted traffic. The fact is that most of the equipment that currently exists supports the transmission of only one type of traffic at one wavelength. As a result, a situation often arises when traffic does not completely fill the fiber. Thus, less “dense” traffic is transmitted over a channel with a formal bandwidth equivalent to, for example, STM-16.
Currently, there is equipment that implements the full load of wavelengths. In this case, one wavelength can be "filled" with heterogeneous traffic, say, TDM, ATM, IP. An example is the Chromatis family of equipment from Lucent Technologies, which can transmit all types of traffic supported by I / O interfaces on the same wavelength. This is achieved with a built-in TDM cross-switch and ATM switch. Moreover, the additional ATM switch is not price-forming. In other words, additional hardware functionality is achieved at virtually the same cost. This makes it possible to predict that the future is universal devices capable of transmitting any traffic from

optimal use of bandwidth.

DWDM tomorrow

Moving smoothly to the trends in the development of this technology, we certainly will not discover America if we say that DWDM is the most promising optical data transmission technology. This can be attributed to a greater extent to the rapid growth of Internet traffic, the growth rates of which are approaching thousands of percent. The main starting points in development will be an increase maximum length transmission without optical signal amplification and the implementation of a larger number of channels (wavelengths) in one fiber. Today's systems transmit 40 wavelengths, which corresponds to a 100 GHz frequency grid. Devices with a 50 GHz mesh, supporting up to 80 channels, which corresponds to the transmission of terabit streams over a single fiber, are next to enter the market. And today you can already hear the statements of the laboratories of development companies, such as Lucent Technologies or Nortel Networks, about the imminent creation of 25 GHz systems.
However, despite such a rapid development of engineering and research ideas, market indicators are making their own adjustments. Last year was marked by a serious drop in the optical market, as evidenced by a significant drop in the price of Nortel Networks shares (29% in one day of trading) after it announced difficulties with the sale of its products. Other manufacturers found themselves in a similar situation.
At the same time, if there is some saturation in the western markets, then the eastern ones are just beginning to unfold. The most striking example is the Chinese market, where a dozen national operators are racing to build backbone networks. And if “they” have already practically solved the issues of building backbone networks, then in our country, sadly, there is simply no need for thick channels for transmitting our own traffic. Nevertheless, the exhibition “Departmental and corporate networks Communications” revealed a huge interest of domestic telecom operators in new technologies, including DWDM. And if such monsters as Transtelecom or Rostelecom already have transport networks scale of the state, then the current power engineers are just starting to build them. So, despite all the troubles, optics is the future. And DWDM will play a significant role here.

Literature

1. http://www. *****/production. php4?&rubric97

2. Magazine ComputerPress №1 2001

There are not many articles on Habré devoted to the technologies of optical communication lines. More recently, there have been articles on powerful DWDM systems, and a brief article on the application of a CWDM system. I will try to supplement these materials and tell you briefly about all the most common and affordable ways in Russia to use the resource of fiber-optic communication lines in data transmission networks and - quite a bit - cable television.

Start. Properties of standard G.652 single-mode fiber

Wikipedia tells us the following distribution:

In real life, the picture is now better, in particular, the specific attenuation in the 1310nm window usually falls within 0.35dB / km, in the 1550nm window it is about 0.22-0.25dB / km, and the so-called "water peak" in the region of 1400-1450nm for modern fibers does not so pronounced, or absent altogether.

Nevertheless, one must keep in mind this picture and the very existence of this dependence.

Historically, the wavelength range that is transmitted by optical fiber is divided into the following ranges:

O - 1260…1360
E - 1360…1460
S - 1460…1530
C - 1530…1565
L - 1565…1625
U - 1625…1675
(I quote from the same article on Wikipedia).

With an acceptable approximation, the properties of the fiber within each range can be considered approximately the same. The water peak, as a rule, falls on the long-wave end of the E-band. We will also keep in mind that the specific (kilometer) attenuation in the O-band is approximately one and a half times higher than in the S- and C-bands, the specific chromatic dispersion, on the contrary, has a zero minimum at a wavelength of 1310 nm and non-zero in C -range.

The simplest sealing systems - bidirectional transmission over a single fiber

Therefore, as soon as technology began to allow, solutions began to appear for transmitting information in both directions over a single fiber. Titles similar decisions- "single fiber transceivers", "WDM", "bi-directional".

The most common options use wavelengths of 1310 and 1550 nm, respectively from the O- and C-bands. "In the wild" transceivers for these wavelengths are found for lines up to 60 km. More "long-range" options are made for other combinations - 1490/1550, 1510/1570 and similar options using transparency windows with lower specific attenuation than in the O-band.

In addition to the above pairs of wavelengths, it is possible to meet a combination of 1310 / 1490nm - it is used if a cable television signal at a wavelength of 1550nm is transmitted simultaneously with data on the same fiber; or 1270/1330nm - it is used to transmit 10Gbps streams.

Multiplexing of data and cable television

Optics is now also used to deliver a cable television signal from the headend to the apartment building. For it, either a wavelength of 1310nm is used - here is the minimum chromatic dispersion, that is, signal distortion; or a wavelength of 1550nm - here the minimum specific attenuation and it is possible to use purely optical amplification using EDFA. If there is a need to deliver both a data stream (Internet) and a cable TV signal to one house at the same time, you must either use two separate fibers or a simple passive device - an FWDM filter.

This is a reversible device (that is, the same device is used for both multiplexing and demultiplexing streams) with three outputs: for cable TV, a single-fiber transceiver and a common output (see diagram). Thus, it is possible to build a PON or Ethernet network using 1310/1490 wavelengths for data transmission, and 1550nm for cable TV.

CWDM and DWDM

The DWDM system is fundamentally no different from CWDM, but - as they say - "the devil is in the details." If the channel pitch in CWDM is 20nm, then for DWDM it is much narrower and is measured in gigahertz (the most common option now is 100GHz, or about 0.8nm; an outdated version with a 200GHz band is also possible and more modern ones are gradually spreading - 50 and 25GHz). frequency range DWDM lies in the C- and L-band, 40 channels at 100 GHz each. Several important properties of DWDM systems follow from this.

First, they are much more expensive than CWDM. Their use requires lasers with strict wavelength tolerance and very high selectivity multiplexers.

Secondly, the used ranges lie in the working areas of the EDFA optical amplifiers. This makes it possible to build long lines with purely optical amplification without the need for optoelectronic signal conversion. It is this property that has led to the fact that many at the word "DWDM" immediately imagine exactly complex systems monsters of the telecom market, although similar equipment can be used in simpler systems.
And thirdly, the attenuation in the C- and L-bands is minimal from the entire transparency window of the optical fiber, which makes it possible to build longer lines even without amplifiers than when using CWDM.

DWDM multiplexers are just as passive devices as CWDM multiplexers. For the number of channels up to 16, they are also arranged from separate filters, and this is quite simple devices. However, multiplexers for a larger number of channels are made using Arrayed Wavelength Grating technology, which is extremely sensitive to temperature changes. Therefore, such multiplexers are produced either with electronic circuit thermal stabilization (Thermal AWG), or using special self-compensation methods that do not require energy (Athermal AWG). This makes such multiplexers more expensive and gentler to operate.

Practical Limitations in Fiber Optic Communication

As comrade saul quite rightly pointed out, the first limitation is the optical budget.
I'll add some clarifications to it.

If we are talking about two-fiber communication lines, it is enough to calculate the optical budget for one wavelength - the one on which the transmission will be carried out.

As soon as we have wave compression (especially in the case of single-fiber transceivers or CWDM systems), we immediately need to remember about the uneven specific attenuation of the fiber at different wavelengths and about the attenuation introduced by multiplexers.

If we are building a system with intermediate taps on OADM, do not forget to calculate the attenuation on OADM. By the way, it is different for the through channel and output wavelengths.

Don't forget to leave a few decibels of headroom.

The second thing to deal with is chromatic dispersion. It really becomes relevant for 10 Gbit / s lines, and generally speaking, the equipment manufacturer first of all thinks about it. By the way, it is the dispersion that gives physical meaning to the mention of kilometers in the marketing names of transceivers. It is simply useful for the operation specialist to understand that there is such a property of the fiber and that, in addition to signal attenuation in the fiber, dispersion also spoils the picture. Add tags

THE WORLD OF NUMBERS AND GLASS

INTRODUCTION

Fiber optics has many well-known advantages over twisted-pair and coaxial cables, such as immunity to electrical noise and unrivaled bandwidth.

Over the past quarter century, fiber optic communications have become a widespread method for transmitting video, audio, other analog signals, and digital data. Fiber optics has many well-known advantages over twisted-pair and coaxial cables, such as immunity to electrical interference and unrivaled bandwidth. For these and many other reasons, fiber-optic information transmission systems are increasingly penetrating into various areas of information technology.

Digital systems provide very high performance, flexibility and reliability, and cost no more than the analog solutions they replace

However, despite these advantages, until recently fiber optic systems used the same analog signal transmission technologies as their copper predecessors. Now, when a new generation of equipment has appeared, based solely on digital methods signal processing, fiber optic communication once again takes telecommunications to a whole new level. Digital systems offer very high performance, flexibility and reliability, and cost no more than the analog solutions they replace.

This manual discusses the technique of digital signal transmission over fiber optic cables and its economic and technological advantages.

ANALOG TRANSMISSION OVER FIBER

To fully appreciate the benefits of digital technology, let's first look at the traditional methods of transmitting analog signals over fiber. To transmit analog signals, amplitude (AM) and frequency (FM) modulation are used. In both cases, the input of the optical transmitter receives a low-frequency analog audio and video signal or data, which is converted into an optical signal. This is done in different ways.

In amplitude modulated systems, the optical signal is a light flux with an intensity that changes in accordance with changes in the input electrical signal. Either LEDs or lasers are used as the light source. Unfortunately, both are non-linear, that is, in the full range of brightness from no radiation to maximum value there is no proportionality between the input signal and the light intensity. However, it is this control method that is used in systems with amplitude modulation. As a result, there are various distortions transmitted signal:

• decrease in the signal-to-noise ratio as the cable length grows;
• non-linear differential gain and phase errors in video signal transmission;
• limitation dynamic range audio signal.

To improve the quality of fiber optic signal transmission systems, it was proposed to use frequency modulation, in which the light source is always either completely turned off or turned on at full power, and the pulse repetition rate changes in accordance with the amplitude of the input signal. For those who are familiar with frequency modulation signals in radio engineering, the use of this term here may seem unreasonable, since in the context of fiber optic systems it is perceived as a method of controlling the frequency of the light radiation itself. This is not the case, and indeed it would be more correct to use the term "pulse phase modulation" (PPM), but in the field of fiber optic technology, such terminology has become established. It should always be remembered that the word "frequency" in the name of the modulation method means the frequency of the pulses, and not the frequency of the light waves carrying them.

With amplitude modulation, the input signal level is represented by the intensity of the light beam

With frequency modulation, the input signal level is represented by the repetition rate of light pulses
Rice. 1. Comparison of amplitude and frequency modulation

Although frequency modulation eliminates many of the radiance control problems inherent in AM systems, it has its own challenges. One of them is the known crosstalk in FM systems. They are observed, in particular, when transmitting several signals with frequency modulation over one fiber, for example, when using a multiplexer. Crosstalk occurs in a transmitter or receiver as a result of tuning instability in important signal filtering circuits designed to separate carrier frequencies. If the filters are poorly tuned, then the frequency-modulated carriers interact with each other and are distorted. Fiber optic engineers can design FM systems that minimize the chance of crosstalk, but any design improvement comes at a cost.

Another type of distortion is called intermodulation. Like crosstalk, intermodulation occurs in systems designed to transmit multiple signals at once over a single fiber. Intermodulation distortion occurs in a transmitter most often as a result of non-linearities in circuits common to different FM carriers. As a result, before combining several carriers into one optical signal, they act on each other, reducing the accuracy of the transmission of the original signal.

DIGITAL SYSTEMS

As with analog systems, digital system transmitters receive low-frequency analog audio and video signals or digital data, which are converted to an optical signal. The receiver receives the optical signal and outputs the native format electrical signal. The difference lies in how the signals are processed and transmitted from the transmitter to the receiver.

Rice. 2. Digital transmission system analog signal

In purely digital systems, the input low-frequency signal is immediately fed to the analog-to-digital converter, which is part of the transmitter. There, the signal is converted into a sequence of logical levels - zeros and ones, called the digital stream. If the transmitter is multichannel, that is, designed to work with several signals, then several digital streams are combined into one, and it controls the on and off of one emitter, which occurs at a very high frequency.

At the receiving end, the signal is reverse-converted. From the combined digital stream, individual streams are extracted corresponding to the individual transmitted signals. They are fed to digital-to-analogue converters, after which they are output to the outputs in the original format (Fig. 2).

Purely digital transmission signal has a lot of advantages over traditional AM and FM systems - from versatility and better signal quality to lower installation costs. Let's take a look at some of the benefits in more detail, and along the way discuss the benefits to both the system installer and system user.

SIGNAL ACCURACY

In analog systems with amplitude modulation, the signal loses quality in proportion to the path traveled through the fiber. This fact, combined with the fact that AM systems work only with multimode fibers, limits the use of such systems to relatively short transmission distances. FM systems work somewhat better: in them, although the signal quality decreases, it remains approximately constant in not very long lines, dropping sharply only when a certain limiting length is reached. Only in fully digital systems is guaranteed the preservation of signal quality during transmission over a fiber optic communication line, regardless of the distance between the transmitter and receiver and the number of transmitted channels (of course, within the capabilities of the system).

In analog systems with amplitude modulation, the signal loses quality in proportion to the path traveled through the fiber. This fact, combined with the fact that AM systems work only with multimode fibers, limits the use of such systems to relatively short transmission distances.

The accuracy of reproduction of the transmitted signal is a significant problem in the development of systems for organizing several transmission channels over a single optical fiber (multiplexers). For example, in an analog system designed to transmit four channels of video or audio, in order to keep within the system bandwidth, you have to limit the bandwidth allocated to individual channels. In digital systems, there is no need to make such a compromise: one, four, or even ten signals can be transmitted through one fiber without reducing quality.

BETTER SIGNAL QUALITY

Rice. 3

Transmission of analog signals in digital form provides higher quality than pure analog. Signal distortion with this method of transmission can only occur with analog-to-digital and inverse digital-to-analog conversion. While no conversion is perfect, modern technology is so advanced that even inexpensive ADCs and DACs provide much better video and audio quality than can be achieved with analog AM and FM systems. This is easily seen from a comparison of the signal-to-noise ratios and harmonics (differential phase and differential gain) of digital and analog systems designed to transmit signals of the same format over the same type of fiber at the same wavelength.

Digital technologies provide engineers with unprecedented flexibility when creating fiber optic systems. Now it's easy to find the right level of performance for different markets, tasks and budgets. For example, by changing the bit depth of an analog-to-digital converter, one can influence the system bandwidth required for signal transmission, and, as a result, overall performance and cost. At the same time, other properties of the digital system - the absence of distortion and the independence of the quality of work from the length of the line - are preserved up to the maximum transmission distance. When designing analog systems, engineers are always caught in the middle between the cost of a system and its technical specifications, trying to balance them without compromising the critical parameters of the transmitted signals. In digital systems, scaling systems and managing their performance and cost is much less of a challenge.

UNLIMITED TRANSMISSION DISTANCE

Another advantage of digital systems over analog predecessors is their ability to restore the signal without introducing additional distortion to it. Such restoration is performed in a special device called a repeater or linear amplifier.

The advantage provided by digital systems is obvious. In them, the signal can be transmitted over distances far exceeding the capabilities of AM and FM systems, while the developer can be sure that the received signal exactly matches the transmitted one and meets the requirements of the technical specifications.

As light travels through the fiber, its intensity gradually decreases and eventually becomes insufficient for detection. If, however, a little before reaching the point where the light becomes too weak, a linear amplifier is installed, then it will amplify the signal to its original power, and it will be possible to transmit it further to the same distance. It is important to note that the digital stream is restored in the linear amplifier, which does not have any effect on the quality of the analog video or audio signal encoded in it, regardless of how many times the restoration was performed in linear amplifiers along the signal path along a long fiber optic line.

The advantage provided by digital systems is obvious. In them, the signal can be transmitted over distances far exceeding the capabilities of AM and FM systems, while the developer can be sure that the received signal exactly matches the transmitted one and meets the requirements of the technical specifications.

LESS COST

Considering the many advantages that digital fiber optic systems have, it can be assumed that they should cost much more than traditional analog systems. However, this is not the case, and users of digital systems, on the contrary, save their money.

In a competitive market, there will always be a manufacturer that offers digital quality at the price of an analog system.

The cost of digital components has dropped significantly in recent years, and OEMs have been able to develop and market products that cost the same or even less than previous generation analog instruments. Of course, some firms want to convince the public that the superior quality of digital systems can only be obtained for additional fee, but in reality they simply decided not to share the savings with their customers. But in a competitive market there will always be a manufacturer that offers digital quality at the price of an analog system.

Digital systems allow more information to be transmitted over a single cable, thereby reducing the need for it.

Other factors also affect the cost of installing and operating a fiber optic system. The most obvious of these is cable costs. Digital systems allow more information to be transmitted over a single cable, thereby reducing the need for it. The advantage is particularly noticeable where it is necessary to simultaneously transmit signals of different types, for example, video and sound or sound and data. Without too much trouble, engineers can design a cost-effective digital system that can carry different types of signals, such as two channels of video and four channels of audio, over a single fiber. With analog technology, you would most likely have to make two separate systems, or at least use two separate cables for transmitting audio and video signals.

Due to fewer components that can fail over time, digital systems are much more stable and reliable.

Even in cases where several signals of the same type must be transmitted over a single fiber, digital systems are preferable because they work more reliably and provide a higher signal quality. For example, in a digital video multiplexer, ten channels can be transmitted with the same high quality, and in an analog system this is generally impossible.

The inevitable costs of maintenance and repair over the years of operation of fiber optic systems should also be taken into account. And here the advantage lies with digital systems. First, they do not require initial setup after installation - the transmitter and receiver are simply connected by fiber optic cable, and the system is ready to go. Analog systems typically require tuning for a particular transmission line, taking into account its length and signal strength. Additional time for adjustment entails additional costs.

Transmitters and receivers for digital systems are cheaper, cable consumption is less, operating costs are lower

Due to fewer components that can fail over time, digital systems are much more stable and reliable. They do not require retuning and troubleshooting is much faster because they do not have the crosstalk, drift, and other disadvantages of traditional analog systems.

Summarize. Transmitters and receivers for digital systems are cheaper, cable consumption is less, and operating costs are lower. Digital fiber optic systems provide a clear economic advantage at all levels.

CONCLUSIONS

Just as fiber optic technology has many advantages over traditional copper wires and coaxial cables, digital transmission is taking fiber optic technology several steps higher, giving users a whole new set of benefits. useful qualities. Digital systems have unique characteristics: signal transmission accuracy over the entire length of the communication line, minimal introduced distortions (including the absence of crosstalk and intermodulation), the ability to repeatedly restore the digital stream when it is transmitted over a long line without compromising the quality of the analog signal encoded in it. This guarantees a level of fidelity in analog signal reproduction that analog systems cannot achieve.

Component prices for digital and analog fiber systems are comparable, and when combined with installation, operation, and maintenance costs, digital systems offer clear economic benefits.

When designing a new fiber optic system, don't waste time analyzing the advantages and disadvantages of digital versus analog systems, because the choice is clear: digital systems are better in every way. It will be much more useful to limit yourself to only them and choose those products that best suit your needs. Even among digital systems, there is a huge variety of solutions. Here are some questions to help you assess them:

• How easy is the installation of the system?
  • if the transmitter and receiver are user configurable, how easy is it to do and what are the problems?
• Is the instrument design compact, robust and reliable?
• Are the instruments available in desktop cases or are they designed for rack mounting? Are there options in both case types?
  • Are the devices suitable for use with both single-mode and multi-mode fibers?
  • Does the manufacturer have sufficient experience and reputation in the market for the products it offers?
  • how does the price of the product compare with the price of traditional analog systems? (Digital devices in production are not more expensive than analog ones and their cost should not be higher).

Analyzing the market and comparing the characteristics of similar products will allow you to finally select the elements of digital fiber optic systems that will faithfully serve you for many years.