1. General concepts electromagnetic radiation
2. The concept of "Light"

A. Story
b. General information
V. Development
4. Conclusion

1. General concepts of electromagnetic radiation.
Electromagnetic radiation is the movement of disturbances electromagnetic field in space. There are invisible and visible electromagnetic radiation. Electromagnetic radiation is generated by moving electric charges, and extends in all directions and in almost all environments. They are transferred without attenuation over long distances.

Electromagnetic radiation is divided into:
. radio waves (starting from ultra-long ones);
. infrared radiation;
. visible light;
. ultraviolet radiation;
. X-rays and hard (gamma radiation).

The electromagnetic scale (spectrum) is the totality of all frequency ranges of electromagnetic radiation. The following quantities are used as spectral characteristics:
. Wavelength;
. Oscillation frequency;
. Photon energy.

The spectrum is divided into the following sections:
. Low frequency vibrations;
. Radio waves;
. Infrared radiation;
. Visible radiation (light);
. Ultraviolet radiation;
. X-ray radiation;
. Gamma radiation.
Electromagnetic waves are widely used in our time in radio and electrical engineering, modern devices. Radio waves are used for radio communications, television, and radar. Infrared radiation is used in ovens, heaters and all heating and drying devices. Ultraviolet radiation is used for disinfection of premises, studies and research of atoms and molecules. Widely used in forensics to find biological traces. X-rays are used in medicine to diagnose diseases and to treat certain diseases.

2. The concept of "Light".
Light is visible electromagnetic radiation, emitted by a heated or excited substance. But the adjacent broad spectrum regions: ultraviolet and infrared radiation are also mistaken for light. Visible wavelengths range from 380 to 780 nanometers. Light is studied by a branch of physics called optics. Light can be considered either as an electromagnetic wave, the speed of propagation in a vacuum of which is constant, or as a stream of photons - particles with a certain energy, momentum, intrinsic angular momentum and zero mass.
Light has all the properties of electromagnetic waves:
. Reflection;
. Refraction;
. Interference;
. Diffraction;
. Polarization.
Light can exert pressure on a substance, be absorbed by the medium, and cause the photoelectric effect. Light deviates from a straight direction. It has a final speed of propagation in a vacuum of 300,000 km/s, and in the medium the speed decreases. In addition to the drop in speed, light begins to refract and can begin to split into the light spectrum under certain circumstances. This is explained by the phenomenon of interference. It is the interference of light that explains the color soap bubbles and thin oil films on water. Light waves are partially reflected from the surface of a thin film, partially transmitted into it, and we observe a rainbow pattern on the surface.
Diffraction of light is the deviation of a light wave from linear propagation. This can be clearly seen when in a room curtained with dark, thick curtains, make a small hole in the curtain, the light will come out like a cone, the top of which will be in our hole. We can observe the refraction of light by placing a spoon in a glass of water. It will be divided at the border between air and water.
We observe the world around us only because humans can perceive the visible spectrum of electromagnetic waves. This occurs due to the fact that special receptors located in the retina of the eye can react to light radiation. And we can distinguish visual images: color, shape, size, distance to an object and much more. Human vision has a number of properties:
. Light sensitivity;
. Sharpness;
. Field of view;
. Binocularity;
. Contrast and adaptation.

3. Application of light in optical fiber.
A. Story
Light is widely used in technology, but has received particular development these days in fiber optic networks. The history of transmitting data over a distance using light and transparent materials began in 1934. Norman French proposed converting voice into light signals and transmitting it along glass rods. A few years later, Swiss physicist Jean-Daniel Colladon conducted an experiment with the transmission of light through a “parabolic liquid flow,” that is, water.
Optical fiber of the modern type was invented in 1954. This was done by two English physicists Narinder Singh Kapani, Harold Hopkins and the Dutch researcher Abraham Van Heel. They announced their inventions at the same time, so all three are considered the founders of this technology. By the way, optical fiber was called optical fiber two years after its invention.
The first fiber optic cables had high light loss. Lawrence Curtis managed to reduce losses in the late 50s. After laser technology was discovered in 1962, fiber optics received another boost in development.
b. General information
Fiber optical communication- a type of wired telecommunication that uses electromagnetic radiation of the optical (near-infrared) range as an information signal carrier, and fiber-optic cables as guide systems. Thanks to the high carrier frequency and wide multiplexing capabilities, the throughput of fiber-optic lines is many times higher than the throughput of all other communication systems and can be measured in terabits per second. But let's return from history to modern times. Today, fiber optic cable is the most... quick way data transmission. This is not surprising. Light acts as a carrier of information, and it is known to have the highest travel speed in the Universe (300 thousand kilometers per second). Low attenuation of light in optical fiber allows the use of fiber-optic communications over significant distances without the use of amplifiers. Fiber optic communications are free from electromagnetic interference and difficult to access for unauthorized use—it is technically extremely difficult to surreptitiously intercept a signal transmitted over an optical cable. When compared with other methods of information transmission, the order of magnitude TB/s is simply unattainable. Another advantage of such technologies is transmission reliability. 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 imagine how information is transferred over optical fiber in general, and even more so are not familiar with specific implementations of technologies. First, let's look at how information is transmitted over optical fiber in general. An optical fiber is a waveguide through which electromagnetic waves with a wavelength of about a thousand nanometers (10-9 m) propagate. This is a region of infrared radiation that is 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 external environment, most of the energy is reflected back into the fiber. This ensures that radiation passes 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 enormous and difficult work of many people. Do not think that such material is easy to create or that this effect is obvious. On the contrary, it should be treated as a great discovery, since today it provides a better way of transmitting 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 ensure that the outward energy output is minimal. Specifically speaking about a technology called “multiplexing,” this means that you transmit multiple wavelengths at the same time. They do not interact with each other, and when receiving or transmitting information, interference effects (superposition of one wave on another) are insignificant, since they manifest themselves most strongly at multiple wavelengths. Here we are talking about using close frequencies (frequency is inversely proportional to the wavelength, so it doesn’t matter what you talk about). A device called a multiplexer is a device for encoding or decoding information into waveforms and back.
V. Development
Smoothly moving on to the development trends 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 associated to a greater extent with 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 implementation of a larger number of channels (wavelengths) in one fiber. Today's systems provide transmission of 40 wavelengths, corresponding to a 100-gigahertz frequency grid. Devices with a 50-GHz network supporting up to 80 channels are next in line to enter the market, which corresponds to the transmission of terabit streams over a single fiber. And today you can already hear statements from laboratories of development companies such as Lucent Technologies or Nortel Networks about the imminent creation of 25-GHz systems.
However, despite such rapid development of engineering and research, market indicators make their own adjustments. The past year has been marked by a serious decline in the optical market, as evidenced by the significant drop in Nortel Networks' share price (29% in one day of trading) after it announced difficulties in selling its products. Other manufacturers found themselves in a similar situation.
At the same time, while Western markets are experiencing some saturation, Eastern markets are just beginning to unfold. The most striking example is the Chinese market, where a dozen national-scale operators are racing to build backbone networks. One cannot help but envy the Chinese - they will now build houses only in close proximity to the fiber optic cable. China's Ministry of Industry and Information Technology recently issued a circular to this effect. In addition, according to this new policy, in order to maintain healthy competition, connection services must be provided to subscribers by several providers at once. True, the connection speed is not specified in any way.
Such a policy is of course beneficial for Chinese operators. In 2012, China Unicom (Hong Kong) Ltd (China's second largest telecommunications company) provided connectivity to 10 million Chinese households on its FTTH networks. And according to Economic Information Daily, approximately 40 million more will join them in 2015. The Chinese government regulation comes into force on April 1, 2013. Meanwhile, in the United States, Google's initiative called "Google Fiber" is being discussed. The bottom line is that Google is going to offer FTTH connections at speeds of 1 gigabit per second to the end consumer. Previously, 1 Gbps speeds were used only in some scientific, government and military institutions. And now we are talking about a nationwide network with such communication speeds. As a pilot version, Google fiber began to be implemented in Kansas. And although work in this direction continues, wait for the emergence of a nationwide fiber optic Google network it will take a long time. Goldman Sachs estimates the cost of this project at more than $140 billion.
Let me remind you that a lot of fiber-optic networks have already been built in the United States. The most famous example is Verizon, which has been building its own fiber optic infrastructure for many years and has already spent $15 billion on it, providing connectivity to approximately 15 million homes. But Verizon offers speeds of 50 Mbps, which can only be increased to 100 Mbps for now. And if “they” have practically resolved the issues of building backbone networks, then in our country, sad as it may be, there is simply no need for thick channels for transmitting our own traffic.
Today on Russian market There are two main competing areas for high-speed Internet connections - home fiber optic networks and ADSL connections.
Home networks are a certain type of “dedicated connection” that connects your home computer to the network through a fiber optic cable that the provider connects to each apartment. ADSL technology, in turn, refers to a type of broadband connection that operates on the principle of a telephone modem, converting analog telephone line into a high-speed transmission channel using special technology. Thus, the main difference between the two competing technologies is technological.
However, the exhibition “Departmental and Corporate Communication Networks”, held in early December, revealed the huge interest of domestic telecom operators in new technologies, including DWDM. And if such monsters as Transtelecom or Rostelecom already have state-scale transport networks, then the current energy sector is just beginning to build them. So, despite all the troubles, optics are the future. And DWDM will play a significant role here. The cost of using fiber optic technology is decreasing, making the service competitive with traditional services. Fiber optic data transmission technology will continue to evolve until an alternative is found. Of the future competitors, only a quantum network is seen, but this technology is still in its infancy and is not yet afraid of optical fiber.
As for the disadvantages, there is only one - the high cost of equipment and tools for installing fiber optics. The cable itself costs tens of times less than transmitters, receivers and signal amplifiers. In addition, special inverters are used to solder cables, some of which cost as much as an expensive car.

4. Conclusion.
In our time of information technology, the state has begun to pay special attention to the process of informatization of society. This process could not but affect such an aspect of public life as education. Today, more and more budget funds are spent on raising the level of technical equipment in schools to improve the information education of young people. These improvements also apply to the quality of Internet connections in educational institutions. And the most progressive and fastest way to connect to the Internet is fiber optic systems. Their introduction into education will make it possible to achieve a huge leap in the information education of students and schoolchildren, which in the future will allow us to train excellent specialists in the field of international Internet systems who will raise our country to a higher level of development in the world. In parallel with this, the development of telecommunications will help educate people capable of maintaining the stability and security of our Internet resources.
From my point of view, the study of the problem posed has a great future and I expect to continue working on this topic as a student. I believe that by studying modern technologies, participating in various levels of research and conferences, you can become a competitive specialist.

Literature:
1) Great Russian Encyclopedia.
2) "White Paper" newspaper.
3) Magazine "ComputerPress No. 1 2001."
4) Kudryashov Yu. B., Perov Yu. F. Rubin A. B. Radiation biophysics: radio frequency and microwave electromagnetic radiation.
5) Listvin A.V., Listvin V.N., Shvyrkov D.V. Optical fibers for communication lines. M.: LESARart, 2003.
6) Alcatel-Lucent report for SEPTEMBER 28, 2009.
7) Soviet encyclopedia.
8) Tarasov K.I. Spectral devices.

There are not many articles on Habré devoted to optical communication line technologies. More recently, there have been articles on high-power DWDM systems, and a short 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 accessible ways in Russia of using the resource of fiber-optic communication lines in data networks and - just a little - cable television.

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

The most common single-mode optical fiber is SMF G.652 different modifications. It's almost certain that if you have a fiber optic line, it's made of G.652 fiber. It has a number of important characteristics that you need to keep in mind.
Specific (also called kilometric) attenuation - that is, the attenuation of one kilometer of fiber - depends on the wavelength of the radiation.

Wikipedia tells us the following distribution:

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

Nevertheless, we must keep in mind this picture and the very presence of this dependence.

Historically, the range of wavelengths that an optical fiber carries 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 Wikipedia article).

To a reasonable approximation, the fiber properties within each range can be considered approximately the same. The water peak usually occurs at 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 compaction systems - bidirectional transmission along a single fiber

Initially, a duplex fiber-optic communication line required two fibers to operate: one fiber transmitted information in one direction, and the other fiber transmitted information in the other direction. This is convenient in its obviousness, but rather wasteful in relation to the use of the resource of the laid cable.

Therefore, as soon as technology began to allow, solutions began to appear for transmitting information in both directions over one 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-band. “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 find a combination of 1310/1490 nm - it is used if, simultaneously with data, a cable television signal at a wavelength of 1550 nm is transmitted along the same fiber; or 1270/1330nm - it is used to transmit 10 Gbit/s streams.

Data and cable multiplexing

Since I touched on the topic of CTV, I’ll tell you a little more about it.

To deliver a cable television signal from the headend to an apartment building, optics are now also used. It uses either a wavelength of 1310 nm - here there is minimal chromatic dispersion, that is, signal distortion; or a wavelength of 1550 nm - here there is a minimum specific attenuation and it is possible to use pure optical amplification using EDFA. If there is a need to deliver both a data stream (Internet) and a CATV signal to one home at the same time, you need to 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 of streams) with three outputs: for CATV, a single-fiber transceiver and a common output (see diagram). In this way, you can build a PON or Ethernet network using wavelengths 1310/1490 for data transmission, and 1550 nm for CATV.

CWDM and DWDM

theslim has already briefly talked about CWDM compaction. On my own behalf, I will only add that the channels for receiving and transmitting data indicated in the article are purely arbitrary; the multiplexer does not care at all which direction the signal goes in each channel; A optical receivers- broadband, they respond to radiation of any wavelength. One of the important points that must be kept in mind when designing a CWDM line is the difference in specific attenuation in the fiber on different channels (see the first section of this article), as well as the difference in the attenuation introduced by the multiplexer itself. The multiplexer is made of series-connected filters, and if for the first channel in the chain the attenuation can be less than one decibel, then for the last it will be closer to four (these values ​​are given for a 1x16 multiplexer, for 16 wavelengths). It is also useful to remember that no one prohibits building two-fiber CWDM lines by simply combining two pairs of multiplexers into one functional block.
In addition, I note that it is quite possible to allocate part of the frequency resource for CATV, transmitting up to seven duplex data streams over one fiber simultaneously with analog television.

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

Firstly, they are significantly more expensive than CWDM. Their use requires lasers with strict wavelength tolerances and multiplexers of very high selectivity.

Secondly, the ranges used lie in the working areas of 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 many, when they hear the word “DWDM,” to immediately imagine the complex systems of telecom market monsters, although such equipment can be used in simpler systems.
And thirdly, attenuation in the C- and L-bands is minimal across the entire transparency window of the optical fiber, which makes it possible to build lines of greater length even without amplifiers than when using CWDM.

DWDM multiplexers are just as passive devices as CWDM multiplexers. For up to 16 channels they are also made up of separate filters, which 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 an electronic thermal stabilization circuit (Thermal AWG), or using special ways automatic compensation that does not require energy (Athermal AWG). This makes such multiplexers more expensive and more difficult to operate.

Practical Limitations in Fiber Optic Communications

Finally, I'll talk a little about the limitations that you have to deal with when organizing optical communications.

As comrade saul quite rightly noted, the first limitation is the optical budget.
I will 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 at which the transmission will be carried out.

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

If we are building a system with intermediate branches on OADM, do not forget to calculate the attenuation on OADM. By the way, it differs for the end-to-end channel and output wavelengths.

Don't forget to leave a few decibels of operational reserve.

The second thing you have to deal with is chromatic dispersion. It really becomes relevant for 10 Gbit/s lines, and generally speaking, the equipment manufacturer thinks about it first of all. By the way, it is dispersion that gives physical meaning to the mention of kilometers in the marketing names of transceivers. It is simply useful for the operating 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

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 constitute serious competition (according to at least, in the TSB segment) coaxial cable and twisted pair. Despite such disadvantages 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 fundamental. 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 optics.
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 currently makes it possible to offer security systems customers not only a reliable, but also a cost-effective solution. Using a light beam to transmit a signal, a wide bandwidth allows 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 that of copper cable (up to 20 MHz);
– immunity to electrical interference, absence of “ground loops”;
– low losses during signal transmission, signal attenuation is about 0.2–2.5 dB/km (for RG59 coaxial cable – 30 dB/km for a 10 MHz signal);
– does not cause interference in neighboring copper or other fiber optic cables;
– long transmission range;
– increased security data transmission;
– good quality of the transmitted signal;
– fiber optic cable is miniature and lightweight.

Operating principle of a fiber optic line
Fiber optics is a technology that uses light as an information carrier, and it does not matter what type of information we are talking about: analog or digital. Typically, infrared light is used and the transmission medium is fiberglass.
Fiber optic equipment can be used to transmit analogue or digital signal various types.
In its simplest form, 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;
– a 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 source of light distributed through optical cables is a light emitting diode (LED) (or semiconductor laser - LD). At the other end of the cable, a receiving detector converts the light signals into electrical signals. Fiber optics relies on a special effect - refraction at the maximum angle of incidence, when total reflection occurs. This phenomenon occurs when a ray of light leaves a dense medium and enters a less dense medium at a certain angle. The inner core (strand) of a fiber optic cable has a higher refractive index than the cladding. Therefore, a beam of light, passing through the internal core, cannot go beyond its limits due to the effect of total reflection (Fig. 1). Thus, the transported signal travels 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 varying aggressiveness.

WORLD OF DIGITAL AND GLASS

INTRODUCTION

Fiber optic communications have many well-known advantages over twisted pair and coaxial cables, such as immunity to electrical interference 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 optic communications have many well-known advantages over twisted pair and coaxial cables, such as immunity to electrical noise and unmatched 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 analogue solutions they replace.

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

This tutorial examines the technology of digital signal transmission over fiber optic cables and its economic and technological advantages.

ANALOG TRANSMISSION OVER FIBER

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

In amplitude modulation systems, the optical signal is a luminous flux with intensity that changes in accordance with changes in the input electrical signal. Either LEDs or lasers are used as a light source. Unfortunately, both are nonlinear, that is, in the full range of brightness from no radiation to maximum value proportionality between the input signal and light intensity is not maintained. However, this is precisely the control method used in systems with amplitude modulation. As a result, various distortions of the transmitted signal occur:

• decrease in signal-to-noise ratio as cable length increases;
• nonlinear differential gain and phase errors in video signal transmission;
• limiting the dynamic range of the audio signal.

To improve the quality of operation 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 familiar with frequency modulation of signals in radio engineering, the use of this term here may seem unfounded, 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 true, and in fact it would be more correct to use the term “pulse phase modulation” (PPM), but in the field of fiber optic technology this is the terminology that has been established. You should always remember that the word “frequency” in the name of the modulation method means the repetition rate of the pulses, and not the frequency of the light waves carrying them.

In 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

While frequency modulation eliminates many of the driver brightness control problems associated with AM systems, it does have its own challenges. One of them is crosstalk, which is known in FM systems. They are observed, in particular, when transmitting several frequency modulated signals over one optical fiber, for example, when using a multiplexer. Crosstalk occurs at a transmitter or receiver as a result of instability in the tuning of important signal filtering circuits designed to separate carrier frequencies. If the filters are poorly tuned, the frequency-modulated carriers interact with each other and become distorted. Fiber optic engineers can design FM systems that minimize the potential for crosstalk, but any improvements to the design will increase the cost of the devices.

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

DIGITAL SYSTEMS

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

Rice. 2. Digital analog signal transmission system

In purely digital systems, the low-frequency input signal is immediately sent to an 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 a 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 switching on and off of one emitter, which occurs at a very high frequency.

At the receiving end, the signal is reversely converted. Individual streams corresponding to individual transmitted signals are separated from the combined digital stream. They are fed to digital-to-analog converters, after which they are output to original format(Fig. 2).

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

SIGNAL TRANSMISSION ACCURACY

In analog systems with amplitude modulation, the signal loses quality in proportion to the path traveled along the optical fiber. This fact, combined with the fact that AM systems work only with multimode fibers, limits the use of such systems relatively short distances transfers. FM systems work somewhat better: although the signal quality in them decreases, it remains approximately constant in not very long lines, sharply decreasing only when a certain maximum length is reached. Only fully digital systems guarantee the preservation of signal quality when transmitted 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 along the optical fiber. This fact, combined with the fact that AM systems only work 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 when developing systems for organizing several transmission channels over one optical fiber (multiplexers). For example, in an analog system designed to transmit four channels of video or audio, in order to meet the system bandwidth, it is necessary to limit the bandwidth allocated individual channels. In digital systems, this compromise does not have to be made: one, four, or even ten signals can be transmitted along a single fiber without loss of quality.

HIGHER SIGNAL QUALITY

Rice. 3

Transmitting analog signals in digital form provides higher quality than pure analog. Signal distortion with this transmission method can only occur during analog-to-digital and reverse digital-to-analog conversion. While no conversion is perfect, today's technology is so advanced that even inexpensive ADCs and DACs provide much higher quality video and audio signals than can be achieved with analog AM and FM systems. This can be easily seen by comparing the signal-to-noise ratios and nonlinear distortions (differential phase and differential gain) of digital and analog systems designed to transmit signals of the same format over optical fiber same type at the same wavelength.

Digital technologies give engineers 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 width of an analog-to-digital converter, you 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 - absence of distortion and 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 between system cost and performance, trying to balance the two without compromising critical signal parameters. In digital systems, scaling systems and managing their performance and cost is much less challenging.

UNLIMITED TRANSMISSION DISTANCE

Another advantage of digital systems over analog predecessors is their ability to restore a signal without introducing additional distortion into it. This 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 that significantly exceed 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 place where the light becomes too weak, you install a linear amplifier, then it will amplify the signal to its original power, and it can be transmitted further over the same distance. It is important to note that the linear amplifier reconstructs the digital stream, which does not have any effect on the quality of the encoded analog video or audio signal, regardless of how many times the restoration is performed in the 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 significantly 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.

LOWER COST

Assessing 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 offering digital quality at the price of an analog system

The cost of digital components has dropped significantly over last years, and equipment manufacturers have been able to develop and offer products that cost the same or even less than previous generation analog products. Of course, some firms want to convince the public that the superior quality of digital systems can only be obtained at a premium, but in reality they have simply chosen 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 one is cable costs. Digital systems allow more information to be transmitted over a single cable, thereby reducing the need for it. The advantage is especially noticeable where signals of different types need to be transmitted simultaneously, for example, video and audio or audio and data. Without special problems Engineers will be able to design a digital system at an affordable cost in which a single fiber can carry different types of signals, such as two channels of video and four channels of audio. Using analog technologies, most likely, you would have to make two separate systems, or, at a minimum, use two separate cables to transmit 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 need to be transmitted over one optical fiber, digital systems are preferable because they operate more reliably and provide higher signal quality. For example, in a digital video multiplexer it is possible to transmit ten channels with equally high quality, but in an analog system this is not possible at all.

It is also necessary to take into account the inevitable costs for the years of operation of fiber-optic systems. Maintenance and repairs. And here the advantage lies with digital systems. Firstly, they do not require initial setup after installation - the transmitter and receiver are simply connected via fiber optic cable, and the system is ready to use. Analog systems usually require adjustment to the parameters of a specific transmission line, taking into account its length and signal intensity. Additional time adjustment entails additional costs.

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

Because there are fewer components that can fail over time, digital systems are much more stable and reliable. They do not require re-tuning, and troubleshooting will take much less time, since they do not have crosstalk, parameter drift and other disadvantages inherent in traditional analog systems.

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

CONCLUSIONS

Just as fiber optic technology has many advantages over traditional copper wires and coaxial cables, digital transmission takes fiber optic technology several notches up, giving users a whole new set of benefits. Digital systems have unique characteristics: accuracy of signal transmission over the entire length of the communication line, minimal introduced distortion (including the absence of cross-distortion and intermodulation), the ability to repeatedly restore a digital stream when transmitting it over a long line without compromising the quality of the analog signal encoded in it. This guarantees a level of analog signal fidelity unattainable with analog systems.

Component prices for digital and analog fiber optic systems are comparable, and when considering 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 and analog systems, because the choice is quite clear: digital systems are better in every way. It will be much more useful to limit yourself to only them and select 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 evaluate them:

• How easy is it to install the system?
  • If the transmitter and receiver are user configurable, how easy is it to do this and what are the challenges?
• Is the design of the devices compact, durable and reliable?
• Are the devices available in desktop cases or designed for rack mounting? Are there options in both types of housings?
  • Are the devices suitable for use with both single-mode and multimode fibers?
  • Does the manufacturer have sufficient experience and reputation in the market for the products he offers?
  • How does the price of the product compare with the price of traditional analog systems? (Digital devices in production are no more expensive than analog ones and their cost should not be higher).

Market analysis and comparison of the characteristics of similar products will allow you to ultimately select elements of digital fiber optic systems that will serve you faithfully for many years.

Fiber optic communication- a method of transmitting information that uses electromagnetic radiation of the optical (near-infrared) range as a carrier of the information signal, and fiber-optic cables as guide systems. Thanks to the high carrier frequency and wide multiplexing capabilities, the throughput of fiber-optic lines is many times higher than the throughput of all other communication systems and can be measured in Terabits per second. Low attenuation of light in optical fiber allows the use of fiber-optic communications over significant distances without the use of amplifiers. Fiber optic communications are free from electromagnetic interference and are difficult to access for unauthorized use: it is technically extremely difficult to surreptitiously intercept a signal transmitted over an optical cable.

Physical basis

Fiber-optic communication is based on the phenomenon of total internal reflection of electromagnetic waves at the interface between dielectrics with different refractive indices. An optical fiber consists of two elements - the core, which is the direct light guide, and the cladding. The refractive index of the core is slightly greater than the refractive index of the cladding, due to which the light beam, experiencing multiple reflections at the core-cladding interface, propagates in the core without leaving it.

Application

Fiber-optic communications are increasingly used in all areas - from computers and on-board space, aircraft and ship systems, to long-distance information transmission systems, for example, the fiber-optic communication line Western Europe - Japan, large part of which passes through the territory of Russia. In addition, the total length of underwater fiber-optic communication lines between continents is increasing.

Fiber to every home Fiber to the premises, FTTP or Fiber to the home, FTTH) is a term used by telecommunications Internet providers to designate broadband telecommunication systems based on the installation of a fiber channel and its termination at the end user's territory by installing optical terminal equipment to provide a range of telecommunications services, including:

• high-speed Internet access;
• telephone services;
• television reception services.

The cost of using fiber optic technology is decreasing, making the service competitive with traditional services.

Story

The history of long-distance data transmission systems should begin in ancient times, when people used smoke signals. Since that time, these systems have improved dramatically, first the telegraph, then the coaxial cable. In their development, these systems sooner or later ran into fundamental limitations: for electrical systems this is the phenomenon of signal attenuation at a certain distance, for microwave systems it is the carrier frequency. Therefore, the search for fundamentally new systems continued, and in the second half of the 20th century a solution was found - it turned out that signal transmission using light is much more effective than both electrical and microwave signals.

In 1966, Kao and Hokam of STC Laboratory (STL) introduced optical filaments made of ordinary glass that had an attenuation of 1000 dB/km (while the attenuation of coaxial cable was only 5-10 dB/km) due to impurities, which they contained and which, in principle, could be removed.

There were two global problems when developing optical data transmission systems: light source and signal carrier. The first was resolved with the invention of lasers in 1960, the second with the advent of high-quality optical cables in 1970. It was developed by Corning Incorporated ( English) . The attenuation in such cables was about 20 dB/km, which was quite acceptable for signal transmission in telecommunication systems. At the same time, fairly compact semiconductor GaAs lasers were developed.

After intensive research between 1975 and 1980, the first commercial fiber optic system was developed using a gallium arsenide (GaAs) semiconductor laser at 0.8 μm wavelength. The bitrate of the first generation systems was 45 Mbit/s, the distance between repeaters was 10 km.

On April 22, 1977, in Long Beach, California, General Telephone and Electronics first used an optical link to transmit telephone traffic at 6 Mbps.

The second generation of fiber optic systems was developed for commercial use in the early 1980s. They operated with light with a wavelength of 1.3 microns from InGaAsP lasers. However, such systems were still limited by the dispersion occurring in the channel. However, already in 1987, these systems operated at speeds of up to 1.7 Gbit/s with a distance between repeaters of 50 km.

Basic definitions

Optical fiber is a glass or plastic thread used to carry light within itself through total internal reflection.

The structure of a fiber optic cable is very simple and similar to the structure of a coaxial electrical cable, only instead of a central copper wire, thin glass fiber (with a diameter of about 1-10 microns) is used, and instead of internal insulation, a glass or plastic shell is used, which does not allow light to escape beyond the fiberglass. We are dealing with the regime of the so-called total internal reflection of light from the boundary of two substances with different refractive indices (the glass shell has a much lower refractive index than the central fiber). The metal braiding of the cable is usually absent, since shielding from external electromagnetic interference is not required, but sometimes it is still used for mechanical protection from the environment (such a cable is sometimes called an armored cable; it can combine several fiber optic cables under one sheath).

Fiber optics branch of applied science and mechanical engineering that describes such fibers. Optical fibers are used in fiber optic communications, which allows digital information to be transmitted over long distances and from more high speed data transmission than in electronic communications. In some cases, they are also used to create sensors.

Fiber optic communications communications built on the basis of fiber optic cables. The abbreviation FOCL (fiber-optic communication line) is also widely used. It is used in various fields of human activity, ranging from computing systems to structures for communication over long distances. It is today the most popular and effective method for providing telecommunications services.

Materials

Glass optical fibers are made from quartz glass, but other materials such as fluoro-zirconate, fluoro-aluminate and chalcogenide glasses can be used for far infrared. Like other glasses, these have a refractive index of about 1.5.

Currently, the use of plastic optical fibers is developing.

The following are used as light sources in fiber optic cables:

1. LEDs, or light-emitting diodes (Light Emmited Diode, LED);
2. semiconductor lasers, or laser diodes (Laser Diode).

For single-mode cables, only laser diodes are used, since with such a small diameter of the optical fiber, the light flux created by the LED cannot be directed into the fiber without large losses; it has an overly wide radiation pattern, while laser diode narrow. Therefore, cheaper LED emitters are used only for multimode cables.

Although fiber optics is a widely used and popular means of communication, the technology itself is simple and developed a long time ago. The experiment with changing the direction of a light beam by refraction was demonstrated by Daniel Colladon and Jacques Babinet back in 1840. A few years later, John Tyndall used this experiment in his public lectures in London, and already in 1870 he published a work on the nature of light. The practical application of the technology was found only in the twentieth century. In the 1920s, experimenters Clarence Hasnell and John Berd demonstrated the possibility of transmitting images through optical tubes. This principle was used by Heinrich Lamm for medical examination of patients. It wasn't until 1952 that Indian physicist Narinder Singh Kapany conducted a series of his own experiments that led to the invention of optical fiber. In fact, he created the very same bundle of glass threads, and the shell and core were made of fibers with different refractive indices. The shell actually served as a mirror, and the core was more transparent - this was how the problem of rapid dispersion was solved. If previously the beam did not reach the end of the optical filament, and it was impossible to use such a means of transmission over long distances, now the problem has been solved. Narinder Kapani improved the technology by 1956. A bunch of flexible glass rods transmitted the image with virtually no loss or distortion.

The invention of fiber optics by Corning specialists in 1970, which made it possible to duplicate a telephone signal data transmission system over a copper wire over the same distance without repeaters, is considered to be a turning point in the history of the development of fiber optic technologies. The developers managed to create a conductor that is capable of maintaining at least one percent of the optical signal power at a distance of one kilometer. By today's standards, this is a rather modest achievement, but then it was a necessary condition in order to develop a new type of wired communication.

Initially, optical fiber was multiphase, that is, it could transmit hundreds of light phases at once. Moreover, the increased diameter of the fiber core made it possible to use inexpensive optical transmitters and connectors. Much later, they began to use higher-performance fiber, through which it was possible to transmit only one phase in the optical environment. With the introduction of single-phase fiber, signal integrity could be maintained over greater distances, which facilitated the transfer of considerable amounts of information.

The most popular fiber today is single-phase fiber with zero wavelength offset. Since 1983, it has been the industry's leading fiber optic product, proven to operate over tens of millions of kilometers.

Classification

There are several classes of optical fibers based on their structure and operating principle:

1. Single-mode fibers
2. Multimode fibers
3. Gradient Index Fibers

Optical fibers with a stepped refractive index distribution profile.

Refractive index profile of various types of optical fibers: multimode fiber with a step change in refractive index (a); multimode fiber with a smooth change in refractive index (6); single mode fiber (c).

All optical fibers are divided into two main groups: multimode MMF (multi mode fiber) and single mode SMF (single mode fiber).

The concept of “mode” describes the mode of propagation of light rays in the inner core of the cable. A single-mode cable uses a central conductor of very small diameter, comparable to a light wavelength of 5 to 10 microns. In this case, almost all light rays propagate along the optical axis of the light guide without being reflected from the external conductor. The production of ultra-thin high-quality fibers for single-mode cable is a complex technological process, which makes single-mode cable quite expensive. In addition, it is quite difficult to direct a beam of light into a fiber of such a small diameter without losing a significant part of its energy. Multimode cables use wider inner cores, which are easier to manufacture technologically. The standards define the two most commonly used multimode cables: 62.5/125 μm and 50/125 μm, where 62.5 μm or 50 μm is the diameter of the central conductor, and 125 μm is the diameter of the outer conductor.

Multimode fibers

Multimode fibers are divided into step index multi mode fiber and graded index multi mode fiber.

In a multimode cable, the trajectories of light rays have a noticeable scatter, as a result of which the signal shape at the receiving end of the cable is distorted. The central fiber has a diameter of 62.5 µm, and the outer cladding diameter is 125 µm (this is sometimes referred to as 62.5/125). Transmission uses a regular (non-laser) LED, which reduces the cost and increases the life of the transceivers compared to single-mode cable. The wavelength of light in a multimode cable is 0.85 microns. The permissible cable length reaches 2-5 km. Nowadays, multimode cable is the main type of fiber optic cable as it is cheaper and more accessible.

Multimode stepped profile fibers

The first data fibers were multimode with a stepped refractive index profile. For light to propagate through total internal reflection, it is necessary to have a refractive index of the core glass, n1, slightly greater than the refractive index of the cladding glass, n2. At the interface between two glass media, the following condition must be satisfied: n1 > n2. If the refractive index of the optical fiber core n1 is the same over the entire cross section, then the fiber is said to have a stepped profile. This fiber light guide is multimode. The light pulse propagating in it consists of many components, directed in separate modes of the light guide. Each of these modes is excited at the fiber input at its own specific angle of entry into the fiber and is directed along it along the core, passing with different beam trajectories. Every fashion passes different distance optical path and therefore travels the entire length of the light guide in different times. Moreover, if we apply a short (rectangular) light pulse to the input of the light guide, then at the output of the multimode light guide we will receive a pulse “blurred” in time. These distortions, due to the dispersion of the delay time of individual modes, are called mode dispersion.

Gradient Profile Multimode Fibers

In a multimode stepped profile optical fiber, the modes travel along optical paths of different lengths and therefore arrive at the end of the fiber at different times. This dispersion can be significantly reduced if the refractive index of the core glass decreases parabolically from a maximum value n1 at the axis of the light guide, to a refractive index value n2 at the interface with the cladding. An optical waveguide with such a profile (when the refractive index changes smoothly) is called a gradient fiber light guide. Light rays travel through such a fiber in wave- or helical spirals. The further the light beam deviates from the axis of the light guide, the more it turns back towards the axis. In this case, since the refractive index from the axis to the edge of the core decreases, the speed of light propagation in the medium increases. Thanks to this, longer optical paths are compensated by shorter travel times. As a result, the difference in time delays of various rays almost completely disappears.

Singlemode fibers

Single-mode fibers are divided into step index single mode fiber or standard fiber SF, dispersion-shifted single mode fiber DSF, and non-zero dispersion-shifted fiber NZDSF. dispersion-shifted single mode fiber).

In a single-mode cable, virtually all the rays follow the same path, resulting in them all reaching the receiver at the same time, and the signal shape is virtually undistorted. A single-mode cable has a central fiber diameter of about 1.3 µm and transmits light only at the same wavelength (1.3 µm). The dispersion and loss of the signal are very small, which makes it possible to transmit signals over significantly longer distance than in the case of using multimode cable. For single-mode cable, laser transceivers are used that use light exclusively at the required wavelength. Such transceivers are still relatively expensive and not very durable. However, in the future, single-mode cable should become the main cable due to its excellent characteristics.

Step profile fibers

Mode dispersion in an optical fiber can be eliminated if the structural parameters of the stepped fiber are selected in such a way that only one mode will be guided in it, namely the fundamental (main) mode. However, the fundamental mode also broadens in time as it passes through such a fiber. This phenomenon is called chromatic dispersion. It is a property of the material, therefore, as a rule, it occurs in any optical fiber, but in the wavelength range from 1200 to 1600 nm it is relatively small or absent. To produce a low-attenuation stepped fiber that directs only the fundamental mode in the wavelength range greater than 1200 nm, the mode field diameter must be reduced to 8-10 µm. Such a stepped fiber light guide is called a standard single-mode optical fiber.

Multi-step profile fibers

The refractive index profile of a conventional single-mode fiber has a stepped profile. For such a profile structure, the sum of the material dispersion in the waveguide dispersion at a wavelength of about 1300 nm is zero. For modern optical fiber communications devices using wavelengths of 1550 nm or simultaneous transmission of signals at several wavelengths, it is desirable to have zero dispersion at other wavelengths. And for this it is necessary to change the wave dispersion and, consequently, the structure of the refractive profile of the fiber light guide. This results in multi-step or segmented refractive index profiles. Using these profiles, it is possible to produce optical fibers in which the zero-dispersion wavelength is shifted to 1550 nm (dispersion-shifted fiber) or the dispersion values ​​are very small throughout the entire wavelength range from 1300 nm to 1550 nm (dispersion smoothed or compensated fiber).

The core diameter of single-mode fibers is 7-9 microns. Due to the small diameter, only one mode of electromagnetic radiation is transmitted through the fiber, thereby eliminating the influence of dispersion distortions. Currently, almost all fibers produced are single-mode.

Fiber Optic Line Elements

1. Optical receiver

Optical receivers detect signals transmitted through a fiber optic cable and convert them into electrical signals, which are then amplified and further reconstructed, as well as clock signals. Depending on the transmission speed and system specifics of the device, the data stream can be converted from serial to parallel.

1. Optical transmitter

The optical transmitter in a fiber optic system converts the electrical data sequence supplied by the system components into an optical data stream.

1. Preamplifier

The amplifier converts the asymmetric current from the photodiode sensor into an asymmetric voltage, which is amplified and converted into a differential signal.

1. Data synchronization and recovery chip

This chip must restore the clock signals from the received data stream and their clocking. The phase-locked loop circuitry required for clock recovery is also fully integrated into the clock chip and does not require external control clock pulses.

1. Optical cable, consisting of optical fibers located under a common protective sheath.

Fiber Optic Transceivers

To transmit data over optical channels, signals must be converted from electrical to optical, transmitted over a communications link, and then converted back to electrical at the receiver. These transformations occur in the transceiver device, which contains electronic components along with optical components.

Widely used in transmission technology, the time division multiplexer allows the transmission speed to be increased to 10 Gb/s. Modern high-speed fiber optic systems offer the following transmission speed standards.

SONET standard	SDH standard	Transmission speed
		51.84 Mb/sec
		155.52 Mb/s
		622.08 Mb/sec
		2.4883 Gb/sec
		9.9533 Gb/sec

New methods of multiplexing wavelength division or wavelength division multiplexing make it possible to increase data transmission density. To achieve this, multiple multiplexed streams of information are sent over a single fiber optic channel using each stream's transmission at a different wavelength. The electronic components in the WDM receiver and transmitter are different from those used in a time division system.

Advantages of fiber optic communication

1. Broadband optical signals due to extremely high carrier frequency. This means that information can be transmitted over a fiber optic line at a speed of about 1 Tbit/s;
2. Very low attenuation of the light signal in the fiber, which makes it possible to build fiber-optic communication lines up to 100 km or more in length without signal regeneration;
3. Resistance to electromagnetic interference from surrounding copper cable systems (power lines, electric motor installations, etc.) and weather conditions;
4. Protection against unauthorized access. Information transmitted over fiber-optic communication lines is practically impossible to intercept in a non-destructive manner;
5. Electrical safety. Being, in fact, a dielectric, optical fiber increases the explosion and fire safety of the network, which is especially important at chemical and oil refineries, when servicing high-risk technological processes;
6. Durability of fiber-optic communication lines the service life of fiber-optic communication lines is at least 25 years.

Disadvantages of fiber optic communication

1. The relatively high cost of active line elements that convert electrical signals into light and light into electrical signals;
2. Relatively high cost of splicing optical fiber. This requires precision, and therefore expensive, technological equipment. As a result, if an optical cable breaks, the cost of restoring a fiber-optic line is higher than when working with copper cables.

Application of fiber optic communication lines

Optical fiber is actively used to build city, regional and federal communication networks, as well as to install connecting lines between city automatic telephone exchanges. This is due to speed, reliability and high throughput fiber networks. Also, through the use of fiber optic channels, there are cable television, remote video surveillance, video conferences and video broadcasts, telemetry and other information systems. In the future, it is planned to use the conversion of speech signals into optical signals in fiber-optic networks.

Federal state budget educational institution higher professional education

St. Petersburg National Research University

information technology, mechanics and optics

Faculty of IKVO Department of MIPiU

Direction (specialty) 090900 “Information security” Group 2750

Qualification (degree) bachelor

For the course “Concepts of modern natural science”

Fiber optic communication.

Completed:

2nd year student

Bogopolskaya E.A.

candidate of technical sciences, associate professor of the department of PBKS

Komarova I.E.

G.S. Petersburg

1. Basic concepts……………………………1

2.Materials………………………………………………………..2

3.History……………………………………………………...2

4.Classification…………………………………...3

5. Elements of fiber optic lines………7

6.Advantages of fiber optic communication......9

7. Disadvantages of fiber optic communication.........9

8. Application of fiber optic communication lines…….9



Optical communication

communication via electromagnetic oscillations of the optical range (usually 10 13 -10 15 Hz). The use of light for the simplest (low-information) communication systems has a long history (see, for example, Optical telegraph). With the advent of lasers the opportunity arose to transfer to the optical range various means and principles of receiving, processing and transmitting information developed for the radio range. Huge volume growth transmitted information and at the same time, the almost complete exhaustion of the capacity of the radio range has given the problem of mastering the optical range for communication purposes exceptional importance. Main advantages of O. s. compared to communications at radio frequencies, determined by the high value of the optical frequency (short wavelength): large frequency bandwidth for information transmission, 10 4 times greater than the frequency band of the entire radio range, and high directivity of radiation with input and output apertures x, significantly smaller apertures antennas in the radio range. The last advantage of O. s. allows the use of generators with relatively low power in transmitters of optical communication systems and provides increased noise immunity and communication secrecy.

Structurally, the line O. s. similar to a radio communication line (See Radio communication). To modulate the radiation of an optical generator, either the generation process is controlled by influencing the power source or the optical resonator of the generator, or additional external devices are used that change the output radiation according to the required law (see Light modulation). Using the output optical unit, the radiation is formed into a low-divergence beam that reaches the input optical unit, which focuses it onto the active surface of the photoconverter. From the output of the latter, electrical signals enter information processing nodes. Selection of carrier frequency in the O.S. system. - a complex complex problem in which the conditions of propagation of optical radiation in the transmission medium, technical characteristics of lasers, modulators, light receivers (See Light receivers), and optical units must be taken into account. In O. systems. Two methods of receiving signals are used - direct detection and heterodyne reception. The heterodyne reception method, having a number of advantages, the main ones being increased sensitivity and discrimination of background interference, is technically much more complicated than direct detection. A serious disadvantage of this method is the significant dependence of the signal value at the output of the photodetector on the characteristics of the path.

Depending on the operating range of the O. s. can be divided into the following main classes: open ground-based short-range systems that use the passage of radiation in the surface layers of the atmosphere; ground-based systems using closed light guide channels (fiber light guides, light guide mirror-lens structures) for highly informative communication between automatic telephone exchanges, computers, for long distance communication; highly informative communication lines (mainly relay lines) operating in near outer space; long-distance communication lines.

In the USSR and abroad, a certain amount of experience has been accumulated in working with open O.S. lines. in the surface layers of the atmosphere using lasers. It has been shown that the strong dependence of communication reliability on atmospheric conditions (determining optical visibility) along the propagation path limits the use of open optical communication lines. over relatively short distances (several kilometers) and only for duplicating existing cable communication lines, use in low-information mobile systems, alarm systems, etc. However, the open lines of O. s. promising as an affinity connection between the Earth and space. For example, using a laser beam you can transmit information over a distance. Optical communications10 8 km at speeds up to 10 5 bit V sec, while microwave technology at these distances provides transmission speeds only Optical communications10 bit V sec. In principle, O. s. in space possible at distances up to 10 10 km, which is unthinkable for other communication systems; however, the construction of O.'s cosmic lines. technically very difficult.

Under terrestrial conditions, the most promising optical systems are those that use closed light-guide structures. In 1974, the possibility of manufacturing glass light guides with attenuation of transmitted signals of no more than a few was shown. db/km. With the current level of technology, using semiconductor diode emitters operating in both laser (coherent) and incoherent modes, cables with optical fiber cores and semiconductor receivers, it is possible to build communication lines for thousands of telephone channels with repeaters located at distances of about 10 km from each other. Intensive work on creating laser emitters with service life Optical communications 10-100 thousand. h, the development of broadband, highly sensitive receiving devices, more efficient light-guide structures, and technology for manufacturing long-distance light guides will apparently make O.S. competitive with communications via existing cable and relay highways in the next decade. It can be expected that O. s. will occupy an important place in the national communications network along with other means. In the future of the O. system. with fiber-optic lines, in terms of their information capabilities and cost per unit of information, can become the main type of trunk and intracity communications.

Lit.: Chernyshev V.N., Sheremetyev A.G., Kobzev V.V., Lasers in communication systems, M., ; Pratt W.K., Laser communication systems, trans. from English, M., 1972; Application of lasers, trans. from English, M., 1974.

A. V. Ievsky, M. F. Stelmakh.

Great Soviet Encyclopedia . - M.: Soviet Encyclopedia. 1969-1978 .

See what “Optical communications” is in other dictionaries:

Transmitting information using light. The simplest (uninformative) types of O. s. used with con. 18th century (e.g. semaphore alphabet). With the advent of lasers, it became possible to transfer to optical technology. range of means and principles of production, processing... ... Physical encyclopedia

OPTICAL COMMUNICATIONS CM- Optical communication... Big Polytechnic Encyclopedia

Big Encyclopedic Dictionary

optical communication- See optical communications. The difference in the use of the two terms is as follows: the concept optical most often refers to optical communications equipment, and the term lightwave refers to optical signal processing equipment. [L.M. Nevdyaev... ... Technical Translator's Guide

Communication between two or more points through light, light signals. The use of light to convey simple messages has a long history. Since ancient times, the lights of bonfires warned of the approach of enemies, showed the way... ... Encyclopedia of technology

Communication via electromagnetic oscillations in the optical range (1013-1015 Hz), usually using lasers. Optical communication systems are structurally similar to radio communication systems. Open space and ground-based optical communication lines are promising... ... encyclopedic Dictionary

optical communication- optinis ryšys statusas T sritis automatika atitikmenys: engl. optical communication vok. optische Kopplung, f; optische Nachrichtenübertragung, f rus. optical communication, f pranc. communication optique, m … Automatikos terminų žodynas

Communication between two or more. points by means of electromagnetic waves optical. range. Optical capacity The communication channel significantly exceeds the capacity of radio frequency channels, since optical radiation has frequencies of the order of 10 1000 THz (1012 1015 Hz) ... Big Encyclopedic Polytechnic Dictionary

Fiber optical communication is a type of wired telecommunication that uses electromagnetic radiation of the optical (near-infrared) range as an information signal carrier, and fiber as guide systems... ... Wikipedia

WORLD OF DIGITAL AND GLASS

INTRODUCTION

Fiber optic communications have many well-known advantages over twisted pair and coaxial cables, such as immunity to electrical noise and unmatched 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 optic communications have many well-known advantages over twisted pair and coaxial cables, such as immunity to electrical noise and unmatched 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 analogue solutions they replace.

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

This tutorial examines the technology of digital signal transmission over fiber optic cables and its economic and technological advantages.

ANALOG TRANSMISSION OVER FIBER

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

In amplitude modulation systems, the optical signal is a luminous flux with intensity that changes in accordance with changes in the input electrical signal. Either LEDs or lasers are used as a light source. Unfortunately, both are nonlinear, that is, in the full range of brightness from no radiation to the maximum value, proportionality between the input signal and light intensity is not observed. However, this is precisely the control method used in systems with amplitude modulation. As a result, various distortions of the transmitted signal occur:

• decrease in signal-to-noise ratio as cable length increases;
• nonlinear differential gain and phase errors in video signal transmission;
• limiting the dynamic range of the audio signal.

To improve the quality of fiber optic signal transmission systems, it has been 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 familiar with frequency modulation of signals in radio engineering, the use of this term here may seem unfounded, 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 true, and in fact it would be more correct to use the term “pulse phase modulation” (PPM), but in the field of fiber optic technology this is the terminology that has been established. You should always remember that the word “frequency” in the name of the modulation method means the repetition rate of the pulses, and not the frequency of the light waves carrying them.

In 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

While frequency modulation eliminates many of the driver brightness control problems associated with AM systems, it does have its own challenges. One of them is crosstalk, which is known in FM systems. They are observed, in particular, when transmitting several frequency modulated signals over one optical fiber, for example, when using a multiplexer. Crosstalk occurs at a transmitter or receiver as a result of instability in the tuning of important signal filtering circuits designed to separate carrier frequencies. If the filters are poorly tuned, the frequency-modulated carriers interact with each other and become distorted. Fiber optic engineers can design FM systems that minimize the potential for crosstalk, but any improvements to the design will increase the cost of the devices.

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

DIGITAL SYSTEMS

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

Rice. 2. Digital analog signal transmission system

In purely digital systems, the low-frequency input signal is immediately sent to an 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 a 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 switching on and off of one emitter, which occurs at a very high frequency.

At the receiving end, the signal is reversely converted. Individual streams corresponding to individual transmitted signals are separated from the combined digital stream. They are sent to digital-to-analog converters, after which they are output in the original format (Fig. 2).

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

SIGNAL TRANSMISSION ACCURACY

In analog systems with amplitude modulation, the signal loses quality in proportion to the path traveled along the optical fiber. This fact, combined with the fact that AM systems only work with multimode fibers, limits the use of such systems to relatively short transmission distances. FM systems work somewhat better: although the signal quality in them decreases, it remains approximately constant in not very long lines, sharply decreasing only when a certain maximum length is reached. Only fully digital systems guarantee the preservation of signal quality when transmitted 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 along the optical fiber. This fact, combined with the fact that AM systems only work 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 when developing systems for organizing several transmission channels over one optical fiber (multiplexers). For example, in an analog system designed to transmit four channels of video or audio, in order to meet the system bandwidth, the bandwidth allocated to individual channels must be limited. In digital systems, this compromise does not have to be made: one, four, or even ten signals can be transmitted along a single fiber without loss of quality.

HIGHER SIGNAL QUALITY

Rice. 3

Transmitting analog signals in digital form provides higher quality than pure analog. Signal distortion with this transmission method can only occur during analog-to-digital and reverse digital-to-analog conversion. While no conversion is perfect, today's technology is so advanced that even inexpensive ADCs and DACs provide much higher quality video and audio signals than can be achieved with analog AM and FM systems. This can be easily seen by comparing the signal-to-noise ratios and nonlinear distortions (differential phase and differential gain) of digital and analog systems designed to transmit the same signal format over the same type of optical fiber at the same wavelength.

Digital technologies give engineers 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 width of an analog-to-digital converter, you 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 - absence of distortion and 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 between the cost of the system and its technical characteristics, 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 challenging.

UNLIMITED TRANSMISSION DISTANCE

Another advantage of digital systems over analog predecessors is their ability to restore a signal without introducing additional distortion into it. This 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 that significantly exceed 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 place where the light becomes too weak, you install a linear amplifier, then it will amplify the signal to its original power, and it can be transmitted further over the same distance. It is important to note that the linear amplifier reconstructs the digital stream, which does not have any effect on the quality of the encoded analog video or audio signal, regardless of how many times the restoration is performed in the 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 significantly 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.

LOWER COST

Assessing 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 offering digital quality at the price of an analog system

The cost of digital components has dropped significantly in recent years, and equipment manufacturers have been able to develop and offer products that cost the same or even less than previous generation analog products. Of course, some firms want to convince the public that the superior quality of digital systems can only be obtained at a premium, but in reality they have simply chosen 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 one is cable costs. Digital systems allow more information to be transmitted over a single cable, thereby reducing the need for it. The advantage is especially noticeable where signals of different types need to be transmitted simultaneously, for example, video and audio or audio and data. Without much difficulty, engineers can design a digital system at an affordable cost in which a single fiber can carry different types of signals, such as two channels of video and four channels of audio. If analog technologies were used, it would most likely be necessary to make two separate systems, or at a minimum, use two separate cables to transmit 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 need to be transmitted over one optical fiber, digital systems are preferable because they operate more reliably and provide higher signal quality. For example, in a digital video multiplexer you can transmit ten channels with equally high quality, but in an analog system this is not possible at all.

You should also take into account the inevitable maintenance and repair costs over the years of operation of fiber optic systems. And here the advantage lies with digital systems. Firstly, they do not require initial setup after installation - the transmitter and receiver are simply connected via fiber optic cable, and the system is ready to use. Analog systems usually require adjustment to the parameters of a specific transmission line, taking into account its length and signal intensity. Additional adjustment time incurs additional costs.

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

Because there are fewer components that can fail over time, digital systems are much more stable and reliable. They do not require re-tuning, and troubleshooting will take much less time, since they do not have crosstalk, parameter drift and other disadvantages inherent in traditional analog systems.

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

CONCLUSIONS

Just as fiber optic technology has many advantages over traditional copper wires and coaxial cables, digital transmission takes fiber optic technology several notches up, giving users a whole new set of benefits. Digital systems have unique characteristics: accuracy of signal transmission over the entire length of the communication line, minimal introduced distortion (including the absence of cross-distortion and intermodulation), the ability to repeatedly restore a digital stream when transmitting it over a long line without compromising the quality of the analog signal encoded in it. This guarantees a level of analog signal fidelity unattainable with analog systems.

Component prices for digital and analog fiber optic systems are comparable, and when considering 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 and analog systems, because the choice is quite clear: digital systems are better in every way. It will be much more useful to limit yourself to only them and select 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 evaluate them:

• How easy is it to install the system?
  • If the transmitter and receiver are user configurable, how easy is it to do this and what are the challenges?
• Is the design of the devices compact, durable and reliable?
• Are the devices available in desktop cases or designed for rack mounting? Are there options in both types of housings?
  • Are the devices suitable for use with both single-mode and multimode fibers?
  • Does the manufacturer have sufficient experience and reputation in the market for the products he offers?
  • How does the price of the product compare with the price of traditional analog systems? (Digital devices in production are no more expensive than analog ones and their cost should not be higher).

Market analysis and comparison of the characteristics of similar products will allow you to ultimately select elements of digital fiber optic systems that will serve you faithfully for many years.