At present, laser technology opens up new possibilities for improving communication, location and radio control systems. These capabilities are associated with the huge gain of the transmitting optical antennas, which makes it possible to obtain a large signal-to-noise ratio in the receiver in a wide frequency band with low-power transmitters and with the ability to use very wide frequency bands in the transmission and reception of optical signals.

Laser information transmission systems have the following advantages over radio systems.

The possibility of transmitting information at a very high speed with a relatively low transmitter power and small overall dimensions of the antenna. Today, laser communication lines can provide information transmission at speeds up to 102 Gbit / s or more. With time multiplexing of channels, it is possible to obtain a resulting pulse repetition rate of more than 100 GHz in a multi-channel communication line, which exceeds the entire band of the radio frequency spectrum in use today.

Secrecy of information transmission and protection from organized interference (due to very narrow directional patterns of transmitting and receiving antennas, which are units of arc seconds).

However, there are also disadvantages, the main of which are: the dependence of work on weather conditions and the need to use light guides (quartz, glass fibers).

Real prospects for laser communication systems open up in the space communication systems "ISZ-AIS" due to the lack of an atmosphere. In such systems, broadband and narrowband information from low-orbit spacecraft will be transmitted via laser communication lines to stationary satellites and from them to ground stations. Of great importance will be satellite communication systems "Earth-to-Earth" through a satellite relay with laser communication lines.

Calculations show that in such a communication channel an information transfer rate of more than 1 Mbps from the Mars region is feasible. For comparison, it can be said that in the existing telemetric radio links for communication with spacecraft in the region of Mars, the information transfer rate does not exceed 10 bit/s.

Before discussing the issue of choosing a system for space communications, let's evaluate the advantages and disadvantages of the systems used:

with direct detection (Fig. 8, a);

with a heterodyne receiver (Fig. 8, b).

Rice. 8

Note that the noise immunity of both systems is approximately the same, and for the same frequency and the same level of development of laser technology, the first system has clear advantages, which are as follows:

Has a simpler receiving device;

Insensitive to Doppler frequency shift, which eliminates the need to search for a signal by frequency in the receiver (as is the case in the second system);

Insensitive to signal wavefront distortion (originated in turbulent atmosphere), so simple ground antennas with large apertures are possible. In a heterodyne receiver, atmospheric turbulence limits the size of the receiving antenna, and to increase it (antenna area), it is necessary to use an antenna array consisting of a plurality of antennas with a device for adding output signals;

It has a receiving antenna, which does not require high optical quality, which makes it possible to implement lighter and cheaper airborne antennas;

Allows you to implement more efficient methods of mutual guidance of transmitting and receiving antennas (compared to single-stage raster scanning in the second system).

The only advantage of systems with a heterodyne receiver is more effective background suppression in the receiver (compared to the former).

Let's analyze frequency suitability of lasers for space communications.

Due to the long communication range, transmitters with an average power from fractions to units of watts are required. Such lasers with acceptable efficiency are available in three main ranges:

10 μm - CO 2 gas laser with = 10.6 μm, in single-mode mode at P = 1 W = 10%, t slave = 10 thousand hours of continuous operation (suitable for on-board equipment and due to the high frequency stability it is quite can work in a system with a heterodyne receiver);

1 µm - solid-state laser on yttrium-aluminum garnet (YAG) activated by neodymium (J-Al / Nd) = 1.06 µm, = 1.5 2%, P max = n0.1 W (such a laser can successfully work on stationary satellites, since pumping is carried out by LED arrays or solar pumping devices.In the latter case, the solar energy collector through an optical filter focuses the pumping energy on the laser rod, providing its excitation.Potassium-rubidium pumping lamps provide t slave up to 5 thousand h at = 10% Resulting = 10 LEDs have a longer life, but their power is low and therefore they are suitable only for low-power transmitters up to 0.1 W);

0.5 µm - a promising Nd: YAG laser operating in the frequency doubling mode = 0.53 µm (bright green), with a converter efficiency close to unity, is promising here.

For low-speed laser communication lines, pulsed metal vapor gas lasers are promising. In the pulsed mode, the copper vapor laser has = 0.5106 and 0.5782 μm and = 5% (in the Q-switched mode) at an average power of one watt.

The possibilities of receiving equipment in these three ranges are as follows:

10.6 μm - there are photodetectors with high quantum efficiency (40-50%) when cooled to 77-100 K, but since photodetectors do not have internal amplification, they are not suitable for systems with direct detection;

1.06 µm - PMTs or avalanche photodiodes can be used for systems with direct detection. But the PMT quantum efficiency at this wavelength is only 0.008, so this range is significantly inferior to the first;

0.53 µm turns out to be a more acceptable range in the direct detection mode, because its performance due to the increase in the efficiency of the PMT is significantly higher.

So, there are two space communication systems:

With direct signal detection at a wavelength of 0.53 µm;

With a heterodyne receiver in the IR range at 10.6 µm.

Moreover, the system with = 10.6 μm has:

Lower level of quantum noise (because the spectral density of quantum noise is proportional to the value of hf, then by = = 10.6 μm it is 20 times less than by = 0.53 μm);

The efficiency of the laser transmitter in the range = 10.6 µm is higher than by = 0.53 µm.

The first two properties of the system allow the use of wider transmitter radiation patterns compared to the visible range system, which simplifies the guidance system.

The disadvantages here are the same as those of the heterodyne method.

The visible range system = 0.53 µm, having a higher level of quantum noise, lower transmitter efficiency, can have significantly reduced transmitting antenna RPs. So, if the apertures of the transmitting antennas are the same (at = 0.53 and 10.6 μm), then the transmitting antenna at = 0.53 μm will have a gain 400 times greater than at = 10.6 μm, which compensates with a margin the disadvantages mentioned above. Narrower beams of transmitting antennas complicate the system of mutual guidance of transmitting and receiving antennas, however, the use of effective multi-stage search methods can significantly reduce the time of getting into communication. Moreover, in a heterodyne receiver, only simple raster scanning is possible when searching for a signal, and the search time increases significantly due to the need to simultaneously search for a signal by frequency.

An important advantage of the visible range antenna is the ability to build a satellite communication system for multiple access. In this case, several (according to the number of communication lines) simple direct detection receivers are placed on board the satellite-RRS. For systems in the 10.6 µm range, this is practically impossible due to the complexity of heterodyne receivers with bulky photomixer cooling devices.

Thus, according to the existing technical level, systems with direct detection (= 0.53 µm) have significant advantages:

for deep space communication "KA-Earth" through the atmosphere;

for satellite system with multiple access.

For a satellite communication system, when the receiving (or transmitting) beam of an artificial satellite repeater is “switched” from one subscriber to another according to the program, a communication system with a high bandwidth of = 0.53 and 10.6 μm has comparable characteristics at information transfer rates up to several hundred megabits per second. Higher information transfer rates (more than 10 Gbit/s) in a system with = 10.6 μm are difficult to implement, while in the visible range they can simply be provided due to the time multiplexing of the channels.

An example of the implementation of a communication system of three synchronous satellites (Fig. 9):

transmitter wavelength = 0.53 µm (direct detection);

modulation is carried out by an electro-optical modulator, and the modulation signal is a microwave subcarrier with a center frequency m = 3 GHz and a sideband from min = 2.5 10 9 to max = 3.5 10 9 Hz (i.e. = 10 9 Hz );

Rice. 9

the electro-optical modulator (crystal) operates in the transverse mode with an electro-optical coefficient r 4·10 -11 at a microwave dielectric constant = 55 0 . The maximum depth of modulation - Г m = /3;

collimating and receiving lenses have dimensions of 10 cm;

the signal-to-noise ratio at the output of the amplifier following the PMT is 10

Let us determine the total power of the direct current source that the satellite must be supplied with in order to satisfy the requirements of the design task (we first determine the optical power level of the transmitted radiation, and then the modulation power required for operation).

Solution: A synchronous satellite has an orbital period of 24 hours. The distance from the Earth to the satellite is determined from the equality of centrifugal and gravitational forces

mV 2 /R ES \u003d mg (R Earth) 2 / (R ES) 2,

where V is the speed of the satellite; m is its mass; g - gravitational acceleration near the Earth's surface; R ES - distance from the center of the Earth to the satellite; R Earth - radius of the Earth.

Synchronous orbital rotation frequency (24 hours) allows you to determine

V/R ES = 2/(246060), then R ES = 42,222 km.

The distance between the satellites R = 73 12 km at a spacing of 120 O. If an optical signal with a power P T is transmitted in a solid angle T and the received aperture provides a solid angle R , then the received power

P R = P T (R / T).

The transmitted optical beam (Fig. 35) diffracts with the beam divergence angle, which is related to the minimum beam radius 0 by the expression

beam = / 0 .

Corresponding solid angle T = (beam) 2 .

If we take 0 equal to the radius d t of the transmitting lens, then

The solid angle of the receiver is equal to

R \u003d d 2 R / R 2,

R is the distance between transmitter and receiver.

From (42), (44), (45) we have

P T = P R R 22 / 22 T 2 R .

Let us write the signal-to-noise ratio at the PMT output operating in the quantum confinement mode (i.e., when the main source of noise is the shot noise of the signal itself):

s / w \u003d 2 (P R e / h) 2 G 2 / G 2 ei d \u003d P R / h,

where P R is the optical power, G is the current gain, i d is the dark current. At = 0.53 µm, = 0.2 - power conversion efficiency, = 10 9 Hz s/w = 10 3 we get P R 2·10 -6 . In this case, the required power in accordance with (46) at R = 7.5·10 4 m will be Р t 3 W.

From the middle of the 20th century, active research into microwaves began. American physicist Charles Townes decided to increase the intensity of the microwave beam. Having excited the ammonia molecules to a high energy level by heating or electrical stimulation, the scientist then passed a weak microwave beam through them. The result was a powerful amplifier of microwave radiation, which Townes in 1953 called a "maser". In 1958, Townes and Arthur Shavlov took the next step: Instead of microwaves, they tried to amplify visible light. It was on the basis of these experiments that Maiman created the first laser in 1960.

The creation of the laser made it possible to solve a wide range of problems that contributed to the significant development of science and technology. This made it possible at the end of the 20th and the beginning of the 21st centuries to obtain such developments as: fiber-optic communication lines, medical lasers, laser processing of materials (heat treatment, welding, cutting, engraving, etc.), laser guidance and target designation, laser printers, barcode readers and more. All these inventions greatly simplified, like the life of an ordinary person, and allowed the development of new technical solutions.

This article will provide answers to the following questions:

1) What is wireless laser communication? How is it implemented?

2) What are the conditions for the use of laser communication in space?

3) What equipment is needed for laser communication?

Definition of wireless laser communication, methods for its implementation.

Wireless laser communication is a type of optical communication that uses electromagnetic waves in the optical range (light) transmitted through the atmosphere or vacuum.

Laser communication between two objects is carried out only through a point-to-point connection. The technology is based on the transmission of data by modulated radiation in the infrared part of the spectrum through the atmosphere. The transmitter is a powerful semiconductor laser diode. Information enters the transceiver module, where it is encoded by various noise-immune codes, modulated by an optical laser emitter and focused by the transmitter's optical system into a narrow collimated laser beam and transmitted into the atmosphere.

On the receiving side, the optical system focuses the optical signal onto a highly sensitive photodiode (or avalanche photodiode), which converts the optical beam into an electrical signal. At the same time, the higher the frequency (up to 1.5 GHz), the greater the amount of information transmitted. The signal is then demodulated and converted into output interface signals.

The wavelength in most implemented systems varies between 700-950 nm or 1550 nm, depending on the laser diode used.

It follows from the above that the key instrumental elements for laser communication are a semiconductor laser diode and a highly sensitive photodiode (avalanche photodiode). Let's take a closer look at how they work.

Laser diode - a semiconductor laser built on the basis of a diode. His work is based on the occurrence of inverse populations in the region of the p-n junction during the injection of charge carriers. An example of a modern laser diode is provided in Figure 1.

Avalanche photodiodes are highly sensitive semiconductor devices that convert light into an electrical signal due to the photoelectric effect. They can be considered as photodetectors providing internal amplification through the effect of avalanche multiplication. From a functional point of view, they are solid-state analogues of photomultipliers. Avalanche photodiodes are more sensitive than other semiconductor photodetectors, which allows them to be used to detect low light powers (≲1 nW). An example of a modern avalanche photodiode is provided in Figure 2.

Conditions for the use of laser communication in space.

One of the promising areas for the development of space communication systems are systems based on the transmission of information over a laser channel, since these systems can provide greater bandwidth, with less power consumption, overall dimensions and weight of transceiver equipment than currently used radio communication systems.

Potentially, space laser communication systems can provide an extremely high data flow rate - from 10-100 Mbps to 1-10 Gbps and higher.

However, there are a number of technical problems that need to be solved in order to implement laser communication channels between a spacecraft (SC) and the Earth:

• high accuracy of guidance and mutual tracking is required at distances from half a thousand to tens of thousands of kilometers and when carriers move at cosmic velocities.
• The principles of receiving and transmitting information over a laser channel become much more complicated.
• Optoelectronic equipment is becoming more complex: precise optics, precision mechanics, semiconductor and fiber lasers, and highly sensitive receivers.

Experiments on the implementation of space laser communication

Both Russia and the United States of America are experimenting with the implementation of laser communication systems for transmitting large amounts of information.

RF Laser Communication System (SLS)

In 2013, the first Russian experiment was conducted to transmit information using laser systems from the Earth to the Russian Segment of the International Space Station (ISS RS) and back.

The space experiment "SLS" was carried out with the aim of testing and demonstrating Russian technology and equipment for receiving and transmitting information over a space laser communication line.

The objectives of the experiment are:

• development in space flight conditions on the ISS RS of the main technological and design solutions incorporated into the standard equipment of the inter-satellite laser information transmission system;
• development of technology for receiving and transmitting information using a laser communication line;
• study of the possibility and operating conditions of laser communication lines "spacecraft board - ground point" in different atmospheric conditions.

The experiment is planned to be carried out in two stages.

At the first stage, the system of receiving and transmitting information flows over the lines "ISS RS-Earth" (3, 125, 622 Mbit/s) and "Earth-ISS RS board" (3 Mbit/s) is being worked out.

At the second stage, it is planned to develop a high-precision guidance system and a system for transmitting information along the line "ISS RS - relay satellite".

The laser communication system at the first stage of the SLS experiment includes two main subsystems:

• onboard laser communication terminal (BTLS) installed on the Russian segment of the International Space Station (Figure 3);
• ground laser terminal (LLT) installed at the Arkhyz optical observation station in the North Caucasus (Figure 4).

Objects of study at stage 1 of CE:

• equipment of the onboard laser communication terminal (BTLN);
• ground laser communication terminal (LLT) equipment;
• atmospheric channel of propagation of radiation.

Figure 4. Ground-based laser terminal: astropavilion with an optical-mechanical unit and an adjusting telescope

Laser communication system (SLS) — stage 2.

The second stage of the experiment will be carried out after the successful completion of the first stage and the readiness of a specialized spacecraft of the Luch type for GSO with an onboard terminal of an inter-satellite laser data transmission system. Unfortunately, information about whether the second stage was held or not was not found in open sources. Perhaps the results of the experiment were classified, or the second stage was never carried out. The information transfer scheme is shown in Figure 5.

Project OPALS USA

Almost simultaneously, the American space agency NASA begins the deployment of the OPALS (Optical Payload for Lasercomm Science) laser system.

“OPALS is the first experimental site for the development of laser space communications technologies, and the International Space Station will act as a test site for the OPALS system,” says Michael Kokorowski, OPALS Project Manager and NASA Jet Propulsion Laboratory ( Jet Propulsion Laboratory, JPL) - "Future laser communication systems, which will be developed based on OPALS technologies, will be able to provide the exchange of large amounts of information, which will eliminate the bottleneck that in some cases holds back scientific research and commercial enterprises."

The OPALS system is a sealed container that contains electronics connected via an optical cable to a laser transceiver (Figure 6). This device includes a laser collimator and a tracking camera mounted on a moving platform. The OPALS facility will be sent to the ISS aboard the Dragon spacecraft, which will launch into space this December. Upon delivery, the container and transmitter will be installed outside the station and a 90-day field test program for the system will begin.

How OPALS works:

From the Earth, specialists from the Optical Communications Telescope Laboratory will send a beam of laser light towards the space station, which will act as a beacon. The equipment of the OPALS system, having caught this signal, with the help of special drives, will aim its transmitter at the ground-based telescope, which will serve as a receiver, and transmit a response signal. If there is no interference in the path of laser light beams, the communication channel will be established and the transmission of video and telemetric information will begin over it, which for the first time will last about 100 seconds.

European data transmission system (European Data Relay System abbr. EDRS).

The European Data Relay System (EDRS) is a project planned by the European Space Agency to create a constellation of advanced geostationary satellites that will transmit information between satellites, spacecraft, unmanned aerial vehicles (UAVs) and ground stations, providing faster than traditional transmission methods data speed, even in the face of natural and man-made disasters.

EDRS will use the new Laser Communication Terminal (LCT) laser communication technology. The laser terminal will allow information to be transmitted at a speed of 1.8 Gbps. LCT technology will enable EDRS satellites to transmit and receive about 50 terabytes of data per day in near real time.

The first EDRS communication satellite is to be launched into geostationary orbit in early 2016 from the Baikonur Cosmodrome on a Russian Proton launch vehicle. Once in geosynchronous orbit over Europe, the satellite will lay laser links between the four Sentinel-1 and Sentinel-2 satellites of the Copernicus Earth Observation Space Program, unmanned aerial vehicles, and ground stations in Europe. , Africa, Latin America, the Middle East and the northeast coast of the USA.

A second, similar satellite will be launched in 2017, with a third satellite scheduled for launch in 2020. In sum, these three satellites will be able to cover the entire planet with laser communications.

Prospects for the development of laser communication in space.

Advantages of laser communication compared to radio communication:

• transmission of information over long distances
• high transfer rate
• compactness and lightness of equipment for data transmission
• energy efficiency

Disadvantages of laser communication:

• the need for precise targeting of receiving and transmitting devices
• atmospheric problems (cloudiness, dust, etc.)

Laser communication makes it possible to transmit data over much greater distances relative to radio communication, the transmission rate is also higher due to the high concentration of energy and a much higher carrier frequency (by orders of magnitude). Energy efficiency, low weight and compactness are also many times or orders of magnitude better. Difficulties in the form of the need for precise targeting of receiving and transmitting devices can be solved by modern technical means. In addition, receiving ground devices can be located in regions of the Earth where the number of cloudy days is minimal.

In addition to the problems presented above, there is another problem - this is the divergence and attenuation of the laser beam when passing through the atmosphere. The problem is especially aggravated when the beam passes through layers with different densities. When passing through interfaces, a beam of light, including a laser beam, experiences particularly strong refraction, scattering, and attenuation. In this case, we can observe a kind of light spot, which is obtained precisely when passing through such an interface between the media. There are several such boundaries in the Earth's atmosphere - at an altitude of about 2 km (active weather atmospheric layer), at an altitude of about 10 km, and at an altitude of about 80-100 km, i.e. already at the edge of space. Layer heights are given for middle latitudes for the summer period. For other latitudes and other times of the year, the heights and the very number of media interfaces can differ greatly from those described.

Thus, when entering the Earth's atmosphere, the laser beam, which had previously calmly traveled millions of kilometers without any loss (perhaps a slight defocusing), loses the lion's share of its power over some unfortunate tens of kilometers. However, this, at first glance, bad fact, we can turn to our advantage. Since this fact allows us to do without any serious pointing of the beam at the receiver. For as such a receiver, or rather the primary receiver, we can just use these very boundaries between layers and environments. We can point the telescope at the resulting spot of light and read information from it. Of course, this will noticeably add the amount of interference and reduce the data transfer rate. And make it generally impossible in the daytime. But this will make it possible to reduce the cost of the spacecraft by saving on the guidance system. This is especially true for satellites in non-stationary orbits, as well as for spacecraft for deep space research.

At the moment, if we consider the connection "Earth - SC and SC - Earth", the optimal solution is the synergy of laser and radio communications. Quite convenient and promising is the transmission of data from the spacecraft to the Earth using laser communication, and from the Earth to the spacecraft by radio communication. This is due to the fact that the laser receiving module is a rather bulky system (most often a telescope), which captures laser radiation and converts it into electrical signals, which are then amplified and converted into useful information using known methods. It is not easy to install such a system on a spacecraft, since most often there are requirements for compactness and low weight. At the same time, the laser signal transmitter has small dimensions and weight compared to antennas for transmitting a radio signal.

Optical fibers and laser communication

Since antiquity, light has been used to convey messages. China, Egypt, and Greece used smoke during the day and fire at night to transmit signals. Among the first historical evidence of optical communication, we can recall the siege of Troy. In his tragedy "Agamemnon", Aeschylus gives a detailed description of the chain of signal lights on the tops of the mountains of Ida, Anthos. Masisto, Egiplanto and Aracnea, as well as on the cliffs of Lemno and Kifara, to transmit to Argo the news of the capture of Troy by the Achaeans.

In later but ancient times, the Roman Emperor Tiberius, while in Capri, used light signals to communicate with the coast.

On Capri, you can still see the ruins of the ancient "Faro" (light) near the villa of Emperor Tiberius on Mount Tiberio.

In North America, one of the first optical communication systems was installed about 300 years ago in the colony of New France (now the province of Quebec in Canada). The regional government, fearful of the possibility of an attack by the English fleet, set up a number of beacon positions in many villages along the St. Lawrence River. There were at least 13 points in this circuit, which began at Il Verte, about 200 km downstream from Quebec. Since the beginning of the 1700s. in each of these villages, every night of the navigation period, there was a sentry whose task was to observe the signal sent from the village downstream and transmit it further. With such a system, the message of the British attack in 1759 reached Quebec before it was too late.

In 1790 a French engineer, Claude Chappe, invented semaphores (optical telegraph) placed on towers mounted within sight of one another, allowing messages to be sent from one tower to another. In 1880, Alexander Graham Bell (1847-1922) received a patent for a "photophone" device that used reflected sunlight to transmit sound to a receiver. The reflected light was modulated in intensity by vibrating a reflective membrane placed at the end of the tube into which Bell spoke. The light traveled a distance of about 200 m and hit a selenium cell (photodetector) connected to the telephone. Although Bell regarded the photophone as his most important invention, its use was limited by the weather. However, this circumstance did not prevent Bell from writing to his father:

“I heard intelligible speech produced by sunlight!... One can imagine that this invention has a future!... We will be able to talk with the help of light at any distance within sight without any wires... In a war such communications cannot be interrupted or intercepted.”

The invention of the laser stimulated an increased interest in optical communications. However, it was soon demonstrated that the Earth's atmosphere distorts the propagation of laser light in an undesirable way. Various systems were considered, such as gas lens tubes and dielectric waveguides, but they were all abandoned in the late 1960s when low-loss optical fibers were developed.

The understanding that thin glass fibers could conduct light through total internal reflection was an old idea known since the 19th century. thanks to the English physicist John Tyndall (1820-1893) and used in tools and for lighting. However, in the 1960s. even the best glasses had a large attenuation of the light transmitted through the fiber, which severely limited the propagation length. At that time, the typical attenuation value was one decibel per meter, meaning that after a 1 m travel, the transmitted power is reduced to 80%. Therefore, only propagation along a fiber several tens of meters long was possible, and the only application was medicine, such as endoscopes. In 1966, Charles Kao and George Hockham from the Standard Telecommunications Laboratory (UK) published a fundamental work in which they showed that if impurities are carefully eliminated in fused silica, and the fiber is surrounded by a cladding with a lower refractive index, then attenuation can be reduced to -20 dB/km. This means that when passing a length of 1 km, the beam power is attenuated to one hundredth of the input power. Although this is a very small value, it is acceptable for a number of applications.

As is often the case in such situations, intense efforts have begun in the UK, Japan and the US to produce fibers with improved performance. The first success was achieved in 1970 by E. P. Capron, Donald Keck and Robert Mayer of the Corning Glass Company. They made fibers that had a loss of 20 dB/km at a wavelength of 6328 A° (the wavelength of a He-Ne laser). In the same year, I. Hayashi and co-workers reported on a laser diode operating at room temperature.

In 1971, I. Jacobs was appointed director of the Digital Communications Laboratory at AT&T Bell Laboratories (Holmdel, New Jersey, USA) and was tasked with developing systems with high information transfer rates. Its chiefs W. Danielson and R. Kompfner transferred part of the staff to another laboratory, led by S. Miller, in order to "keep an eye" on what was happening in the field of optical fibers. Three years later, Danielson and Kompfner commissioned Jacobs to form a research team to study the feasibility of fiber-based communication. It was clear that the most economical, initial application of systems using light was the communication of telephone exchanges in large cities. Then cables were used for this, and information was transmitted in digital form, by encoding it with a series of pulses. Fibers, with their ability to transmit vast amounts of information, seemed to be the ideal replacement for electrical cables. Offices and telephone exchanges in large cities are located at distances of several kilometers from each other, and even at that time they could be connected without problems, even using fibers with relatively high losses.

So, a preliminary experiment was done in mid-1976 in Atlanta with optical fiber cables placed in tubes of conventional cables. The initial success of these attempts resulted in a system that linked two telephone exchanges in Chicago. Based on these first results, in the fall of 1977, Bell Labs decided to develop an optical system for general use. In 1983, a connection was established between Washington and Boston, although this was associated with many difficulties. This communication system operated at a transmission rate of 90 Mbps. It used multimode fiber at a wavelength of 825 nm.

Meanwhile, NTTC (Japanese telegraph and telephone company) was able to pull fibers with a loss of only 0.5 dB / km at wavelengths of 1.3 and 1.5 microns, and the Lincoln Laboratory at MIT demonstrated the operation of an InGaAsP laser diode capable of continuous operation in the range between 1.0 and 1.7 µm at room temperature. The use of low-loss fibers at 1.3 µm has led to more advanced systems. Systems were built with a throughput of 400 Mbps in Japan and 560 Mbps in Europe. The European system could transmit 8000 telephone channels simultaneously. More than 3.5 million kilometers of fiber have been produced in the US. The only part that still uses copper wire is the connection between the house and the telephone exchange. This "last mile", as it has come to be called, is also becoming the target of fiber communications.

The first transatlantic telegraph cable was put into operation in 1858. Almost a hundred years later, in 1956, the first telephone cable was laid, called TAT-1. In 1988, the first generation of transatlantic cables on optical fibers began to operate (they began to be called TAT-8). They operate at a wavelength of 1.3 microns and link Europe, North America and the Eastern Pacific. In 1991, the installation of the second generation of fiber-optic communication, TAT-9, which operates on 1.3 microns and links the US and Canada with the UK, France and Spain, began. Another line operates between the US and Canada and Japan.

There are a number of other fiber optic lines in the world. For example, the optical submarine link between England and Japan covers 27,300 km in the Atlantic Ocean, Mediterranean Sea, Red Sea, Indian Ocean, Pacific Ocean, and has 120,000 intermediate amplifiers per fiber pair. In comparison, the first transatlantic telephone cable in 1956 used 36 converters, while the first optical cable laid across the Atlantic Ocean used 80,000.

Today, after 30 years of research, optical fibers have reached their physical limits. Silica fibers can transmit infrared pulses at a wavelength of 1.5 microns with a minimum loss of 5% per kilometer. These losses cannot be reduced due to the physical laws of light propagation (Maxwell's laws) and the fundamental nature of glass.

However, there is one achievement that can radically improve the situation. This is the ability to directly amplify optical signals in the fiber, i.e. without having to first extract them from the fibers. By adding suitable elemental impurities such as erbium to the fiber material and excitating them with suitable pump light through the fiber itself, a population inversion between two levels of erbium can be obtained with a transition that is exactly 1.5 µm. As a result, it is possible to obtain an amplification of the light pulse at this wavelength as it propagates through the fiber. A piece of such an active fiber is placed between the two ends of the fibers through which the signal propagates. With the help of an optical coupler, the pump radiation is also directed to this piece. At the output, the remainder of the pump radiation escapes, and the amplified signal continues to propagate through the fiber. Using this approach, intermediate electronic amplifiers can be eliminated. In older electronic amplifier systems, light exited the fiber, was recorded by a photoelectric receiver, the signal was amplified and converted into light that continued to propagate in the next section of the fiber.