The era of laser communication is approaching: details from the creator of the technology. Space laser communication system

Active research into microwaves began in the mid-20th century. American physicist Charles Townes decided to increase the intensity of the microwave beam. Having excited the ammonia molecules to high energy levels through heat or electrical stimulation, the scientist then passed a weak microwave beam through them. The result was powerful amplifier microwave radiation, which Townes called a “maser” in 1953. In 1958, Townes and Arthur Schawlow took the next step: instead of using microwaves, they tried to amplify visible light. Based on these experiments, 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 significant developments in science and technology. Which made it possible at the end of the 20th and 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 much more. All these inventions have greatly simplified the life of an ordinary person and allowed the development of new technical solutions.

This article will answer the following questions:

1) What is wireless laser communication? How was it accomplished?

2) What are the conditions for using laser communications in space?

3) What equipment is needed to implement laser communication?

Definition of wireless laser communication, methods of 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 data transmission using modulated radiation in the infrared part of the spectrum through the atmosphere. The transmitter is a powerful semiconductor laser diode. The information enters the transceiver module, in which it is encoded with various noise-resistant codes, modulated by an optical laser emitter and focused by the optical system of the transmitter into a narrow collimated laser beam and transmitted into the atmosphere.

On the receiving side, the optical system focuses optical signal to a highly sensitive photodiode (or avalanche photodiode), which converts the optical beam into an electrical signal. Moreover, the higher the frequency (up to 1.5 GHz), the greater the volume of transmitted information. 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.

From the above it follows that the key instrument elements for laser communication are a semiconductor laser diode and a highly sensitive photodiode (avalanche photodiode). Let's look at the principle of their operation in a little more detail.

Laser diode is a semiconductor laser built on the basis of a diode. His work is based on the occurrence of population inversion in the region p-n junction upon 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 that provide internal amplification through the avalanche multiplication effect. From a functional point of view, they are solid-state analogs of photomultipliers. Avalanche photodiodes have greater sensitivity compared to other semiconductor photodetectors, which allows them to be used for recording low light powers (≲ 1 nW). An example of a modern avalanche photodiode is provided in Figure 2.

Conditions for using laser communications in space.

One of the promising directions for the development of space communication systems are systems based on information transmission via a laser channel, since these systems can provide greater throughput, with less energy consumption, overall dimensions and the mass of the transceiver equipment than the currently used radio communication systems.

Potentially, space laser communication systems can provide extremely high speed information flow - from 10-100 Mbit/s to 1-10 Gbit/s and higher.

However, there are a number of technical problems that need to be solved in order to implement laser communication channels between the 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 speeds.
• The principles of receiving and transmitting information via a laser channel are becoming significantly more complicated.
• Optical-electronic equipment is becoming more complex: precision optics, precision mechanics, semiconductor and fiber lasers, highly sensitive receivers.

Experiments on the implementation of space laser communications

Experiments on the implementation of laser communication systems for transmitting large amounts of information are being conducted by both Russia and the United States of America.

RF Laser Communication System (SLS)

In 2013, the first Russian experiment on transmitting information using laser systems from Earth to the Russian segment of the International Space Station (RS ISS) and back.

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

The objectives of the experiment are:

• testing, under conditions of space flight on the ISS RS, the main technological and design solutions incorporated into the standard equipment of the intersatellite 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 “from the spacecraft to the ground point” under different atmospheric conditions.

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

At the first stage, the reception-transmission system is worked out information flows via the lines “on board the RS ISS–Earth” (3, 125, 622 Mbit/s) and “Earth–on board the RS ISS” (3 Mbit/s).

At the second stage, it is planned to develop a high-precision guidance system and an information transmission system along the line “on board the ISS RS – relay satellite.”

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

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

Objects of study at stage 1 of FE:

• on-board laser communication terminal equipment (BTLN);
• ground laser communication terminal (GLT) equipment;
• atmospheric radiation propagation channel.

Figure 4. Ground laser terminal: astro pavilion with optical-mechanical unit and alignment telescope

Laser communication system (LCS) - 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 on the GEO with an on-board terminal of the inter-satellite laser information transmission system. Unfortunately, information about whether the second stage was carried out or not could not be 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 deploying the OPALS (Optical Payload for Lasercomm Science) laser system.

“OPALS represents the first experimental site for the development of laser space communications technologies, and the International Space Station will serve as a test site for OPALS,” said Michael Kokorowski, OPALS project manager and member of NASA's Jet Propulsion Laboratory (JPL). Jet Propulsion Laboratory, JPL, "Future laser communications systems that will be developed based on OPALS technologies will be able to exchange large volumes of information, eliminating the bottleneck that has in some cases held back research and commercial enterprise."

The OPALS system is a sealed container containing electronics connected via an optical cable to a laser transmitting and receiving device (Figure 6). This device includes a laser collimator and a tracking camera mounted on a moving platform. The OPALS installation will be sent to the ISS aboard the Dragon spacecraft, which will launch into space in December this year. Once delivered, the container and transmitter will be installed outside the station and a 90-day field testing program for the system will begin.

Operating principle of OPALS:

From 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, using special drives, will aim its transmitter at a ground-based telescope, which will serve as a receiver, and transmit a response signal. If there is no interference on the path of propagation of laser light beams, a communication channel will be established and the transmission of video and telemetric information will begin, which will last about 100 seconds for the first time.

European Data Relay System abbreviated EDRS.

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

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

The first EDRS communications satellite is set to go to geostationary orbit in early 2016 from the Baikonur Cosmodrome on a Russian Proton launch vehicle. Once in geosynchronous orbit over Europe, the satellite will carry laser communications links between the four Copernicus Earth observation program Sentinel-1 and Sentinel-2 satellites, unmanned aerial vehicles, and ground stations in Europe , Africa, Latin America, the Middle East and the northeast coast of the United States.

A second, similar satellite will be launched in 2017, and the launch of a third satellite is planned for 2020. Together, these three satellites will be able to cover the entire planet with laser communications.

Prospects for the development of laser communications in space.

Advantages of laser communication compared to radio communication:

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

Disadvantages of laser communication:

• the need for precise pointing 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 speed due to the high concentration of energy and much more high frequency carrier (by orders of magnitude) is also higher. Energy efficiency, low weight and compactness are also several times or orders of magnitude better. Difficulties in the form of the need for precise guidance of receiving and transmitting devices can be solved with modern technical means. In addition, ground-based receiving devices can be located in areas of the Earth where the number of cloudy days is minimal.

In addition to the problems presented above, there is another problem - 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 the interface between media, a light beam, including a laser beam, experiences particularly strong refractions, scattering and attenuation. In this case, we can observe a kind of light spot resulting precisely from passing 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 approximately 10 km, and at an altitude of approximately 80-100 km, i.e. already at the boundary of space. The heights of the layers are given for mid-latitudes in the summer. For other latitudes and other seasons, the heights and the number of interfaces between the media may differ greatly from those described.

Thus, when entering the Earth’s atmosphere, a laser beam, which had previously calmly traveled millions of kilometers without any losses (except perhaps a slight defocusing), loses the lion’s share of its power within some unfortunate tens of kilometers. However, we can turn this fact, bad at first glance, to our advantage. Since this fact allows us to do without any serious aiming of the beam at the receiver. Because as such a receiver, or rather a primary receiver, we can use precisely these very boundaries between layers and media. We can point the telescope at the resulting spot of light and read information from it. Of course, this will significantly increase the amount of interference and reduce the data transfer rate. And it will make it completely impossible during 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 Earth-spacecraft and spacecraft-Earth communications, the optimal solution is the synergy of laser and radio communications. It is quite convenient and promising to transmit data from the spacecraft to the Earth using laser communications, and from the Earth to the spacecraft using radio communications. 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 using known methods and converted into useful information. Such a system is not easy to install on a spacecraft, since most often the requirements are compactness and low weight. At the same time, the laser signal transmitter is small in size and weight compared to antennas for transmitting radio signals.

The advantages of a laser channel over a radio channel are that, firstly, it does not create radio interference; secondly, it is more confidential; thirdly, it can be used under conditions of exposure to high levels of electromagnetic radiation.

The schematic diagram of the transmitter is shown in Fig. 1. The transmitter consists of a command encoder made on an ATtiny2313 microcontroller (DD1), an output block on BC847V transistors (VT1, VT2) and an RS-232 interface, which, in turn, consists of a DB9-F connector (for cable) (XP1) and level converter - on MAX3232 (DD3).

The microcontroller reset circuit consists of elements DD2 (CD4011B), R2, C7. The output unit is an electronic switch made on transistor VT1, in the collector circuit of which a laser pointer is connected through a current limiter on transistor VT2. The transmitter is powered by a constant stabilized voltage of 9 - 12 V. Microcircuits DD1, DD2, DD3 are powered by a voltage of 5V, which is determined by the 78L05 stabilizer (DA1).

The DD1 controller is programmed in the BASCOM environment, which allows it to send commands from personal computer(PC) via RS-232 interface, from the Bascom terminal, using the “echo” function.

The microcontroller has clock frequency 4 MHz from internal oscillator. Packs of pulses with a frequency of about 1.3 KHz from the OS0A (PB2) output are supplied to the output block. The number of pulses in a packet is determined by the number of the command received from the PC.
To enter a command, you need to press any key on the PC keyboard, then when the words “Write command” and “Enter No. 1...8” appear, enter a number from 1 to 8 and press the “Enter” key.

The program for the transmitter microcontroller “TXlaser” consists of a main loop (DO...LOOP) and two interrupt processing subroutines: for reception (Urxc) and for timer 0 overflow (Timer0).

To obtain an output frequency of 1.3 KHz, the timer is configured with a frequency division factor (Prescale) = 1024. In addition, counting starts from the lower value Z = 253 (at a high level on PB2) and reaches 255. A timer overflow interrupt occurs when the processing of which switches the output of PB2, and the timer is again set to the value Z = 253. Thus, a signal with a frequency of 1.3 KHz appears at the output of PB2 (see Fig. 2). In the same subroutine, the number of pulses on PB2 is compared with the specified one, and if they are equal, the timer stops.

In the reception interrupt processing subroutine, the number of pulses that need to be transmitted is set (1 – 8). If this number is greater than 8, the message “ERROR” is displayed in the terminal.

During the operation of the subroutine, the PD6 pin is present low level(HL1 LED is off) and the timer is stopped.
In the main loop at pin PD6 - high level, and the HL1 LED is on.
Text of the "TXlaser" program:

$regfile = "attiny2313a.dat"
$crystal = 1000000
$hwstack = 40
$swstack = 16
$framesize = 32

Config Pind.0 = Input "UART - RxD
Config Portd.1 = Output "UART - TxD
Config Portd.6 = Output "LED HL1
Config Portb.2 = Output "output OC0A

"timer configuration 0-division factor=1024:
Config Timer0 = Timer, Prescale = 1024
Stop Timer0 "stop the timer

Dim N As Byte "variable definition"
Dim N0 As Byte

Const Z = 253 "lower limit of the timer count for output frequency = 1.3 KHz
Timer0 = Z

On Urxc Rxd "receive interrupt processing subroutine
On Timer0 Pulse "overflow interrupt routine"

Enable Urxc
Enable Timer0

Do "main loop
Set Portd.6 "turn on the HL1 LED
Loop

Rxd: "receive interrupt processing subroutine
Stop Timer0
M1:
Print "Write commad"
Input "Enter No. 1...8:" , N0 "command input
If N0 > 8 Then "limit the number of commands
Print "Error"
Goto M1
End If
N0 = N0 * 2
N0 = N0 - 1 "set value of the number of pulses in a packet
Toggle Portb.2
Start Timer0 "start the timer
Return

Pulse: "overflow interrupt processing routine"
Stop Timer0
Toggle Portb.2
Reset Portd.6 "turn off the LED
Timer0 = Z
N = N + 1 "increment in the number of pulses
If N = N0 Then "if the number of pulses = specified
N=0
N0 = 0
Waitms 500 "delay 0.5s
Else
Start Timer0 "otherwise, continue counting
End If
Return
End "end program

The transmitter is made on a printed circuit board measuring 46x62 mm (see Fig. 3). All elements, except the microcontroller, are SMD type. The ATtiny2313 microcontroller is used in a DIP package. It is recommended to place it in the panel for DIP chips TRS (SCS) - 20 in order to be able to “painlessly” reprogram it.

The transmitter circuit board TXD.PCB is located in the "FILE PCAD" folder.
The schematic diagram of the laser channel receiver is shown in Fig. 4. At the input of the first amplifier DA3.1 (LM358N), a low-pass filter formed by elements CE3, R8, R9 and having a cutoff frequency of 1 KHz attenuates background noise of 50 -100 KHz from lighting fixtures. Amplifiers DA3.2 and DA4.2 amplify and increase the duration of received pulses of the useful signal. The comparator on DA4.1 generates an output signal (one), which is supplied through the inverters of the CD4011D (DD2) chip - DD2.1, DD2. The signal synchronously arrives at the contacts of the microcontroller ATtiny2313 (DD1) – T0 (PB4) and PB3. Thus, Timer0, operating in the mode of counting external pulses, and Timer1, measuring the time of this counting, are launched synchronously. Controller DD1, which acts as a decoder, displays received commands 1…8 by setting log.1 on the PORTB pins, respectively РВ0…РВ7, while the arrival of a subsequent command resets the previous one. When the command “8” arrives at PB7, log.1 appears, which, with the help of electronic key on transistor VT1, turns on relay K1.

The receiver is powered constant voltage 9 -12V. The analog and digital parts are powered by 5V voltages, which are determined by stabilizers of type 78L05 DA5 and DA2.

In the RXlaser program, Timer0 is configured as a counter of external pulses, and Timer1 as a timer that counts the period of passage of the maximum possible number of pulses (command 8).

In the main cycle (DO...LOOP), Timer1 is turned on when the first command pulse is received (K=0), the condition for enabling the inclusion of timer Z=1 is reset.
In the interrupt processing subroutine, when the Timer1 count coincides with the value of the maximum possible count, the command number is read and set in PORTB. The condition for enabling inclusion of Timer1 is also set - Z=0.
Text of the RXlaser program:

$regfile = "attiny2313a.dat"
$crystal = 4000000
$hwstack = 40
$swstack = 16
$framesize = 32

Ddrb = 255 "PORTB - all outputs
Portb = 0
Ddrd = 0 "PORTD-input
Portd = 255" pull-up PORTD
Config Timer0 = Counter , Prescale = 1 , Edge = Falling "as pulse counter
Config Timer1 = Timer, Prescale = 1024, Clear Timer = 1" as timer
Stop Timer1
Timer1 = 0
Counter0 = 0

"variable definition:
Dim X As Byte
Dim Comm As Byte
Dim Z As Bit
Dim K As Bit

X =80
Compare1a = X "number of pulses in the match register
Z=0

On Compare1a Pulse "interrupt routine by coincidence

Enable Interrupts
Enable Compare1a

Do "main loop
If Z = 0 Then "first condition for turning on the timer
K = Portd.3
If K = 0 Then "second condition for turning on the timer
Start Timer1
Z=1
End If
End If
Loop

Pulse: "subroutine interrupt processing by coincidence
Stop Timer1
Comm = Counter0 "reading from the external pulse counter
Comm = Comm - 1 "definition of the bit number in the port
Portb = 0 "port zeroing
Set Portb.comm "set the bit corresponding to the command number
Z=0
Counter0 = 0
Timer1 = 0
Return
End "end program

The programs "TXlaser" and "RXlaser" are located in the Lazer_prog folder.

The receiver is located on a board measuring 46x62 mm (see Fig. 5). All components - SMD type, with the exception of the microcontroller, which must be placed in the panel for DIP chips of type TRS(SCS) - 20.

Setting up the receiver comes down to setting the end-to-end transmission coefficient and the response threshold of the comparator. To solve the first problem, you need to connect an oscilloscope to pin 7 of DA4.2 and by selecting the value of R18, set such an end-to-end transmission coefficient at which the maximum amplitude of noise emissions observed on the screen will not exceed 100 mV. Then the oscilloscope switches to pin 1 of DA4.1 and selecting a resistor (R21) sets the zero level of the comparator. By turning on the transmitter and directing the laser beam to the photodiode, you need to make sure that rectangular pulses appear at the output of the comparator.
The receiver circuit board RXD.PCB is also located in the FILE PCAD folder.

It is possible to increase the noise immunity of the laser channel by modulating the signal with a subcarrier frequency of 30 – 36 KHz. Modulation of pulse trains occurs in the transmitter, while the receiver contains a bandpass filter and an amplitude detector.

The diagram of such a transmitter (transmitter 2) is shown in Fig. 6. Unlike transmitter 1 discussed above, transmitter 2 has a subcarrier generator tuned to a frequency of 30 KHz and assembled on slots DD2.1, DD2.4.. The generator provides modulation of bursts of positive pulses.

The laser channel receiver with a subcarrier frequency (receiver 2) is assembled on the domestic K1056UP1 (DA1) microcircuit. The receiver circuit is shown in Fig. 7. To isolate command pulses, an amplitude detector with a low-pass filter and a pulse normalizer, assembled on logic elements DD3.1, DD3.2, a diode assembly DA3 and C9, R24, are connected to the output of the DA1 10 microcircuit. Otherwise, the circuit of receiver 2 coincides with the circuit of receiver 1.

E. N. Chepusov, S. G. Sharonin

Today it is impossible to imagine our life without computers and networks based on them. Humanity stands on the threshold of a new world in which a single information space. In this world, communications will no longer be hampered by physical boundaries, time or distance.

Nowadays there are a huge number of networks all over the world that perform various functions and solving many different problems. Sooner or later, there always comes a time when the network capacity is exhausted and new communication lines need to be laid. This is relatively easy to do inside a building, but difficulties begin when connecting two adjacent buildings. Special permits, approvals, licenses to carry out work are required, as well as the fulfillment of a number of complex technical requirements and the satisfaction of considerable financial requests from organizations managing land or sewerage. As a rule, it immediately becomes clear that the shortest path between two buildings is not a straight line. And it is not at all necessary that the length of this path will be comparable to the distance between these buildings.

Of course, everyone knows a wireless solution based on various radio equipment (radio modems, small-channel radio relay lines, microwave digital transmitters). But the number of difficulties does not decrease. The airwaves are oversaturated and obtaining permission to use radio equipment is very difficult, and sometimes even impossible. And the throughput of this equipment significantly depends on its cost.

We offer you to take advantage of the new economical form wireless communication, which arose quite recently, is laser communication. This technology received the greatest development in the USA, where it was developed. Laser communications provides a cost-effective solution to the problem of reliable, high-speed short-range communications (1.2 km) that can arise when connecting telecommunications systems from different buildings. Its use will allow integration local networks with global ones, integration of local networks remote from each other, and also to provide for the needs of digital telephony. Laser communication supports all interfaces necessary for these purposes - from RS-232 to ATM.

How is laser communication accomplished?

Laser communication vs. GSM communications allows for point-to-point connections with information transfer rates of up to 155 Mbit/s. In computer and telephone networks, laser communication ensures the exchange of information in full duplex mode. For applications that do not require high transmission rates (for example, video and control signals in process and closed-circuit television systems), a special, cost-effective half-duplex solution is available. When you need to combine not only computer, but also telephone networks, models of laser devices with a built-in multiplexer can be used for simultaneous transmission of LAN traffic and digital group telephony streams (E1/ICM30).

Laser devices can transmit any network stream that is delivered to them using optical fiber or copper cable in the forward and reverse directions. The transmitter converts electrical signals into modulated laser radiation in the infrared range with a wavelength of 820 nm and a power of up to 40 mW. Laser communication uses the atmosphere as a propagation medium. The laser beam then hits a receiver that has maximum sensitivity within the wavelength range of the radiation. The receiver converts laser radiation into signals from the electrical or optical interface used. This is how communication is carried out using laser systems.

Families, models and their features

In this section, we would like to introduce you to the three families of the most popular laser systems in the USA - LOO, OmniBeam 2000 and OmniBeam 4000 (Table 1). The LOO family is basic and allows data and voice transmission over distances of up to 1000 m. The OmniBeam 2000 family has similar capabilities, but operates at longer distance(up to 1200 m) and can transmit video images and a combination of data and voice. The OmniBeam 4000 family can provide high-speed data transfer: from 34 to 52 Mbit/s over distances up to 1200 m and from 100 to 155 Mbit/s up to 1000 m. There are other families of laser systems on the market, but they either cover shorter distances, or support fewer protocols.

Table 1.

Each family includes a set of models that support different communication protocols (Table 2). The LOO family includes economical models that provide transmission distances of up to 200 m (the letter "S" at the end of the name).

Table 2.

The undoubted advantage of laser communication devices is their compatibility with most telecommunications equipment for various purposes(hubs, routers, repeaters, bridges, multiplexers and PBXs).

Installation of laser systems

An important stage in creating a system is its installation. The actual switching on takes a negligible amount of time compared to the installation and configuration of laser equipment, which takes several hours if performed by well-trained and equipped specialists. At the same time, the quality of operation of the system itself will depend on the quality of these operations. Therefore, before presenting typical inclusion options, we would like to pay some attention to these issues.

When placed outdoors, transceivers can be installed on roof or wall surfaces. The laser is mounted on a special rigid support, usually metal, which is attached to the wall of the building. The support also provides the ability to adjust the angle of inclination and azimuth of the beam.

In this case, for ease of installation and maintenance of the system, its connection is made through distribution boxes (RK). The connecting cables are usually fiber optic for data transmission circuits and copper cable for power and control circuits. If the equipment does not have an optical data interface, then it is possible to use a model with an electrical interface or an external optical modem.

The power supply unit (PSU) of the transceiver is always installed indoors and can be mounted on a wall or in a rack that is used for LAN equipment or structured cabling systems. A condition monitor can also be installed nearby, which serves to remotely monitor the functioning of transceivers of the OB2000 and OB4000 families. Its use allows for diagnostics of the laser channel, indication of the signal magnitude, as well as looping the signal to check it.

When installing laser transceivers internally, it is necessary to remember that the power of laser radiation decreases when passing through glass (at least 4% on each glass). Another problem is water droplets running down the outside of the glass when it rains. They act as lenses and can cause beam scattering. To reduce this effect, it is recommended to install the equipment near the top of the glass.

To ensure high-quality communication, it is necessary to take into account some basic requirements.

The most important of them, without which communication will be impossible, is that buildings must be within line of sight, and there should be no opaque obstacles in the path of beam propagation. In addition, since the laser beam in the receiver area has a diameter of 2 m, it is necessary that the transceivers are located above pedestrians and traffic at a height of at least 5 m. This is due to ensuring safety regulations. Transport is also a source of gases and dust, which affect the reliability and quality of transmission. The beam must not be projected in close proximity to or cross power lines. It is necessary to take into account the possible growth of trees, the movement of their crowns during gusts of wind, as well as the influence of precipitation and possible disruptions due to flying birds.

The correct choice of transceiver guarantees stable operation of the channel in the entire range of climatic conditions in Russia. For example, a larger beam diameter reduces the likelihood of precipitation-related failures.

Laser equipment is not a source electromagnetic radiation(AMY). However, if placed near devices with EMR, the laser's electronics will pick up this radiation, which can cause a change in the signal in both the receiver and transmitter. This will affect the quality of communication, so it is not recommended to place laser equipment near EMR sources such as powerful radio stations, antennas, etc.

When installing a laser, it is advisable to avoid oriented laser transceivers in the east-west direction, since several days a year sun rays can block the laser radiation for several minutes, and transmission will become impossible, even with special optical filters in the receiver. Knowing how the sun moves across the sky in a specific area, you can easily solve this problem.

Vibration can cause the laser transceiver to shift. To avoid this, it is not recommended to install laser systems near motors, compressors, etc.

Figure 1. Placement and connection of laser transceivers.

Several typical inclusion methods

Laser communication will help solve the problem of short-range communication in point-to-point connections. As examples, let's look at several typical options or methods of inclusion. So, you have a central office (CO) and a branch (F), each of which has a computer network.

Figure 2 shows a variant of organizing a communication channel for the case in which it is necessary to combine the F and CO, using Ethernet as the network protocol, and physical environment- coaxial cable (thick or thin). In the CO there is a LAN server, and in F there are computers that need to be connected to this server. With laser systems such as the LOO-28/LOO-28S or OB2000E models, you can easily solve this problem. The bridge is installed in the central center, and the repeater in F. If the bridge or repeater has an optical interface, then an optical minimodem is not required. Laser transceivers are connected via dual fiber optics. The LOO-28S model will allow you to communicate at a distance of up to 213 m, and the LOO-28 - up to 1000 m with a “confident” reception angle of 3 mrad. The OB2000E model covers a distance of up to 1200 m with a “confident” reception angle of 5 mrad. All these models operate in full duplex mode and provide a transfer speed of 10 Mbit/s.

Figure 2. Connecting a remote Ethernet LAN segment based on coaxial cable.

A similar option for combining two Ethernet networks, using as a physical medium twisted pair(10BaseT) is shown in Figure 3. Its difference is that instead of a bridge and a repeater, concentrators (hubs) are used that have the required number of 10BaseT connectors and one AUI or FOIRL interface for connecting laser transceivers. In this case, it is necessary to install a LOO-38 or LOO-38S laser transceiver, which provides the required transmission speed in full duplex mode. The LOO-38 model can support communication distances up to 1000 m, and the LOO-38S model can communicate up to 213 m.

Figure 3. Connecting a remote Ethernet LAN segment based on twisted pair.

Figure 4 shows a variant of combined data transmission between two LANs (Ethernet) and a group digital stream E1 (PCM30) between two PBXs (in the CO and F). To solve this problem, the OB2846 model is suitable, which provides data and voice transmission at a speed of 12 (10+2) Mbit/s over a distance of up to 1200 m. The LAN is connected to the transceiver using dual optical fiber through a standard SMA connector, and telephone traffic is transmitted via 75 Ohm coaxial cable via BNC connector. It should be noted that multiplexing data and speech streams does not require additional equipment and is performed by transceivers without reducing the throughput of each of them individually.

Figure 4. Integration of computer and telephone networks.

Embodiment high speed transmission data between two LANs (LAN "A" in the CO and LAN "B" in the F) using ATM switches and laser transceivers is shown in Figure 5. The OB4000 model will solve the problem of high-speed short-range communication in an optimal way. You will have the opportunity to transmit E3, OC1, SONET1 and ATM52 streams at the required speeds over a distance of up to 1200 m, and 100 Base-VG or VG ANYLAN (802.12), 100 Base-FX or Fast Ethernet (802.3), FDDI, TAXI 100/ 140, OC3, SONET3 and ATM155 with the required speeds - over a distance of up to 1000 m. The transmitted data is delivered to the laser transceiver using a standard dual optical fiber connected via an SMA connector.

Figure 5. Consolidation of high-speed telecommunications networks.

The examples given are not exhaustive possible options use of laser equipment.

Which is more profitable?

Let's try to determine the place of laser communication among other wired and wireless solutions, briefly assessing their advantages and disadvantages (Table 3).

Table 3.

On January 30, the Eutelsat 9B satellite was launched into orbit. It became the first satellite equipped with the EDRS (European Data Relay System) system. Wanting to know more about new technology, a Mediasat correspondent went to the office of the developer of the EDRS module, Tesat, which is located in the small German town of Backnang. The head of the laser technology department, Matthias Motsigemba, gave us a tour of the enterprise and talked about laser communication technology, which is still little known in the world.

With support from the German Space Agency, Tesat has developed the Laser Communications Terminal (LCT), which provides support for high-speed data transmission between Low Earth Orbit (LEO) and Geostationary Earth (GEO) satellites. The terminal does possible transfer data at a speed of 1.8 Gbps over a distance of up to 45,000 kilometers. These LCT terminals should become the basis of the main data transmission channels in the EDRS system, which should ensure data transmission between LEO and GEO satellites.

Mathias Motsigemba: “Now we have the opportunity to provide services high quality in a mode close to real time. This makes a huge difference! The LEO satellite takes a picture and sends it to the GEO satellite, which in turn sends it to the ground via radio frequency. A laser beam is an excellent solution in a vacuum, however, in atmospheric conditions it is not the best choice, since clouds can create interference. For protection TV signal you can use high speeds data transmission and interference-resistant optical technology in the feeder line. The advent of laser communications technology can be compared to the beginning of the use of optical fiber instead of copper.”

The Earth Observation System teleport may be a foreign service using terrestrial unsecured lines.
Service optical transmission data (LEO to GEO and GEO to ground station).
The ground station can be located in its own country within the line of sight of the GEO satellite.
S/C – sovereignty of your information assets.

The need to develop this technology was dictated by the growing demand for data transmission capacity for civil and military surveillance satellites, HALE missions. The idea of ​​​​creating the EDRS system was put forward by the European Commission, which is already involved in the Sentinel satellite constellation, the Copernicus program. The next step should be the creation of inter-satellite communication channels. Eutelsat offered capacities for the communication module on Eutelsat satellite 9B. After seven years of development of the first and second generation LCTs, the LCT system was launched on Alphasat in July 2013. The LCT system on the Sentinel-1A satellite was successfully integrated in December 2013. In December 2014, the Sentinel 1A satellite was launched and put into operation. In November 2014, the European Space Agency and Tesat held a joint live presentation, during which a radar image on the Sentinel-1A satellite was sent in near real time via Alphasat over a distance of 41,700 kilometers to a ground station.

“Technically, there is no difference between the laser communications equipment installed on Alphasat and similar equipment on Eutelsat 9B. Alphasat demonstrated technical capabilities project, while the EDRS system on the Eutelsat 9 B satellite is a commercial service offered by Airbus Defense and Space. Typically, an Earth observation satellite has 10 minutes to contact a ground station and 90 minutes to orbit the Earth. This means that you can only use 10% of the space asset, and in the event of an emergency or natural disaster, too much time is spent waiting for contact with a ground monitoring station. Now, while observing ships, for example, you can detect a problem within 15 minutes.” , says Mathias Motsigemba.

The key element of the product line is the LCT-135 (135 mm telescope) for the GEO/LEO intersatellite link. As with the previous model, LCT-125, the device combines in one unit all the optical, mechanical and electrical submodules of the terminal, such as the power distribution system, on-board processor, tracking and data acquisition modules, and data processing system. Data from the satellite's AOCS sensors is easily transferred to the LCT via a standard interface - LIAU (Laser Interface Adaptation Unit).

LCT parameters:

• Range – 45,000 km.
• Weight: 53 kg.
• Data transfer rate (full duplex):
  for EDRS – 1.8 Gbit/s, for other missions – 5.65 Gbit/s.
• Transmission power: 2.2W
• Maximum power consumption: 160W
• Dimensions: 0.6 x 0.6 x 0.7 m.

This week the results of a kind of lunar laser communication became known. The test took place over 30 days under difficult conditions due to lunar dust. A special conductor was working, which is currently located in the orbit of the moon. This test showed that the communication system is fully operational despite the distance. It communicates as successfully as any radio signal from NASA.

This technology demonstrates the practical use of broadband lasers for interconnection and communications. This communication, or rather its loading, is performed much faster than a similar radio communication. This method allows you to receive a signal on Earth at a speed of 622 Mbit and send with 20 Mbit. This speed was recorded on October 20. It was transmitted from the Moon to Earth using a pulsed laser beam. This signal was received by a station in New Mexico, which is part of collaboration USA and Spain.

Lasers have a great advantage over radio signals. They are the ones who have the greatest throughput. It is also important to transmit data using a specific coherent beam. This contributes to less energy consumption when transmitting signals over long distances.

Researchers at NASA say the program's test was a great success. They did not expect this kind of results. The laser message was received and transmitted back into orbit even in the most difficult conditions. This confirms the theory that no matter what interference there is, the signal will arrive on Earth. Neither cosmic dust nor distance is an obstacle to the laser signal. Even at moments when the layer of atmosphere increased, signal transmission was carried out without any problems, which indicates efficiency of this device. There was no trace of mistrust among the skeptics at NASA when even clouds did not become an obstacle to signal transmission.

Surprisingly, there was not a single error in the signal. The procedure is reminiscent of communicating on a mobile phone. Moreover, it works without human intervention. The system can even lock itself when for a long time There is no signal coming from ground stations.