What are the components of an atmospheric laser communication system?

The atmospheric laser communication system is mainly composed of atmospheric channels, optical transmitters, optical receivers, optical antennas (lenses or mirrors), electric transmitters, electrical receivers, terminal equipment, power supplies, etc., and some are also equipped with auxiliary equipment such as remote control and telemetry. The information is converted into the corresponding electrical signal by the electric transmitter, modulated to the optical carrier generated by the laser with a modulator, and then the modulated optical signal is emitted into the atmosphere through the optical transmitting antenna. The optical signal is transmitted through the atmospheric channel and reaches the receiving end, and the optical receiving antenna focuses on the received optical signal, and then sends it to the optical detector, which is amplified and restored into the original electrical signal, and sent to the electric receiver to be demodulated into the original information.

In outer space, if lasers are used to communicate, they can be unaffected by the atmosphere. At present, the carrier wave of satellite communication is microwave, and the data transmission rate is difficult to reach more than 50 megabits per second, the main reason is that the communication satellite cannot accommodate a large antenna, and the future satellite communication data rate is required to work in hundreds or thousands of gigabit ratios per second, therefore, it can only be realized by laser communication. The use of laser communication between communication satellites can achieve unexpected advantages.

Under ideal circumstances, the laser carrier has a 4 kHz circuit belt that can accommodate 10 billion channels; If the bandwidth of color TV is 10 MHz, it can transmit 10 million sets of programs at the same time without interfering with each other.

The frequency of the laser is simple, the energy is highly concentrated, the beam is very fine, the wavelength is between microwave and infrared, if the laser is used to carry out inter-satellite link communication and many other characteristics such as high intensity, high monochromaticity, high coherence and high directionality, you can obtain a series of advantages such as larger capacity, narrower beam, higher gain, faster speed, stronger anti-interference and better confidentiality, so that the laser becomes the most ideal carrier for the development of space communication satellites.

The laser communication system consists of two parts: sending and receiving. The transmitting part mainly includes lasers, optical modulators and optical transmitting antennas. The receiving part mainly includes optical receiving antenna, optical filter and light detector.

The information to be transmitted is sent to an optical modulator connected to the laser, which modulates the information on the laser and sends it out through an optical transmitting antenna. At the receiving end, the optical receiving antenna receives the laser signal and sends it to the photodetector, which turns the laser signal into an electrical signal, which becomes the original information after amplification and demodulation.

Atmospheric laser communication can transmit information such as language, text, data, and images. It has the advantages of large communication capacity, no electromagnetic interference, strong confidentiality, light equipment, good mobility, etc., but it is difficult to align the lead in front of the optical transceiver antenna when used, and the communication distance is limited to the visual distance (thousands of meters to tens of kilometers in the first mode), which is susceptible to the influence of the climate, and can even cause communication interruption under severe climatic conditions.

Atmospheric laser communication can transmit language, text, data, images and other information.

The advantages of laser communication are:

1) Large communication capacity. Theoretically, laser communication can transmit 10 million TV programs and 10 billion channels at the same time**.

2) Strong confidentiality. The laser is not only highly directional, but also can use invisible light, so it is not easy to be intercepted by the enemy, and has good secrecy performance.

3) The structure is light and the equipment is economical. Due to the small divergence angle of the laser beam and good directionality, the transmitting antenna and the receiving antenna required for laser communication can be very small, the general antenna diameter is tens of centimeters, and the weight is only a few kilograms, while the microwave antenna with similar functions weighs several tons and more than ten tons.

Some of the weaknesses of laser communication are:

1) The communication distance is limited to the line-of-sight (a range of several kilometers to tens of kilometers), which is susceptible to climate influences, and even causes communication interruption under harsh climatic conditions. Atmospheric molecules such as oxygen, nitrogen, carbon dioxide, and water vapor in the atmosphere have an absorption effect on light signals; The uneven molecular density of the atmosphere and the dust, smoke, ice crystals, salt particles, microorganisms and tiny water droplets suspended in the atmosphere have a scattering effect on the light signal. Clouds, rain, fog, snow, etc., severely attenuate the laser.

Atmospheric turbulence caused by air convection on the Earth's surface can affect laser transmission with effects such as beam defection, beam diffusion, beam scintillation (random variation of bright and dark spots within a beam cross-section), and effects like jitter (random jumping of beam convergence points).

Different wavelengths of laser light have different reductions in the atmosphere. Theoretical and practical results show that the attenuation of lasers with wavelength and wavelength is smaller, and the laser with wavelength is stronger in fog penetration.

Atmospheric laser communication can be used for communication in rivers and lakes, border defense, islands, mountains and valleys, etc.; It can also be used for temporary replacement equipment when microwave communication or coaxial cable communication is interrupted and repaired. Blue-green lasers with a wavelength of nearby can be used for underwater communications or communications against submarines.

2) Difficulty in aiming. The laser beam has an extremely high directivity, which makes it difficult to aim between the transmitting and receiving points. In order to ensure aiming between the transmitting and receiving points, not only high requirements are put forward for the stability and accuracy of the equipment, but also the operation is complicated.

Laser is a light wave that also has the properties of electromagnetic waves. However. Lasers are significantly different from ordinary radio waves, with a frequency of several hundred million trillion cycles, which is more than 100,000 times the frequency of microwaves (ultra-high frequency electromagnetic waves).

From the relationship between wavelength and wave velocity c and frequency, it can be seen that the wavelength of a laser is very short, so its volatility is much worse than that of radio waves. On the contrary, the laser has peculiar particle properties, which makes it an eye-catching "rising star" in military communications.

Laser communication is basically similar to radio communication, with the laser emitted by the laser being used as the carrier at the transmitting end. The voice signal is changed into an electrical signal through the transmitter and sent to the modulator, which controls a certain parameter (frequency, amplitude or phase) of the carrier to send the voice signal on the laser light wave according to the change of voice, and sends it out through the transmitting telescope (also known as the transmitting antenna) to propagate in the medium. At the receiving end, the receiving telescope (also known as the receiving antenna) converts the laser signal into a voice signal in the opposite direction of the transmitting end.

According to the different transmission media, laser communication can be divided into cosmic communication (laser propagation in space outside the atmosphere), atmospheric communication (laser propagation within the atmosphere), underwater communication (laser propagation under water) and optical fiber communication (laser propagation in optical fibers).

Laser communication is a communication method that uses lasers to transmit information. Laser is a new type of light source, which has the characteristics of high brightness, strong directionality, good monochromacy, and strong coherence. According to the transmission medium, it can be divided into atmospheric laser communication and optical fiber communication.

Atmospheric laser communication is a laser communication that uses the atmosphere as a transmission medium. Optical fiber communication is a communication method that uses optical fibers to transmit optical signals.

The atmospheric laser communication system is mainly composed of a large air channel, an optical transmitter, an optical receiver, an optical antenna (lens, trousers, vertical or reflector), an electric transmitter, an electric receiver, a terminal equipment, a power supply, etc.