Visual light communication is a type of optical data transmission between two points (source and receiver), which are on a line of direct visibility.
Optical data transmission is usually realized by means of optical fibers, since they allow to transfer information over considerable distances without major power losses, matching issues, and atmospheric noise (disturbance). There is a possibility of data transmission over the spaces of free visibility without using any waveguide structures. This type of optical communication is known since Alexander Bell patented a «photo phone» in 1870.
Today, it is used more and more often both on Earth and in space. Typically, this type of communication requires direct visibility between the transmitter and the receiver, and special atmospheric optical systems, such as telescopes. As the light source, a laser is often used (sometimes with the amplifier). However, there still are problems to be solved.
Energy is transferred with a well-collimated laser radiation, but at the present stage, there is a significant loss of power in transmitting information over long distances. With the tightening of the requirements for the quality of the communication channel, the range is reduced. To reduce the divergence of the laser beam, a particular ratio of the external and internal diameters exceeding the boundaries of the range of gain over losses must be adjusted.
Large efficient optical telescopes and diffraction-limited sources are used to increase a large beam collimation. Since the wavelength of a laser beam is small, divergence of the optical transmitter is often significantly smaller than the transmitter of the microwave source or radio of similar size. The antenna gain in the optical telescope may be considerable – more than 99 dB, even with an average diameter telescope of, for example, 30 cm, which is still significantly higher than any small size microwave antenna.
High directionality of the receiver can be also considered an advantage: it is important both to obtain the greatest possible capacity of the transmitter and to bring disturbances to its possible minimum, for example, ambient light that increases the noise and thereby reduce the capacity of data transmission. You can achieve high directionality and sensitivity by using a large telescope on the reception side.
To achieve high directionality of transition, a high accuracy placement of the transmitter and the receiver is required. To reduce the impact of atmospheric noise and improve the accuracy of the radiation, adaptive optics is used.
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