## 4 Discussion  
In this paper, an FSO system that operated in the third-atmosphere transmission window (8-12 \(\upmu\)m) was presented. According to our studies [35; 36; 38], in the case of worse  
Figure 12: Experimental results in the real conditions for a light haze (violet) with marked theoretical calculations (pink) in the case of (**a**) attenuation of the wavelength 1.5 \(\upmu\)m, (**b**) attenuation of the wavelength 10 \(\upmu\)m (**c**) visibility. weather conditions (moderate fog or mist), the designed link can effectively sustain the continuity of the connection. In Reference [41], the dependence on transmission efficiency and the position of the turbulence source within the chamber was presented. In Reference [42], the FSO system analysis in tropical weather was investigated, while the test in the area surrounding the city with a Mediterranean climate is shown in Reference [43]. In Reference [44], FSO efficiency in the maritime environment was investigated. A mathematical model of the received signal strength, taking into account a single-point measurement system for atmospheric parameters, was proposed. On the other hand, in Reference [45] the influence of spatial and temporal characteristics of atmospheric turbulence was analyzed. It turned out that temporal characteristics were slightly stronger than spatial ones. To guarantee good communication quality with increasing spatial characteristics, it is necessary to provide a wide bandwidth. The results obtained in this work indicate that the proposed FSO link works as desired both in the laboratory and in real conditions. The data from theoretical calculations were compared with the experimental results taking into account the atmospheric conditions. It turned out that similar attenuation values for individual wavelengths were obtained. Analytical calculations have shown that the FSO link operating at a wavelength of 10 \(\upmu\)m, under the conditions of visibility from 0.8 to 1.0 km, relative humidity of 70% and at a temperature of 0 \({}^{\circ}\)C, for a distance between the transmitter and receiver of about 2 km will have a larger optical power reserve than the system operating at 1.55 \(\upmu\)m wavelength, despite that, the assumed transmitter power for 10 \(\upmu\)m wavelength was lower than for a 1.55 \(\upmu\)m transmitter. This means that it is possible to reduce the radiation power generated by the laser operating in the third-generation link to reduce operating costs and extend its operating time while maintaining link range for commercially available systems. Laboratory and field tests confirmed the results of analytical analyzes. To improve the links turbulence problems may be reduced. Many research activities on beam control and tracking techniques for mitigating deleterious effects of angle-of-arrival fluctuation and pointing error on the performance of free-space optical communications are available. To increase the reliability of the system, in conditions where scintillation or turbulence of the atmosphere can cause signal fading, automatic resumption of packet transmission, appropriate modulation types, multi-beam systems or adaptive optics systems are also used. If the weather conditions are bad during the links operation (i.e., there is dense snow or a rainstorm), the FSO operating at a wavelength of 10 \(\upmu\)m will be able to provide a range similar to commercial systems. Similarly, in a situation of good visibility, i.e., above 2000 m. It is worth emphasizing that the designed link, apart from the fact that it provides better quality of transmission in fog or rain conditions, is also safer for eyesight. Thus, it is reasonable to use radiation sources in the far-infrared range in FSO systems. To summarize, the results showed that optical radiation with a wavelength of approximately 10 \(\upmu\)m was characterized by better transmission properties in the case of limited visibility (e.g., light rain and fogs) than in the case of near-infrared waves. Conceptualization, M.G., A.P., and J.S.