## 1 Introduction  
Free space optics communication (FSO) is data transmission technology based on light propagation in free space. It takes full advantage of linking high-speed connections with energy efficiency; compact size; the low costs of installation, maintenance, and operation to enhance the communication system's accessibility [1, 2]. Moreover, it also provides unprecedented levels of data transmission security [3]. The main advantage of FSOs is their wide application field that includes outdoor wireless access, backhaul, fiber backup, last mile [4, 5], bridging Wide Area Network (WAN) [6], and military access [7]. Several factors can significantly reduce the transmission efficiency, in particular, physical obstructions, rain [8], temperature fluctuations in different air layers [9], geometric losses [10], absorption [11], atmospheric turbulence [12, 13, 14], and atmosphere attenuation and scattering [15]. In commercial FSO communication systems, strictly defined wavelengths of approximately 0.8 \(\upmu\)m and 1.5 \(\upmu\)m are used [16, 17]. Their selection is dependent on the characteristics of the atmosphere's attenuation and the availability of sources and detectors of optical radiation [18].  
The development of quantum cascade (QC) lasers operating in the long-wave infrared radiation and highly sensitive mercury cadmium telluride (MCT) detectors provided new possibilities for FSO links [19; 20]. It has become possible to develop an FSO link operating at wavelengths 8-12 \(\upmu\)m where there is less attenuation caused by mists with fine aerosol particles than in the case of commercially available links. These links are also safe for eyesight [21]. The conducted analyses show that for the construction of an optical link in open space, characterized by lower attenuation in hazy conditions than currently existing ones, QC lasers and MCT photodiode operating in the spectral range of 8-12 \(\upmu\)m can be used [22; 23; 24].  
Currently, no optical links operating in this wavelength range are available on the market. Until recently, one of the most significant barriers to their development was the lack of appropriate radiation sources.  
Due to the development of QC lasers [25; 26], it was possible to initiate work on the constructing FSO links operating in the 8-12 \(\upmu\)m wavelength range. However, the development of such a link was coupled with the need to solve several design problems, both for transmitters and receivers of optical radiation. To obtain a long range of the link, with a low error rate and high data transmission speed, it was necessary to select the optimal operating conditions for the QC laser and minimize the noise of the photoreceiver's input stage, with the possibly wide transmission band.  
In this paper, the theoretical and experimental relationship between atmospheric conditions and the intensity of received radiation in the FSO links was analyzed. Two optical links working with wavelengths 1.5 \(\upmu\)m and 10 \(\upmu\)m were analyzed. The transmission quality was characterized by the realized bit error rate (BER) [27]. It was found that improved transmission properties in the case of limited visibility were obtained for a wavelength of about 10 \(\upmu\)m, compared to the near-infrared waves. Analogous situations related to the selection/determination of appropriate parameters of the sources generating signals being sent through noisy transmission channels to improve the transmission efficiency also occur in classic transmission channels [28; 29; 30; 31; 32; 33].  
The paper is organized as follows: In Section 2, the construction of the detection and transmitter modules is described, respectively. Section 3 shows the laboratory test stand and experimental results and presents the experimental results in real conditions. Section 4 contains the discussion and conclusions.