## 2 Materials and Methods  
In Figure 1, the detection module, which is used in the proposed FSO link, is presented. It is a modified version of the VPAC series of modules by VIGO System S.A. The standard housing of such a module includes an infrared radiation detector, a four-stage thermoelectric cooler (TEC) with a temperature sensor, a broadband preamplifier, electromagnetic noise filters (EMIs), and a miniature fan dissipati  
Figure 1: Broadband detection module of the VPAC series with a biased photodiode [23]. the cooler. Additionally, this equipment includes a TEC controller and a DC power supply. In Figure 2, a block diagram of this module is presented. The current signal obtained from the HgCdTe photodiode is read by a wideband (about 1 GHz) trans-impedance amplifier. This amplifier also provides a constant reverse power supply to the detector with a voltage of 200 mV. This creates the conditions for obtaining the maximum signal-to-noise ratio over a wide frequency range. Table 1 presents the most important parameters of this module, optimized for a wavelength of 10 \(\upmu\)m. More detailed descriptions and experimental research on detection modules, shown in Figure 2, are presented in References [23,24]. In the transmitter module, a cascade laser from Alpes Lasers was used [34]. The laser control system consists of a laboratory laser housing (LLH) in which the laser was placed on a thermoelectric cooler, a laser diode driver (LDD 400), and a thermoelectric cooler controller (TCU 200). A window made of zinc selenide (ZnSe) with a diameter of 11 mm, which is transparent to radiation with a wavelength of 3.5 to 12 \(\upmu\)m, was installed in the head. The LDD 400 module was used to control the laser. It has an output to monitor the current flowing through the laser structure. The LLH laser head was connected to the LDD 400 system with a low-impedance tape. An external generator was required to control the LDD 400 system. A TCU 200 controller was used to regulate the operating temperature of the QC laser. The temperature was measured by a PT100 temperature sensor built into the head. The temperature control range is from \(-40\) to \(+80\)\({}^{\circ}\)C. The built-in display allowed us to read the set and current temperature with a resolution of 0.1 \({}^{\circ}\)C. The construction of the laser head also allowed the use of additional cooling using water circulation. A TPG 200 generator was used to generate the laser control pulses. This generator enables laser control via the LDD 400 system. It allows the generation of pulses with a duration of up to 200 ns at a frequency of 10 kHz to 5 MHz. It is also equipped with a trigger input for synchronization of the generated pulses with an external signal and with two control outputs according to the TTL (Transistor-Transistor-Logic) standard with an impedance \begin{table}
\begin{tabular}{l c c} \hline \hline
**Parameter** & **Unit** & **Value** \\ \hline Transimpedance \(\text{@R}_{\text{LOAD}}=50\)\(\Omega\) & V/A & \(13.5\times 10^{3}\) \\ \hline Output resistance & \(\Omega\) & 50 \\ \hline Gain bandwidth & MHz & 0.001\(\div\)780 \\ \hline Noise voltage & nV/Hz\({}^{1/2}\) & 190 \\ \hline Voltage sensitivity & V/W & \(2.1\times 10^{5}\) \\ \hline Detectivity & cmHz\({}^{1/2}\)/W & \(3\times 10^{10}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The basic parameters of the detection module optimized for a wavelength range of 10 \(\upmu\)m [9]. Figure 2: Block diagram of the detection module. TEC, thermoelectric cooler; EMI, electromagnetic noise filter; SubMiniature version B connector (SMB). of 50 \(\Omega\), enabling to obtain the rise and fall times of the control pulses below 10 ns. An additional water-cooling system for the laser head was used for research purposes. It consisted of an electric water pump, an expansion tank, a radiator with fans, and coolant lines. Figure 3 shows a diagram of the FSO system transmitter module.