## 3 Results  
In the next step, the designed transmitter and receiver modules were used in the research laboratory setup as presented in Figure 4. They were located next to each other and facing the mirror on the opposite side of the laboratory. The optical path was about 70 m. The scheme of the laboratory test stand is presented in Figure 5.  
Figure 4: Photo of the receiving (**a**) and transmitting (**b**) modules of the FSO link. Figure 3: A diagram of the free space optics communication (FSO) system transmitter module. The Tektronix DSA 70404 analyzer software-Jitter and Eye Diagram Analysis Tools was used to analyze the effect of the frequency and duration of laser pulses on the BER [27; 37]. This software allowed for the obtention of an image resulting from the overlapping of successive pulses with a color determinant of the frequency of their occurrence. To verify the correctness of the adopted measurement method, a sequence of 15000 pseudo-random pulses in the return to zero (RZ) code with an amplitude of 2.5 V, frequency of 1 MHz and duration of 100 ns was generated using the Picosecond 12000 generator. The pulse sequence from the generator was visualized by storing the recorded waveforms in the internal memory (WfmDB function) with simultaneous imaging (Infinity Persistance function) on the oscilloscope. Then the following parameters were measured: signal mean value at logical levels \"0\" and \"1\" (EyeTop and EyeBase), \(Q\) factor, signal-to-noise ratio, RMS (Root Mean Square) noise value at logical levels \"0\" and \"1\" (NoiseRMStop and NoiseRMSbase). The measurements were carried out with the limitation of the measurement area, with the use of cursors, to the pulse duration. Figure 6 shows an example of the oscillogram obtained during the measurements. The method of measuring individual signal parameters was also verified. It was found that the RMS noise values at logical levels \"0\" and \"1 were determined in half of the pulse duration, while the signal mean values at logical levels \"0\" and \"1\" were determined and averaged over the range delimited by the cursors. From the presented oscillogram, the values of the \(Q\) and \(\mathrm{S/N}\) coefficients were read. They are 4.918 and 91.05, respectively. The data assessment showed that the measurement of the \(Q\) parameter with the gating turned on was consistent with the value determined from the formula [37]:  
\[Q=\frac{EyeTop-EyeBase}{NoiseRMStop+NoiseRMSbase} \tag{1}\]  
The individual components of this formula were read from the waveform screen. Figure 5: Scheme of the research position for testing the FSO link in laboratory conditions. LDD, laser diode driver; QC, quantum cascade. A Picosecond 12,000 generator was used to test the effect of the laser pulse repetition frequency, the pulse cycle factor, and the laser operating temperature on the BER value. Parameters of the generated pulses are presented in Table 2. The generator used for the research made it possible to create a pseudo-random sequence of bits stored in the device's internal memory. Pulse waveforms with a given frequency, duration, and amplitude were obtained at the generator output according to the previously created pseudo-random sequence. In Figures 7-9, the determined BER dependencies for different values of the pulse duty cycles, frequency values, and laser operating temperatures are presented. \begin{table}
\begin{tabular}{l c} \hline \hline
**Parameter** & **Value** \\ \hline Type of work & Pseudo-random sequence \\ \hline The length of the generated code & 15,000 bits \\ \hline Character length & 14 \\ \hline Code type & RZ (return to zero) \\ \hline Pulse frequency & from 500 kHz to 9.0 MHz \\ \hline Pulse duty cycle & from 0.5\% to 40\% \\ \hline Operating temperature of the laser & from \(-10\)\({}^{\circ}\)C to \(20\)\({}^{\circ}\)C \\ \hline Low-level voltage & 4.5 V \\ \hline High-level voltage & 0 V \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of the pulses generated by the Picosecond 12,000 generator, for which the BER studies were conducted.