## 3 Results
Figure 6: Sample oscillogram obtained during bit error rate (BER) measurements. Figure 8: BER values for different pulse duty factors and frequencies at the laser operating temperature of 0 \({}^{\circ}\)C.  
Figure 7: BER values for different pulse duty factors and frequencies at the laser operating temperature of \(-10\)\({}^{\circ}\)C.  
The conducted research shows that with the increase in the pulse duty factor, the BER value initially decreases and then increases. The lower the laser operating temperature, the lower the BER value for a given frequency. For the laser operating temperature of \(-10\)\({}^{\circ}\)C, and the frequencies of 500 kHz, 1 MHz, and 2 MHz, the lowest BER value was obtained for pulses of 100 ns duration. For the temperature of \(-10\)\({}^{\circ}\)C, the lowest BER, amounting to \(10^{-15}\), was obtained for the frequency of 500 kHz, while for 2 MHz the BER was obtained at the level of \(10^{-12}\). On the other hand, for a temperature of \(0\)\({}^{\circ}\)C, and frequencies of 500 kHz, 1 MHz, and 2 MHz, the lowest BER was obtained for pulses with a duration of about 40 ns. For the laser operating temperature of 20 \({}^{\circ}\)C, the best transmission quality was obtained for the pulse duration of 25 ns. As the laser operating temperature increased, the minimum BER value was obtained for shorter pulse durations. The heating of the laser structure during the long-duration generation causes their overmodulation and an increase in noise at the logical \"1\" level. With the increase in the laser operating temperature, an increase in the fluctuation of the amplitude of the pulses was also observed. This is due to the heating of the QC structure during current flow as well as the change in operating conditions due to the limited efficiency of the cooling system. Along with the increase of the pulse duration, increasing the laser operating temperature causes a further decrease in amplitude and an increase in the noise value. The choice of the link operating point is a compromise between the required amplitude, bandwidth, and noise value. For longer pulses, a several-fold decrease in the S/N value resulting from thermal effects occurring in the laser structure was observed. The use of an additional system stabilizing the temperature of the cooling medium, e.g., at \(0\)\({}^{\circ}\)C, would allow for more stable laser operation at even lower temperatures. However, its use will increase the size of the link, and at the same time will make it difficult to use it outside the laboratory. Based on the conducted laboratory tests it was decided that the field tests would be carried out for the laser operating temperature of \(-10\)\({}^{\circ}\)C, the frequency of 2 MHz, and the pulse duration of 100 ns. The experimental validation of the proposed FSO system in the real conditions is conducted for two measurement tracks. The first one with the designed link, and the  
Figure 9: BER values for different pulse duty factors and frequencies at the laser operating temperature of 20 \({}^{\circ}\)C.  
second one with a laser generating radiation with a wavelength of 1.5 \(\upmu\)m. The third path with laser generating radiation with a wavelength of 550 nm was used to determine visibility. The air temperature was measured with a digital thermometer and the relative humidity with a hygrometer. A diagram of the research stand for examining the influence of atmospheric conditions on the link operation is presented in Figure 10.  
The conditions under which the experiment was carried out are presented in Figure 11. The link was about 80 m, crossing the Military University of Technology from side to side. Figure 12 shows the results obtained during the tests carried out in light haze conditions are presented: the air temperature was 10 \({}^{\circ}\)C, and the relative humidity was 100%. The amplitude of the signals at the output of the photoreceivers was measured every 20 min.