## 3 Results  
During the tests of a free space data link, laser structures were placed in an LHH model housing device (Alpes Lasers S.A., Switzerland) which connected a driving current signal to a laser structure placed on a standard copper submount and controlled the temperature using a two-stage TEC module. The triggering signals for the current driver were generated in a pattern signal generator model Picosecond and collimated with an off-axis parabolic mirror. To monitor laser current pulses, the current probe model CT-1 Tektronix (Tektronix Inc., Beaverton, USA) was also applied. The registered pulses were visualized and processed in an oscilloscope model MSO 6 Tektronix with a built-in eye diagram toolbox. The schematic setup of the FSO system is presented in Figure 8.  
Figure 8: The schematic setup of the free space data link. Figure 7: The signal-to-noise ratio (SNR) value vs. data link range in the case of (**a**) no scintillation and (**b**) medium scintillation \(\text{C}_{\text{n}}{}^{2}=10^{-14}\)\(\text{m}^{-2/3}\). The main tasks of the preliminary tests were to define the limitations of the light pulses concerning pulse power, time duration, and frequency. An analysis was performed in room temperature conditions and the shapes of the driving current signal and the laser light power were compared. The shapes were a coincidence but some differences were observed in the rising pulse part. For both lasers, the light pulses are a bit faster and start later (delay time \(\sim\)10 ns) (Figure 9), and therefore time was needed to exceed the lasing threshold current and to obtain population inversion. These time delays also adversely influenced the modulation rate. Some amplitude oscillations were also observed in both signals generated by impedance mismatching of the current driver and the laser interface. The pulse characteristics of these lasers were determined for time durations of 18 ns (pulsed) and 30 ns (cw). Some registered pulses for different driving currents are presented in Figure 10. For the pulsed laser, it can be observed that there is a strong influence of current on amplitude and time duration of light pulses. By decreasing the current, a decrease in both optical pulse amplitude and its time duration (min. duration time \(\sim\)5 ns) is noted. In the case of the cw laser, the driving current directly influences the radiation pulse power. However, the shape of these pulses (time duration) has not been changed. Although the average power of this laser is 20 mW, the obtained peak power for the duty cycle of 30% is \(\sim\)220 mW. In this case, the pulsed operation mode of this laser results in a better data link. Figure 10: Light pulses for different driving currents for (**a**) pulsed and (**b**) cw lasers. Figure 9: Laser light power and driving current pulses for pulsed and cw lasers. Finally, the influence of the pulse duty cycle on signal amplitude was also determined (Figure 11). The pulsed laser is more sensitive to duty cycle changes and a four-times higher duty cycle decreases the pulse power by about 30%. For the cw laser, the same decrease is measured for a 10-times higher duty cycle. A preliminary test of information signal transfer was to send several pulses in a frame (BURST). The BURST consisted of 16 triggering pulses with a repetition rate up to a frequency of 2.5 MHz (pulsed) and 40 MHz (cw) (Figure 12). That frequency was limited by the efficiency of the cooling system. The measured triggering signal period was ~25 ns for the pulsed laser and ~10 ns for the cw laser. However, for the pulsed laser, the main limitation was the pulse duty cycle that caused the mentioned thermal heating. This decreased the amplitude of the next pulses in the frame signal. The amplitudes of the radiation pulses were constant with some oscillations in the case of the cw laser.