## 3 Results
### QC Laser Research for OWC Application  
The main goal of the preliminary tests was to define the power-time limitations of the emitted light pulses. Increasing their duration caused a transient decrease in the laser pulse amplitude (Figure 8a). This effect is determined by the laser structure heating during supplying the current. The shortest light pulse (~5 ns) was obtained for an 18 ns triggering signal. However, its amplitude was limited by the pulse rise time. The shapes of the driving current and the light pulses were coincidental but some differences in the rising pulse part were observed. The light pulse had a slower rise time and started later (delay time of ~14 ns) (Figure 8b). It is due to the time needed to exceed the lasing threshold current to generate population inversion in the laser structure. This delay also adversely influences the modulation rate and jitter. There also are observed amplitude oscillations  
Figure 6: SNR values versus link range.  
Figure 7: The block scheme of the OWC lab system with the testing setup (DET.—MCT photodetector, AMP.—detector amplifier).  
generated by the impedance mismatching of the current driver and the lasing structure, which is noticeable itself especially for high currents. Practically, these current oscillations generate heat and can damage the laser structure.  
Figure 9 presents some registered pulses for different driving currents and pulse frequencies. The strong influence of both parameters on the shape of emitted pulses is observed. The current decreasing and frequency increasing led to a decrease in the laser pulse amplitude. Additionally, shortening of the optical pulses also was noticed. Regarding 5 MHz and 7 MHz pulse frequencies, the peak pulse level was decreased by about 30% (to the level of 1.27 W) and about 65% (to the level of 0.64 W), respectively.  
Regarding data transmission, laser power stability is also important. This stability was tested for different pulse parameters (power, frequency, time duration). Considering Figure 10, the deviation range in the optical pulse amplitude for a 1.71 A driving current and for three frequencies (20 kHz, 3 MHz, and 5 MHz) are presented. Although for higher frequencies the light pulse level was decreasing, the power stability was improving.  
Preliminary generation of on-off keying signals was performed by sending a frame containing several pulses (BURST mode). The BURST consisted of 16 pulses (time duration of 18 ns, period time of 100 ns) with a repetition rate up to the frequency of 2.5 MHz  
Figure 8: (a) Optical signals registered by the detection module for different pulse times and (b) shapes of driving current and light signals.  
Figure 9: (a) Power of optical signals for different driving currents and (b) shapes of light pulses for different driving frequencies.  
(Figure 11a). That range of repetition rate was limited by the efficiency of the cooling laser setup determined by Peltier module parameters mounted in the LLH housing. During the tests, the maximum current of this module (2 A) was reached. The minimum time period between two pulses was \(\sim\)25 ns (three registered light pulses in the time period of 75 ns) (Figure 11b). The tested configuration allowed generating pulses with a time resolution of 25 ns, but the main limitation was the mentioned laser heating.