## 4 Conclusions  
The paper experimentally determines the capabilities of the currently developed state-of-the-art high-power pulsed QC lasers and photodetectors in OWC systems. The interest in this technology results from the theoretical possibility of obtaining a higher link capacity (a high modulation rate of lasers) and a longer data range (a better atmospheric transparency).  
The performed analyses and tests of the constructed OWC system were provided to define its performance. Its estimated data range was 5 km, which was below the distance reported for some commercially available OWC systems operating in NIR or SWIR spectral ranges. However, in these systems, both a high data rate and long link range are usually ensured by applying a few lasers with low beam divergence in the transmitter and increasing the size of the receiver aperture. Regarding the designed system, the mentioned range was determined for a single QC laser without any advanced modulation techniques (only on-off keying), receiving optics with a 4-inch diameter, and a beam divergence of 1 mrad. Additionally, this result was estimated for the visibility of 4 km.  
The test results showed that for this type of laser, light pulses are shorter than the driving current ones. It results from both the rise time of the driving current pulses and the time needed to ensure population inversion in the lasing structure. This limits the modulation frequency of pulses. Some oscillations of the driving current coming from the no-resistance character of the QC laser were observed. Any inductance and capacitance also are very critical for ultra-high current signals and can damage the lasers. High pulse duty cycles cause a large amount of local heat generation in the laser structure by the high driving currents. It changes the operating point of the laser, usually increasing the threshold current and decreasing the light power. The modulation speed of the laser pulses also was limited. Despite the above-mentioned drawbacks, the described data link can be an effective communication device for many applications in which the data rate is less important than the link range.  
The modulation rate (up to 7 MHz) and peak power (above 0.6 W) of the light pulses are the first results obtained using a high-power QC laser operated at room temperature. They have defined new opportunities for long-range optical wireless communications.  
This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** I would like to thank my colleagues [PERSON] and [PERSON] for their support. Research was carried out in the laboratory of the Institute of Optoelectronics  
Figure 12: Eye diagrams for different modulation speeds: EW-eye width, A–”one” level amplitude.  
MUT and supported by Narodowe Centrum Badari i Rozwoju, grant No. MAZOWSEZ/0196/19-00. Assistance of the Siec Badawcza Lukasiewicz--Instytut Technologii Elektronowej ([PERSON]) in providing support of quantum cascade lasers technology is gratefully acknowledged.  
The author declares no conflict of interest.