## 2 Materials and Methods
For weak and very weak turbulence, the data range is constant (above 3.5 and 4.5 km) for 4.5 and 1.5 \(\upmu\)m wavelengths. If the refractive structure parameter increases above 5 \(\times\) 10\({}^{-16}\) m\({}^{-2/3}\), the range starts decreasing, but the longer radiation wavelength provides an extended range. To analyze the operation of an OWC transmitter with QC lasers in MWIR spectral range, two different quantum cascade structures were designed at the Institute of Electron Technology (Warsaw, Poland). The Laser #356 was a high-power pulsed QC laser (peak power 3 W and max. duty cycle of 8%) with a wavelength of ~4.5 \(\upmu\)m. The second lasing structure, the Laser #253 was designed to operate in cw mode. It was characterized by a mean average power of 20 mW with a wavelength of ~4.8 \(\upmu\)m (Figure 5a). The optical power-current (L-I) and spectral characteristics of these lasers are presented in Figure 5b. For laser radiation, the same current driver was applied. The light beam was formed using an off-axis parabolic mirror with a three-inch diameter and a two-inch focal length. This setup collimated ~75% of the laser output power. The collimated beam was collected on the mercury-cadmium-telluride (MCT) photodetector with a four-inch parabolic mirror. The photodetector was mounted with an amplifier in a detection module fabricated by the VIGO System company. To determine the data link range, it was necessary to define several parameters of the FSO components (Table 5). Figure 4: FSO link range vs. scintillation level for two wavelengths. Figure 5: (**a**) Spectrum and (**b**) optical power-current (L-I) characteristics of both quantum cascade (QC) lasers. To determine radiation attenuation of these two lasers, considering the characteristics of atmosphere absorption spectra with many absorption lines, more precise analyses of light attenuation was performed. The analyzed QC lasers are characterized by broad lines of emitted radiation (~100 nm) in the spectra and the maximum changes with supplying current. The determination of their light transmission in the atmosphere should be based on the average values of the extinction coefficient in these spectra. However, the results cannot be directly referenced as maximum extinction coefficient because of the strong influence of narrow H\({}_{2}\)O absorption lines in this spectral ranges. The averaged wavelength ranges were 4.45-4.55 \(\upmu\)m and 4.75-4.85 \(\upmu\)m, for the pulsed laser and the cw laser, respectively. It was assumed that there were no differences in the scattering effect for these radiations because of their comparable wavelengths. The averaged extinction coefficient values for summer and winter at the latitude corresponding to Poland are presented in Figure 6. The environmental parameters were temperatures of 20.5 and \(-\)1.5 \({}^{\circ}\)C and water vapors of 13.4 and 3.4 g/m\({}^{3}\). According to these characteristics, the values of extinction coefficient used in the link budgets are listed in Table 6.  
The analyses of the data link range were performed using the previously mentioned analytical model, data of the FSO components, and atmospheric attenuation listed in Tables 5 and 6. In Figure 7, some obtained characteristics of SNR values are presented. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Laser** & **Summer** & **Winter** \\ \hline Laser \#356 (4.5 \(\upmu\)m) & 0.75 km\({}^{-1}\) & 0.70 km\({}^{-1}\) \\ Laser \#253 (4.8 \(\upmu\)m) & 0.65 km\({}^{-1}\) & 0.25 km\({}^{-1}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Extinction coefficients for two lasers simulated for two weather seasons. Figure 6: Averaged extinction coefficient for summer and winter.