## 3 Results and Analysis
The longer wavelengths (at 1597.6 nm) in the L-band lead to increased fiber attenuation, chromatic dispersion, self-phase modulation, stimulated Raman scattering, and FWM. The fiber nonlinearities lead to increased pulse broadening, signal distortion, interference, a high error rate, and a low SNR. Also, in FSO, the accumulated absorption loss causes significant power loss, modal crosstalk, and power fluctuations near 1600 nm, causing a high BER. The results obtained by varying the GIMMF fiber distance in both the downlink and uplink wavelengths are shown in Table 2.  
The eye patterns for distinct downlink and uplink wavelength channels operating at the LP[0,1] mode are depicted in Figure 5. After the transmission over a 5 km distance, the signals become worse compared to the signal. However, low modal crosstalk is observed at the fundamental LP[0,1] mode; thus, clear as well as open eye patterns are observed. Figure 6a,b depict the performance of the LP modal NG-PON system using a GIMMF-FSO link at varied focal lengths of the lens for downlink and uplink transmission, respectively. These figures express that higher system performance is achieved (BER \(\leq\) 10\({}^{-9}\)) to transmit bidirectional signals in the GIMMF channels when there is an increase from 0.08 to 0.125 m over a 5 km distance. Here, an acceptable BER is observed for more than a 0.085 m focal length in the downlink and 0.08 to 0.085 m in the uplink direction, as \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Downlink Wavelength** & **Distance** & **Uplink Wavelength** & **Distance** \\
**(nm)** & **(km)** & **(nm)** & **(km)** \\ \hline
1596 & 5.5 & 1524 & 5.5 \\
1596.8 & 5.45 & 1524.8 & 5.5 \\
1597.6 & 5.4 & 1525.6 & 5.5 \\
1598.4 & 5.47 & 1526.4 & 5.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summarized results for varied fiber length for various wavelengths. Figure 4: Distance vs. BER of (**a**) downlink and (**b**) uplink transmission. shown in Figure 6a,b, respectively. To further enhance the system's performance in terms of transmission distance, channel capacity, and transmission speed, an additional receiver lens can be incorporated. However, it is quite challenging to obtain an omnidirectional and flexible transmission by increasing the focal lens and using more lenses. It also shows that longer distances would require sophisticated optical beam coupling schemes at both the transmitter and receiving sides to reduce the beam size and attain lower loss coupling from the wider beam [20]. Nevertheless, the near-infrared 1524 nm wavelength faces minor chromatic dispersion effects, thermal deformations, small beam divergence, and fewer surface imperfections; thus, BER deviations are observed at a larger focal length of the lens. Figure 7 depicts the eye patterns of different wavelengths at a 0.085 m focal length of the thin lens. Figure 5: Eye patterns for different downlink and uplink wavelengths at a 5 km distance. Figure 6: Focal length vs. BER of (**a**) downlink and (**b**) uplink transmission. Figure 8a,b depict the performance of the LP modal NG-PON system at varied lens reflectance values for downlink and uplink transmission, respectively, at the LP[0,1] mode. The phase transformation of the lens is considered basic Fourier optics. It is utilized to derive the lenses' wave transform properties, propagation models, and a diffraction analysis. For a thin lens, the phase transformation is expressed as follows [21]:  
\[J(x,y)=exp[-i\frac{\pi(a^{2}+b^{2})}{\lambda f}] \tag{4}\]  
where \(f\) is the focal length, \(\lambda\) is the operating wavelength, and \(a\) and \(b\) means lens dimension. Note that higher lens reflectance linearly decreases the system's performance in terms of BER. For a downlink transmission with a 5 km fiber distance, the evaluated reflectance is 14.5% at 1596 nm, 13% at 1596.8 nm, 12% at 1597.6 nm, and 13% at 1598.4 nm, as shown in Figure 8a.