# Hybrid Self-Protected Fiber-FSO WDM-PON System with Fiber Breakage Prevention
## 4 Result and Discussion
In the wireless FSO connection, the atmospheric scattering, turbulence, absorption and sky background will cause the power loss of free space. Additionally, the misalignment between two CMs also produces power loss. The abovementioned issues will result in poor OSNR and BER performances. Hence, we assume that the FSO transmission is performed in a clean atmosphere. According to the earlier study of an optical FSO system [27], the FSO link length can achieve 860 (13.9 dB) to 1000 m (16.7 dB) based on the corresponding redundant power without optical amplification in the proposed PON network, while the atmospheric condition is in pure status. In the next case, once the \"B1\" fault is happening in Figure 3, all the FSO signals of \(\lambda_{\text{B1D}}\) to \(\lambda_{\text{B4U}}\) will be interrupted. To restore the FSO traffic connection at this time, the SW\({}_{\text{B}}\) can switch to \"b\" point to route the protection fiber path through fiber WDM-PON architecture. Figure 5: (**a**) Measured 24.3 Gbit/s BER performances of \(\lambda_{\text{A1D}}\) to \(\lambda_{\text{A4U}}\) via FSO fiber path through 25 km fiber link and 2 m free space link. (**b**) Obtained BER measurements of \(\lambda_{\text{B1D}}\) to \(\lambda_{\text{B4U}}\) in the original FSO fiber path after 24 km fiber and 2 m wireless FSO connections. Insets are the corresponding eye diagrams of eight selected wavelengths. All the downstream and upstream FSO wavelengths will transmit through 25 km fiber link for data reconnection, as seen in Figure 3. Hence, Figure 6a exhibits the obtained 24.3 Gbit/s OOK BER measurements of \(\lambda_{\text{A1}}\) to \(\lambda_{\text{A4}}\) signals in original fiber link after 24 km transmission. Figure 6b presents the received FSO BER performances of \(\lambda_{\text{B1D}}\) to \(\lambda_{\text{B4U}}\) through a 25 km fiber link via the protection fiber. The achieved power sensitivities of \(\lambda_{\text{A1}}\) to \(\lambda_{\text{A4}}\) and \(\lambda_{\text{B1}}\) to \(\lambda_{\text{B4}}\) are \(-27.0\), \(-27.0\), \(-27.5\), \(-27.0\), \(-27.0\), \(-27.5\), \(-26.5\) and \(-26.0\) dBm under the FEC target, respectively. The insets of Figure 6 are the obtained corresponding eye diagrams of \(\lambda_{\text{A1}}\) to \(\lambda_{\text{A4}}\) and \(\lambda_{\text{B1}}\) to \(\lambda_{\text{B4}}\) under the BER of \(10^{-8}\) to \(10^{-9}\), respectively. The observed eyes are also pure. In addition, the achievable power budgets of the eight wavelengths are 34.5, 34.5, 35, 34.5, 34.5, 35, 34 and 33.5 dB, respectively, at the wavelengths of 1528.77, 1529.55, 1530.33, 1531.12, 1531.90, 1532.68, 1533.47 and 1534.25 nm. Additionally, the redundant powers of the eight wavelengths are 17.7, 17.7, 18.2, 17.7, 17.5, 18.0, 17.0 and 16.5 dB, respectively. The redundant powers are sufficient to meet with the total insertion loss induced by presented fiber-FSO WDM-PON architecture for signal connection. The measured results that the obtained power penalty differences between Figures 4 and 5, and Figures 4 and 5 are in the ranges of 4.5 to 7 dB and 6 to 7 dB, respectively, under the FEC threshold. The difference in BER measured in Figures 5 and 6 is when the fiber breakage occurs on the fiber connection of the WDM fiber and FSO-PON (as seen in Figures 2 and 3), respectively. So, the four WDM fiber (\(\lambda_{\text{A1D}}\) to \(\lambda_{\text{A4U}}\)) and four FSO signals (\(\lambda_{\text{B1D}}\) to \(\lambda_{\text{B4U}}\)) will reroute the transmission path through the FSO and fiber link as the fault is occurring. Here, there is little difference between the BER measurements obtained in Figures 5 and 6. According to the previous study, the BER performance depends on the total fiber link length. As the total transmission distance of SMF gradually ex  
Figure 6: (**a**) Measured 24.3 Gbit/s BER performances of \(\lambda_{\text{A1D}}\) to \(\lambda_{\text{A4U}}\) in original WDM fiber path through 24 km link.